2.6 Ideal battery capacity
The battery capacity is a measure of the amount of charge or energy stored in the battery. The fundamental units of battery capacity is coulombs (C), although a more common and useful unit is Amp-hrs (Ah) (amps = C/time, so Ah = C/time(sec) x time (hrs)). The battery capacity in Ah can be ideally calculated from the weight/volume or number of moles of materials of the electrode and electrolyte (if it is an active component in the redox reactions) in the battery. The ideal battery capacity under equilibrium conditions (which can differ substantially from the “real” battery capacity under load) is calculated by from the moles of available reactants, from which the moles of electrons can be determined. Using Faraday’s constant, which gives the number of Coulombs for a mole of electrons (F = 96,484.56 C/mol), the total available coulombs (charge) can be determined for the battery. Since the battery capacity, when described in Ah is a measure of the total stored coulombs (Amps are C/sec), then the battery capacity can be determined by the equation:
Since the primary function of a battery is to store electrical energy rather than electrical charge, the energy storage of a battery is also an essential parameter. A simple way to determine the energy storage capacity of the battery is to multiply the Ah capacity by the nominal battery voltage, such that:
3 Battery voltage and capacity in non-equilibrium
The battery voltage described by the Nernst Equation and battery capacity assumes that the battery is in equilibrium. Since a battery under load is not in equilibrium, the measured voltage and battery capacity may differ significantly from the equilibrium values, and the further from equilibrium (ie the high the charge or discharge currents), the larger the deviation between the battery voltage and capacity equilibrium and the realistic battery voltage may be. The difference between the voltage under equilibrium and that with a current flow is termed polarization. Polarization effects have a significant impact on battery operation, both beneficial and detrimental. For example, polarization effects mean that under normal operation of lead acid batteries the electrolysis of water proceeds slowly and to first order can be neglected during discharge (but not charging since the voltage is higher). However, polarization effects also have detrimental effects on performance, by, for example, reducing efficiency and making the battery capacity sensitive to charging and discharging conditions.
The polarization is comprised of three basic mechanisms, relating to resistive drops in the battery, and to two effects relating to the rates at which a reaction can proceed. These two effects are due to kinetic effects caused by the inherent rates of the chemical reaction (called kinetic overvoltage or activation overvoltage), and by the effects related to the movement of reactants to the electrode (called mass transport overvoltage). The overvoltage causes a deviation of the voltage and capacity from the equilibrium values calculated earlier. As shown below, during discharging, the battery voltage is lower than that in equilibrium, while during charging, a higher voltage than the Nernst voltage is required. Polarization effects have significant impact on the battery efficiency and how the battery can be charged and discharged.
3.1 Cell potential away from equilibrium and including polarization effects.
3.1.1 Reaction Rates and Polarization
The equilibrium electrochemical potentials only take into account the initial and final potentials of the materials in the reaction, without considering the rates or kinetics of the reactions themselves. The chemical reaction rates play an important role in determining the operation of a battery and in the processes that control battery behavior. For example, if multiple reactions can occur, then a reaction with a reaction rate significantly lower than all other reaction rates will not proceed to a significant extend and may potentially be ignored.
A single chemical reaction is typically composed of multiple steps, and each of these steps has a particular rate. The reaction rate is controlled by two processes. First, in order for the reaction to proceed, all the reactants must be physically present in one location, which for a battery is the electrode. The processes which involve the transport of the reactants in their appropriate form to the site of the chemical reaction are called mass transport or concentration overpotential. The steps in getting the reactants to the electrode are shown below: all the reactants must be present in their appropriate form (ie in solution or as a solid), those in solution must diffuse to the site of the reaction, the reactant species must absorb on the surface of the electrode (if the electrode is part of the chemical reaction), and finally the electron transfer must occur. Applied to a lead acid battery, this means that both the lead metal and the lead ion must be present. This involve the dissolution of the metallic ion (if it is present in solid form, as in the lead acid case shown below), the transport of the reactants from the electrolyte to the electrode surface, and the adsorption of the necessary components on the electrode surface.
In addition to the transport of the reactant species to the site of the reaction, a second possible rate limiting step for the reaction is the rate at which the chemical reaction proceeds due to the kinetics of the chemical reaction. In many chemical reactions, the reacting species form short-lived intermediate products, and then these intermediate products react to form the final products. If the rate of formation of the intermediate species is slower than the remaining steps, then these intermediate steps control the reaction rate. Further, the energy required to form these intermediate products may be higher than the average energy of the reactants. As the reactants have a distribution of kinetic energy, and only those with higher energy can form the intermediate products. In this case, only a fraction of the initial reactants have sufficient energy to allow the reaction to proceed, thus limiting the reaction rate. The higher energy of the intermediate species gives rise to an activation energy, as shown in the figure below. In order for the reaction to proceed at a rapid rate, the reactants must be given energy greater than the activation energy. As the kinetic energy of the reactants is determined by their temperature, increasing the temperature of the reactants is a simple (but for batteries often impractical or accompanied by numerous other negative aspects) way to increase the reaction rate and decrease the overpotential.
Another way to decrease the activation energy may be reduced for some reactions by the use of a catalyst. In some chemical reactions, the reactant atoms must interact or collide in a particular way, such that a new material forms. For example, the interaction may require that the reactants a physically oriented in a particular way, as shown in the figure below. For such reactions, the addition of other chemical species that tend to orient the molecules in a specific oreintation increase the probability of the reaction proceeding. Materials that have such an effect are called catalysts. This effect makes reaction rates sensitive to the presence of small number of other species, which do not appear in the formula of the chemical reaction.
3.1.2 Mass Transport Overvoltage in Batteries
The mass transport overvoltage has a significant impact on batteries, particularly at high rates of charge and discharge. As the battery discharges, it depletes the region around the electrode of some of the reactants. The concentration gradient between the region surrounding the electrode and further away in the electrolyte causes reactants to diffuse towards the electrode. However, if the discharge rate of the battery causes the reactants to be used at a greater rate than they can diffuse towards electrode, then the concentration near the electrode will continue to drop as the battery discharges. This drop in concentration is greater than that expected voltage drop if the reactants were uniformly distributed through the electrolyte and therefore, according to the Nernst equation, the battery voltage decreases more rapidly than that calculated by equilibrium. The more rapidly a battery is discharged, the more rapid the fall in voltage compared to that from equilibrium. Rapid discharging affects not only the battery voltage, but also battery capacity. Since the some of the reactants are not used in the reaction before the voltage drops below the minimum voltage, then the available battery capacity is also reduced.
During charging, a similar process occurs, except that charging increases the concentration surrounding the electrode. Consequently, a higher voltage is required to charge the battery than expected by equilibrium calculations. The mass transport overvoltage has a significant effect on the battery parameters relevant to a photovoltaic system. The lower voltages during discharge and higher voltages during charging reduce the battery efficiency. Further, mass transport effects alter the available battery capacity, as the battery capacity is reduced under high discharge rates. Because of these effects, mass transport has a significant impact on the optimal use of a battery, limiting both the charge and discharge currents.
3.1.3 Activation Overvoltage in Batteries
The kinetic or activation overvoltage of the reduction and oxidation reactions of the battery should be as small as possible, since during charging the voltage required will greater than the equilibrium voltage by activation energy. The difference in the charging voltage and the discharging voltage (i.e., the overvoltage) reduces the battery efficiency.
If there are secondary or side reactions in the battery, then the kinetic overpotential has different effects between charging and discharging. During discharging, the battery voltage is lower, and therefore there is less possibility that the voltage is sufficient to overcome the activation energy of secondary battery reactions. During charging, the battery voltage is higher, and hence there is the possibility that additional reactions can occur. This effect can give rise to beneficial properties. The hydrolysis of water consists of the redox reaction shown below, which has a electrochemical potential of 1.23 V.
Consequently, if a voltage of more than 1.23V is applied to a battery which has water as a component of the electrolyte, then the electrolysis of water occurs, producing hydrogen and oxygen instead of the charging reaction for the battery. Since most batteries operate at about 2V, this would then make water-based electrolytes unsuitable for batteries. However, the overvoltage of the redox reactions in the electrolysis of water are high enough such that during discharging, gas evolution from the electrolysis of water (or either one of the half reaction involved in the electrolysis of water) is not a dominant consideration. However, during charging, the higher voltage experienced by the battery causes first the hydrogen and then the oxygen half reactions to proceed. In lead acid battery systems, the presence of these two reactions gives rise to gassing. In many battery configurations, gassing leads to numerous undesirable side-effects, including water loss from the electrolyte and physical damage to the electrolyte.
3.1.4 Resistive Drops in Batteries
A final contribution to the overvoltage in a battery is the resistive drops that occur in a battery. There are several components of the total battery resistance. Part of the overall resistance is due to resistance of the components in the path of the electron flow, including the electrode and the connections between the two electrodes. Other components of the resistive polarization include the surface of the electrodes. The resistive polarization may not be linear with applied voltage. Other components of the resistive polarization include the resistance of the surface of the electrode. For example, in a lead acid battery, as the discharge reaction proceeds, lead sulfate builds up on the surface of the electrode, which is non-conductive. The resistive overpolarization has several practical impacts on battery performance and operation. Similar to the concentration polarization, it reduces the efficiency and places limits on how much the battery can be charged or discharged.
3.2 Secondary Reactions
In addition to the central reduction and oxidation reaction which comprise a battery, secondary or side reactions may occur. In most cases, these side reactions give rise to unwanted or detrimental effects. In all cases, these secondary reactions reduce the coulombic efficiency of the battery. If the secondary reaction occurs during discharging, some of the charge (current that would normally flow to the load is used by the secondary reaction). Similarly, during charging, the secondary reactions use charge intended to drive the main battery reactions, thus reducing the couloumbic efficiency. The electrolysis of water described in the activation overpotential is an example of an unwanted secondary reaction. Secondary reactions give rise to several unwanted effects, such a gassing, self-discharge and corrosion of the electrodes.
3.3 Physical State of the Electrodes
The physical state of the electrodes plays an important part in the practical operation of a battery. The key characteristic of a battery electrode is that its surface area should be large. This lowers the series resistance, increases the area over which the chemical reaction can take place (hence also reducing the mass transport overvotlage). In addition, a large surface area helps ensure that the reactants are not completely covered by the products of the chemical reaction. A complete, uniform coverage of the electrode by the product reaction would prevent the redox reactions from proceeding, since the reactant species could not longer reach the electrode. Moreover, even in the reaction products allow the reactant species through, the reaction products are often not conductive, and therefore electrons evolved or required by the redox reactions could not pass through the reaction productions. A large surface area is typically achieved by using porous materials. The figure below shows the porous lead used in a lead acid battery.
During charging and discharging, several processes can occur that change the structure or shape of the electrode. In most battery reactions, the electrode materials undergo a physical change during the discharge/charge cycle. The changes to the electrode, both physical changes as the original electrode material is re-formed and chemical changes of the materials on the electrodes give rise to numerous non-idealities. A key non-ideality is that the material may change its morphology, potentially during deposition of the reaction products on the electrode, but more commonly when the electrode material remains unchanged for long periods of time. For example, in lead-acid batteries, lead sulfate, which forms as the battery is discharged, may form large, relatively insoluble crystals over time. These large crystals are difficult to convert back into lead or lead oxide, and hence they reduce battery capacity if the battery is left in its discharged state.
Other effects that relate to the physical changes experienced by the electrode or electrolyte are that the reactant products seldom have the identical density as the reactants, and hence the electrode undergoes physical changes in its size. If the mechanical stresses are too large, the electrode material may flake off, hence permanently reducing capacity. The relative physical changes in size may be exacerbated at high or low temperatures, as density differences may increase as the temperate changes.
Finally, as the electrode material is re-formed during charging, the electrode may change its shape. In lead acid batteries, this is circumvented by the fact that the solubility of the lead ion Pb2+ is very low, and hence Pb2+ is rapidly converted to Pb in the close physical proximity to where it was dissolved, thus preventing significant changes of shape of the electrode. Alternately, either the products during discharging or the original battery material during charging may form so as isolate regions from charging or discharging, thus permanently reducing battery capacity.
4 Battery Characteristics
The use of batteries in photovoltaic systems differs from the use of batteries in other common battery applications. For photovoltaic systems, the key technical considerations are that the battery experience a long lifetime under nearly full discharge conditions. Common rechargeable battery applications do not experience both deep cycling and being left at low states of charge for extended periods of time. For example, in batteries for starting cars or other engines, the battery experiences a large, short current drain, but is at full charge for most of its life. Similarly, batteries in uninterruptible power supplies are kept at full charge for most of their life. For batteries in consumer electronics, the weight or size is often the most important consideration. This section provides an overview of the critical battery characteristics or specifications, including battery voltage, capacity, charging/discharging regimes, efficiency, etc.
4.1 Battery Voltage
The voltage of a battery is a fundamental characteristic of a battery, which is determined by the chemical reactions in the battery, the concentrations of the battery components, and the polarization of the battery. The voltage calculated from equilibrium conditions is typically known as the nominal battery voltage. In practice the nominal battery voltage cannot be readily measured, but for practical battery systems (in which the overvoltages and non-ideal effects are low) the open circuit voltage is a good approximation to the nominal battery voltage.
Since the electric potential (voltage) from most chemical reactions is on the order of 2V while the voltage required by loads is typically larger, in most batteries, numerous individual battery cells are connected in series. For example, in lead acid batteries, each cell has a voltage of about 2V. Six cells are connected to form a typical 12V lead acid battery.
4.1.1 Voltage Variation with Discharging
Due to the polarization effects, the battery voltage under current flow may differ substantially from the equilibrium or open circuit voltage. A key characteristic of a battery technology is how the battery voltage changes due under discharge conditions, both due to equilibrium concentration effects and due polarization. Battery discharge and charging curves are shown below for several different battery systems. The discharge and charge curves are not necessarily symmetric due to the presence of additional reactions that may be present at the higher voltages encountered in charging.
4.1.2 Cut-Off Voltage
In many battery types, including lead acid batteries, the battery cannot be discharged below a certain level or permanent damage may be done to the battery. This voltage is called the "cut-off voltage" and depends on the type of battery, its temperature and the battery's rate of discharge.
4.1.3 Measuring State of Charge Based on Voltage
While the reduction of battery voltage with discharge is a negative aspect of batteries which reduces their efficiency, one practical aspect of such a reduction, if it is approximately linear, is that at a given temperature, the battery may be used to approximate the state of charge of the battery. In systems where the battery voltage is not linear over some range of state of charge of the battery or in which there are rapid variations in the voltage with the BSOC will be more difficult to determine the BSOC and therefore will be more difficult to charge. However, a battery system that maintains a more constant voltage with discharge rate will have a high voltage efficiency and will be more easily used to drive voltage sensitive loads.
4.1.4 Effect of Temperature on Voltage
Battery voltage will increase with the temperature of the system, and can be calculated by the Nernst Equation for the equilibrium battery votlage.
4.2 Battery Charging and Discharging Parameters
The key function of a battery in a PV system is to provide power when other generating sourced are unavailable, and hence batteries in PV systems will experience continual charging and discharging cycles. All battery parameters are affected by battery charging and recharging cycle.
4.2.1 Battery State of Charge (BSOC)
A key parameter of a battery in use in a PV system is the battery state of charge (BSOC). The BSOC is defined as the fraction of the total energy or battery capacity that has been used over the total available from the battery.
Battery state of charge (BSOC or SOC) gives the ratio of the amount of energy presently stored in the battery to the nominal rated capacity. For example, for a battery at 80% SOC and with a 500 Ah capacity, the energy stored in the battery is 400 Ah. A common way to measure the BSOC is to measure the voltage of the battery and compare this to the voltage of a fully charged battery. However, as the battery voltage depends on temperature as well the state of charge of the battery, this measurement provides only a rough idea of battery state of charge.
4.2.2 Depth of Discharge
In many types of batteries, the full energy stored in the battery cannot be withdrawn (in other words, the battery cannot be fully discharged) without causing serious, and often irreparable damage to the battery. The Depth of Discharge (DOD) of a battery determines the fraction of power that can be withdrawn from the battery. For example, if the DOD of a battery is given by the manufacturer as 25%, then only 25% of the battery capacity can be used by the load.
Nearly all batteries, particularly for renewable energy applications, are rated in terms of their capacity. However, the actual energy that can be extracted from the battery is often (particularly for lead acid batteries) significantly less than the rated capacity. This occurs since, particularly for lead acid batteries, extracting the full battery capacity from the battery dramatically reduced battery lifetime. The depth of discharge (DOD) is the fraction of battery capacity that can be used from the battery and will be specified by the manufacturer. For example, a battery 500 Ah with a DOD of 20% can only provide 500Ah x .2 = 100 Ah.
4.2.3 Daily Depth of Discharge
In addition to specifying the overall depth of discharge, a battery manufacturer will also typically specify a daily depth of discharge. The daily depth of discharge determined the maximum amount of energy that can be extracted from the battery in a 24 hour period. Typically in a larger scale PV system (such as that for a remote house), the battery bank is inherently sized such that the daily depth of discharge is not an additional constraint. However, in smaller systems that have relatively few days storage, the daily depth of discharge may need to be calculated.
4.2.4 Charging and Discharging Rates
A common way of specifying battery capacity is to provide the battery capacity as a function of the time in which it takes to fully discharge the battery (note that in practice the battery often cannot be fully discharged). The notation to specify battery capacity in this way is written as Cx, where x is the time in hours that it takes to discharge the battery. C10 = Z (also written as C10 = xxx) means that the battery capacity is Z when the battery is discharged in 10 hours. When the discharging rate is halved (and the time it takes to discharge the battery is doubled to 20 hours), the battery capacity rises to Y. The discharge rate when discharging the battery in 10 hours is found by dividing the capacity by the time. Therefore, C/10 is the charge rate. This may also be written as 0.1C. Consequently, a specification of C20/10 (also written as 0.1C20) is the charge rate obtained when the battery capacity (measured when the battery is discharged in 20 hours) is discharged in 10 hours. Such relatively complicated notations may result when higher or lower charging rates are used for short periods of time.
The charging rate, in Amps, is given in the amount of charge added the battery per unit time (i.e., Coulombs/sec, which is the unit of Amps). The charging/discharge rate maybe specified directly by giving the current - for example, a battery may be charged/discharged at 10 A. However, it is more common to specify the charging/discharging rate by determining the amount of time it takes to fully discharge the battery. In this case, the discharge rate is given by the battery capacity (in Ah) divided by the number of hours it takes to charge/discharge the battery. For example, a battery capacity of 500 Ah that is theoretically discharged to its cut-off voltage in 20 hours will have a discharge rate of 500 Ah/20 h = 25 A. Furthermore, if the battery is a 12V battery, then the power being delivered to the load is 25A x 12 V = 300W. Note that the battery is only "theoretically" discharged to its maximum level as most practical batteries cannot be fully discharged without either damaging the battery or reducing is lifetime.
4.2.5 Charging and Discharging Regimes
Each battery type has a particular set of restraints and conditions related to its charging and discharging regime, and many types of batteries require specific charging regimes or charge controllers. For example, nickel cadmium batteries should be nearly completely discharged before charging, while lead acid batteries should never be fully discharged. Furthermore, the voltage and current during the charge cycle will be different for each type of battery. Typically, a battery charger or charge controller designed for one type of battery cannot be used with another type.
4.3 Battery Capacity
"Battery capacity" is a measure (typically in Amp-hr) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions. However, the actual energy storage capabilities of the battery can vary significantly from the "nominal" rated capacity, as the battery capacity depends strongly on the age and past history of the battery, the charging or discharging regimes of the battery and the temperature.
Units of Battery Capacity: Ampere Hours
The energy stored in a battery, called the battery capacity, is measured in either watt-hours (Wh), kilowatt-hours (kWh), or ampere-hours (Ahr). The most common measure of battery capacity is Ah, defined as the number of hours for which a battery can provide a current equal to the discharge rate at the nominal voltage of the battery. The unit of Ah is commonly used when working with battery systems as the battery voltage will vary throughout the charging or discharging cycle. The Wh capcity can be approximated from the Ahr capacity by multiplying the AH capcity by the nominal (or, if known, time average) battery voltage. A more accurate approach takes into account the variation of voltage by integrating the AH capacity x V(t) over the the time of the charging cycle. For example, a 12 volt battery with a capacity of 500 Ah battery allows energy storage of approximately 100 Ah x 12 V = 1,200 Wh or 1.2 KWh. However, because of the large impact from charging rates or temperatures, for practical or accurate analysis, additional information about the variation of battery capacity is provided by battery manufacturers.
4.3.1 Impact of Charging and Discharging Rate on Capacity
The charging/discharging rates affect the rated battery capacity. If the battery is being discharged very quickly (i.e., the discharge current is high), then the amount of energy that can be extracted from the battery is reduced and the battery capacity is lower. This is due to the fact the necessary components for the reaction to occur do not necessarily have enough time to either move to their necessary positions. The only a fraction of the total reactants are converted to other forms, and therefore the energy available is reduced. Alternately, is the battery is discharged at a very slow rate using a low current, more energy can be extracted from the battery and the battery capacity is higher. Therefore, the battery of capacity should include the charging/discharging rate. A common way of specifying battery capacity is to provide the battery capacity as a function of the time in which it takes to fully disscharge the battery (note that in practice the battery often cannot be fully discharged).
The temperature of a battery will also affect the energy that can be extracted from it. At higher temperatures, the battery capacity is typically higher than at lower temperatures. However, intentionally elevating battery temperature is not an effective method to increase battery capacity as this also decreases battery lifetime.
4.3.3 Age and history of battery
The age and history of the battery have a major impact on the capacity of a battery. Even when following manufacturers specifications on DOD, the battery capacity will stay at or close to its rated capacity for a limited number of charge/discharge cycles. The history of the battery has an additional impact on capacity in that if the battery has been taken below its maximum DOD, then battery capacity may be prematurely reduced and the rated number of charge/discharge cycles may not be available.
4.4 Battery Efficiency
As with any other component in a PV system, efficiency is an important issue in component selection due to the relatively high cost of power generated by PV modules. The overall battery efficiency is specified by two efficiencies: the columbic efficiency and the voltage efficiency.
4.4.1 Columbic Efficiency
The columbic efficiency of battery the ratio of the number of charges that enter the battery during charging compared to the number that can be extracted from the battery during discharging. The losses that reduce columbic efficiency are primarily due to the loss in charge due to secondary reaction, such as the electrolysis of water or other redox reactions in the battery. In general, the columbic efficiency may be high, in excess of 95%.
4.4.2 Voltage Efficiency
The voltage efficiency is determined largely be the voltage difference between the charging voltage and voltage of the battery during discharging. The dependence of the battery voltage on BSOC will therefore impact voltage efficiency. Other factors being equal, a battery in which the voltage varies linearly with BSOC will have a lower efficiency than one in which the voltage is essentially constant with BSOC.
4.5 Energy, Volumetric and Power Density
Energy density is a parameter used chiefly to compare one type of battery system to another. The energy density of a battery is the capacity of the battery divided by either the weight of the battery, which gives the gravimetric energy density in Wh/kg, or by the volume, which gives a volumetric energy density in Wh/dm3 (or Wr/litre3). A battery with a higher energy density will be lighter than a similar capacity battery with a lower energy density. In portable systems, the energy density is a critical parameter but in conventional PV systems which provide power for a stationary object, the energy density may be less important. Nevertheless, the costs of transporting batteries to remote locations are considerably high, so a high energy density battery is typically an advantage.
The power density of a battery is related to its energy density, as well as the ability of the battery to discharge quickly. While the power density is important in some applications, particularly transport, it is typically not critical in photovoltaic systems.
4.6 Other Electrical Battery Parameters 4.6.1 Internal Series Resistance
The internal series resistance of a battery determines the maximum discharge current of the battery. Consequently, for applications in which the batteries are required to provide high instantaneous power, the internal series resistance should be low. In addition, the series resistance will effect the battery's efficiency but may change as the battery ages.
Self-discharge refers to the fact that even in the absence of a connected load, the discharge reaction will proceed to a limited extent and the battery will therefore discharge itself over time. The rate of self-discharge depends primarily on the materials involved in the chemical reaction (i.e., the type of battery system) and on the temperature of the battery.
4.6.3 Cold Cranking Current
The maximum amount of current a battery can provide for a short period of time is called the cranking current. This parameter is often specified for transport applications, in which the battery must provide enough current to start a large engine. However, it is typically not an important parameter in PV systems.
4.7 Battery Lifetime and Maintenance 4.7.1 Battery Lifetime
The lifetime of a battery may be specified in several different way depending on the application and hence on which mechanism are most significant. For applications in which the battery is regularly charged and discharged (such as in photovoltaic systems), the most appropriate measure of lifetime is the number of charge/discharge cycles over which the battery maintains a given fraction of its capacity.
Since batteries inherently involve chemical reactions that are reactive, the materials used in batteries are suseptible to alternate reactions that degrade battery performamce. While certain catastrophic battery failure mechanisms are possible, battery lifetime is typically controlled by the gradual degradation in battery capacity which accompanies charge/discharge cycles. Consequently, battery lifetime is typically given as the number of charge/discharge cycles which it can undergo and still maintain its original capacity. However, in systems which do not frequently experience charge/discharge cycles (such as in uninterruptable power supplies), battery lifetime is more appropriately specified in years. Improper use of the battery can greatly accelerate battery aging and further decrease the number of cycles over which a battery can be used.
Battery life is defines either in years (if it remains fully charged or in # of cycles under a given set of conditions (including temperature and DOD).
4.7.2 Maintenance Requirements
The type of battery used will also have an important impact on the maintenance requirements of the battery. Some types of battery reactions evolve gasses and other products which change the volume of the components in the battery. In cases in which the volume of a battery changes, it is more difficult to seal the battery, and the battery will need to have certain chemical components (usually simply water) added to compensate for the evolution of gasses. A hermetically sealed battery does not exchange any materials with its surrounding environment. Such a battery will have lower maintenance requirements than a battery in which the various battery elements interact with the surroundings. Nearly all small common primary batteries are hermetically sealed and require no maintenance, but many secondary batteries, particularly lead acid batteries, require a strict maintenance schedule.
4.7.3 Failure Modes
A battery can degrade or can fail catastrophically. Modes are: shorts, degrdation of electrode material, freezing, increases in resistance.
4.7.4 Battery Safety and Disposal
Most battery systems, including those used in renewable energy systems, contain corrosive or dangerous chemicals and the safety regulations for each type of battery should be carefully checked. Additional safety concerns relate to their ability to produce large current. Finally, for lead-acid battery systems, the evolution of hydrogen is a potential issue.
Batteries should not be thrown away as most batteries contain toxic and/or corrosive material.
4.8 Summary and Comparison of Battery Characteristics
There are a large number of battery parameters. Depending on which application the battery is used for, some parameters are more important than others. The following is a list of parameters that may be specified by a manufacturer for a given type of battery. For example, in a typical battery for a general car, the energy density is not relevant - a battery is a small fraction of the total battery weight and consequently this parameter would typically not be listed for a conventional car battery. However, in electric vehicle applications, the battery weight is a significant fraction of the overall weight of the vehicle and so the energy densities will be given.
|Nominal Voltage||All||Usually standardized to 12V|
|Battery Capacity||All||Given as a function of discharge rate|
|Internal Series Resistance||High current applications||Running motors|
|Self Discharge Rate||Applications that are only recharged periodically|
|Energy/Power Density||Mobile applications using significant amounts of battery energy||Electric vehicles, satellites, portable electronics, etc|
|Cut-off voltage||Mobile applications using significant amounts of battery energy||Electric vehicles, satellites, portable electronics, etc|
|Efficiency||Application in which charging is restricted or heat should be minimized||Stand-alone energy systems|
|Depth of discharge||Application which experience regular, deep discharge||Stand alone energy systems, traction batteries|
|Temperature range||Applications which experience large variations in temperature||Stand alone energy systems, some transport applications|
|Battery lifetime||All||Specified as either time or cycle life|
5 Lead Acid Batteries
Lead acid batteries are the most commonly used type of battery in photovoltaic systems. Although lead acid batteries have a low energy density, only moderate efficiency and high maintenance requirements, they also have a long lifetime and low costs compared to other battery types. One of the singular advantages of lead acid batteries is that they are the most commonly used form of battery for most rechargeable battery applications (for example, in starting car engines), and therefore have a well-established established, mature technology base.
5.2 Operation of Lead Acid Batteries
A lead acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in a electrolytic solution of sulfuric acid and water. In case the electrodes come into contact with each other through physical movement of the battery or through changes in thickness of the electrodes, an electrically insulating, but chemically permeable membrane separates the two electrodes. This membrane also prevents electrical shorting through the electrolyte. Lead acid batteries store energy by the reversible chemical reaction shown below.
The overall chemical reaction is:
At the negative terminal the charge and discharge reactions are:
At the positive terminal the charge and discharge reactions are:
As the above equations show, discharging a battery causes the formation of lead sulfate crystals at both the negative and positive terminals, as well as the release of electrons due to the change in valence charge of the lead. The formation of this lead sulfate uses sulfate from the sulfuric acid electrolyte surrounding the battery. As a result the electrolyte becomes less concentrated. Full discharge would result in both electrodes being covered with lead sulfate and water rather than sulfuric acid surrounding the electrodes. At full discharge the two electrodes are the same material, and there is no chemical potential or voltage between the two electrodes. In practice, however, discharging stops at the cutoff voltage, long before this point. The battery should not therefore be discharged below this voltage.
In between the fully discharged and charged states, a lead acid battery will experience a gradual reduction in the voltage. Voltage level is commonly used to indicate a battery's state of charge. The dependence of the battery on the battery state of charge is shown in the figure below. If the battery is left at low states of charge for extended periods of time, large lead sulfate crystals can grow, which permanently reduces battery capacity. These larger crystals are unlike the typical porous structure of the lead electrode, and are difficult to convert back into lead.
5.2.1 Voltage of lead acid battery upon charging.
The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved. During the first part of the charging cycle, the conversion of lead sulfate to lead and lead oxide is the dominant reaction. However, as charging proceeds and most of the lead sulfate is converted to either lead or lead dioxide, the charging current electrolyzes the water from the electrolyte and both hydrogen and oxygen gas are evolved, a process known as the "gassing" of the battery. If current is being provided to the battery faster than lead sulfate can be converted, then gassing begins before all the lead sulfate is converted, that is, before the battery is fully charged. Gassing introduces several problems into a lead acid battery. Not only does the gassing of the battery raise safety concerns, due to the explosive nature of the hydrogen produced, but gassing also reduces the water in the battery, which must be manually replaced, introducing a maintenance component into the system. In addition, gassing may cause the shedding of active material from the electrolyte, thereby permanently reducing battery capacity. For these reasons, the battery should not regularly be charged above the voltage which causes gassing. The gassing voltage changes with the charge rate.
Lead sulphate is an insulator, and therefore the way in which lead sulfate forms on the electrodes determined how easily the battery can be discharged.
5.3 Characteristics of Lead Acid Batteries
For most renewable energy systems, the most important battery characteristics are the battery lifetime, the depth of discharge and the maintenance requirements of the battery. This set of parameters and their inter-relationship with charging regimes, temperature and age are described below.
5.3.1 Depth of Discharge and Battery Capacity
The depth of discharge in conjunction with the battery capacity is a fundamental parameter in the design of a battery bank for a PV system, as the energy which can be extracted from the battery is found by multiplying the battery capacity by the depth of discharge. Batteries are rated either as deep-cycle or shallow-cycle batteries. A deep-cycle battery will have depth of discharge greater than 50%, and may go as high as 80%. To achieve the same useable capacity, a shallow-cycle battery bank must have a larger capacity than a deep-cycle battery bank.
In addition to the depth of discharge and rated battery capacity, the instantaneous or available battery capacity is strongly affected by the discharge rate of the battery and the operating temperature of the battery. Battery capacity falls by about 1% per degree below about 20°C. However, high temperatures are not ideal for batteries either as these accelerate aging, self-discharge and electrolyte usage. The graph below shows the impact of battery temperature and discharge rate on the capacity of the battery.
5.3.2 Battery Lifetime
Over time, battery capacity degrades due to sulfation of the battery and shedding of active material. The degradation of battery capacity depends most strongly on the interrelationship between the following parameters:
- the charging/discharging regime which the battery has experienced
- the DOD of the battery over its life
- its exposure to prolonged periods of low discharge
- the average temperature of the battery over its lifetime
The following graph shows the evolution of battery function as number of cycles and depth of discharge for a shallow-cycle lead acid battery. A deep-cycle lead acid battery should be able to maintain a cycle life of more than 1,000 even at DOD over 50%.
In addition to the DOD, the charging regime also plays an important part in determining battery lifetime. Overcharging or undercharging the battery results in either the shedding of active material or the sulfation of the battery, thus greatly reducing battery life.
The final impact on battery charging relates to the temperature of the battery. Although the capacity of a lead acid battery is reduced at low temperature operation, high temperature operation increases the aging rate of the battery.
Constant current discharge curves for a 550 Ah lead acid battery at different discharge rates, with a limiting voltage of 1.85V per cell (Mack, 1979). Longer discharge times give higher battery capacities.
5.3.3 Maintenance Requirements
The production and escape of hydrogen and oxygen gas from a battery causes water loss and water must be regularly replaced in lead acid batteries. Other components of a battery system do not require maintenance as regularly, so water loss can be a significant problem. If the system is in a remote location, checking water loss can add to costs. Maintenance-free batteries limit the need for regular attention by preventing or reducing the amount of gas which escapes the battery. However, due to the corrosive nature the elecrolyte, all batteries to some extent introduce an additional maintenance component into a PV system.
5.3.4 Battery Efficiency
Lead acid batteries typically have coulombic efficiencies of 85% and energy efficiencies in the order of 70%.
5.4 Lead Acid Battery Configurations
Depending on which one of the above problems is of most concern for a particular application, appropriate modifications to the basic battery configuration improve battery performance. For renewable energy applications, the above problems will impact the depth of discharge, the battery lifetime and the maintenance requirements. The changes to the battery typically involve modification in one of the three basic areas:
- changes to the electrode composition and geometry
- changes to the electrolyte solution
- modifications to the battery housing or terminals to prevent or reduce the escape of generated hydrogen gas.
5.5 Special Considerations for Lead Acid Batteries
Flooded lead acid batteries are characterised by deep cycles and long lifetimes. However, flooded batteries require periodic maintenance. Not only must the level of water in the electrolyte be regularly monitored by measuring its specific gravity, but these batteries also require "boost charging".
Boost or equalization charging involves short periodic overcharging, which releases gas and mixes the electrolyte, thus preventing stratification of the electrolyte in the battery. In addition, boost charging also assists in keeping all batteries at the same capacity. For example, if one battery develops a higher internal series resistance than other batteries, then the lower SR battery will consistently be undercharged during a normal charging regime due to the voltage drop across the series resistance. However, if the batteries are charged at a higher voltage, then this allows all batteries to become fully charged.
Specific Gravity (SG)
A flooded battery is subject to water loss from the electrolyte due to the evolution of hydrogen and oxygen gas. The specific gravity of the electrolyte, which can be measured with a hydrometer, will indicate the need to add water to the batteries if the batteries are fully charged. Alternately, a hydrometer will accurately indicate the SOC of the battery if it is known that the water level is correct. SG is periodically measured after boost charging to insure that the battery has sufficient water in the electrolyte. The SG of the battery should be provided by the manufacturer.
Special Considerations for Gelled, Sealed Lead Acid Batteries
Gelled or AGM lead acid batteries (which are typically sealed or valve regulated) have several potential advantages:
- they can be deep cycled while retaining battery life
- they do not need boost charging
- they require lower maintenance.
However, these batteries typically require a more precise and lower voltage charging regime. The lower voltage charging regime is due to the use of lead-calcium electrodes to minimise gassing, but a more precise charging regime is required to minimise gassing from the battery. In addition, these batteries may be more sensitive to temperature variations, particularly if the charging regime does not compensate for temperature or is not designed for these types of batteries.
5.5.1 Failure Modes for Lead Acid Batteries
The battery for a PV system will be rated as a certain number of cycles at a particular DOD, charging regime and temperature. However, batteries may experience either a premature loss in capacity or a sudden failure for a variety of reasons. Sudden failure may be caused by the battery internally short-circuiting due to the failure of the electrical separator within the battery. A short circuit in the battery will reduce the voltage and capacity from the overall battery bank, particularly if sections of the battery are connected in parallel, and will also lead to other potential problems such as overcharging of the remaining batteries. The battery may also fail as an open circuit (that is, there may be a gradual increase in the internal series resistance), and any batteries connected in series with this battery will also be affected. Freezing the battery, depending on the type of lead acid battery used, may also cause irreversible failure of the battery.
The gradual decline in capacity may be worsened by inappropriate operation, particularly by degrading the DOD. However, the operation of one part of the battery bank under different conditions to another will also lead to a reduction in overall capacity and an increase in the likelihood of battery failure. Batteries may be unintentionally operated under different regimes due either to temperature variations or to the failure of a battery in one battery string leading to unequal charging and discharging in the string.
Battery installation should be conducted in accordance with the relevant standard in the country in which they are being installed. At present there are Australian standards AS3011 & AS2676 for battery installation. There is also a draft standard for batteries for RAPS applications which will eventually become an Australian standard.
Among other factors to be considered in the installation of a battery system are the ventilation required for a particular type of battery bank, the grounding conditions on which the battery bank is to be placed, and provisions taken to insure the safety of those who may have access to the battery bank. In addition, when installing the battery bank care must be taken to ensure that the battery temperature will fall within the allowable operating conditions of the battery and that the temperature of the batteries in a larger battery bank are at the same temperatures. Batteries in very cold conditions are subject to freezing at low states of charge, so that the battery will be more likely to be in a low state of charge in winter. To prevent this, the battery bank may be buried underground. Batteries regularly exposed to high operating temperatures may also suffer a reduced lifetime.
Batteries are potentially dangerous and users should be aware of three main hazards: The sulfuric acid in the electrolyte is corrosive. Protective clothing in addition to foot and eye protection are essential when working with batteries.
Batteries have a high current generating capability. If a metal object is accidentally placed across the terminals of a battery, high currents can flow through this object. The presence of unnecessary metal objects (e.g. jewellery) should be minimised when working with batteries and tools should have insulated handles.
Explosion hazards due to evolution of hydrogen and oxygen gas. During charging, particularly overcharging, some batteries, including most batteries used in PV systems, may evolve a potentially explosive mixture of hydrogen and oxygen gas. To reduce the risk of explosion, ventilation is used to prevent the buildup of these gasses and potential ignition sources (i.e. circuits which may generate sparks or arcs) are eliminated from the battery enclosure.
Batteries introduce a periodic maintenance component into a PV system. All batteries, including "maintenance free" batteries require a maintenance schedule which should ensure that:
- the battery terminals are not corroded
- the battery connections are tight
- the battery housing should be free of cracks and corrosion.
Flooded batteries require extra and more frequent maintenance. For flooded batteries, the level of electrolyte and the specific gravity of the electrolyte for each battery needs to be checked regularly. Checking the specific gravity of a battery by using a hydrometer should be carried out at least 15 minutes after an equalisation or boost charge. Only distilled water should be added to batteries. Tap water contains minerals which may damage the battery electrodes.
5.5.4 Battery Disposal and Recycling
The lead in a lead acid battery presents an environmental hazard if it is not properly disposed of. Lead acid batteries should be recycled so that the lead can be recovered without causing environmental damage.
5.6 Electrode Materials and Configuration
The materials from which the electrodes are made have a major affect on the battery chemistry, and hence affect the battery voltage and its charging and discharging characteristics. The geometry of the electrode determines the internal series resistance and the charging and discharging rate.
5.6.1 Plate Material
The basic anode and cathode materials in a lead acid battery are lead and lead dixodie (PbO2). The lead electrode is in the form of sponge lead. Sponge lead is desirable as it is very porous, and therefore the surface area between the lead and the sulfic acid electrolyte is very large. The addition of small amoints of other elements to the lead electrode to form lead alloys can reduce several of the disadvantages associated with the lead. The main types of electrodes used are lead/antimony (using several percent antimony), lead/calcium alloys, and lead/antimony/calcium alloys.
Antimony lead alloy batteries have several advantages over pure lead electrodes. These advantages include: the lower cost of lead/antimony; the increased strength of the lead/anitmony electrode; and the ability to be deeply discharged for short period of time. However, lead/antimony alloys are prone to sulfation and should not be left at low states of charge for extended periods of time. I addition, lead/antimony alloys increase the gassing of the battery during charging leading to high levels of water loss. Since the water must be addedto these batteries, they have higher maintenance. Furthermore, lead/antimony batteries have a high discharge rate and a short lifetime. These problems (xx- check if both problems are caused by plating)) are caused by the dissolution ofantimony from one electrode and its deposition or plating on the other electrode. (xx the increased adhesion of PbO2 xx)
Lead calcium batteries are an intermediate cost technology. Like antimony, calcium also adds strength to the lead of the negative electrode, but unlike antimony, the addition of calcium reduces the gassing of the battery and also produces a lower self-discharge rate. However, lead calcium batteries should not be deeply discharged. Consequently, these types of batteries may be considered "maintenance-free", but are only shallow cycle batteries.
Adding antimony as well as calcium to the electrodes provides some of the advantages of both antimony and lead, but at an increased cost. Deep discharge batteries such as these can also have a high lifetime. Furthermore, trace amounts of other materials can be added to the electrodes to increase battery performance.
5.6.2 Electrode Configuration
In addition to the material used to make the electrode plates, the physical configuration of the electrodes also has an impact on the charging and discharging rates and on the lifetime. Thin plates will allow faster charging and discharging, but are less robust and more prone to shedding of material from the plates. As high charging or discharging currents are not typically a required feature of batteries for renewable energy systems, thicker plates can be used, which have lower charge and discharge times, but also have longer lifetimes.
5.6.3 Battery Housing
In an open, flooded battery, any gas which is generated can escape to the atmosphere, causing both safety and maintenance problems. A sealed lead acid (SLA), valve-regulated lead acid (VRLA) or recombining lead acid battery prevent the loss of water from the electrolyte by preventing or minimizing the escape of hydrogen gas from the battery. In a sealed lead acid (SLA) battery, the hydrogen does not escape into the atmosphere but rather moves or migrates to the other electrode where it recombines (possibly assisted by a catalytic conversion process) to form water. Rather than being completely sealed, these batteries include a pressure vent to prevent the build-up of excess pressure in the battery. Sealed batteries require stringent charging controls to prevent the build-up of hydrogen faster than it can recombine, but they require less maintenance than open batteries.
Valve regulated lead acid (VRLA) batteries are similar in concept to sealed lead acid (SLA) batteries except that the valves are expected to release some hydrogen near full charge. SLA or VRLA batteries typically have additional design features such as the use of gelled electrolytes and the use of lead calcium plates to keep the evolution of hydrogen gas to a minimum.
5.7 Types of Lead Acid Batteries
Despite the range in battery types and applications, the characteristics particularly important in PV applications are the maintenance requirements of the battery and the ability to deep charge a battery while maintaining a long lifetime. To promote long cycle life with deep discharge, deep cycle batteries may be either of the open-flooded type with an excess of electrolytic solution and thick plates, or of the immobilized electrolytic type. Sealed gelled batteries may be rated as deep cycle batteries, but they will usually withstand fewer cycles and lower discharges than the specially designed flooded plate or AGM batteries. Shallow-cycle batteries typically use thinner plates made from lead calcium alloys and do not typically have a depth of discharge above 25%.
Batteries for PV or remote area power supplies (RAPS)
The stringent requirements for batteries used in photovoltaic systems have prompted several manufacturers to make batteries specifically designed for PV or other remote power systems. The batteries most commonly used in stand-alone photovoltaic systems are either deep-cycle lead acid types, or shallower cycle maintenance-free batteries. Deep-cycle batteries may be open flooded batteries (which are not maintenance-free) or captive electrolyte AGM batteries which are maintenance-free (but which do require care in regulator selection). Special shallow-cycle maintenance-free batteries that withstand infrequent discharging may also be used in PV applications, and provided that the battery bank is appropriately designed, never require a DOD of more than 25%. A long-life battery in an appropriately designed PV system with correct maintenance can last up to 15 years, but the use of batteries which are not designed for long service life, or conditions in a PV system, or are part of a poor system design can lead to a battery bank which fails after only a few years.
Several other types of specific purpose batteries are available and these are described below.
Starting, lighting ignition batteries (SLI).These batteries are used in automotive applications and have high discharge and charge rates. Most often they use electrode plates strengthened with either lead antimony in a flooded configuration, or lead calcium in a sealed configuration. These batteries have a good life under shallow-cycle conditions, but have very poor lifetime under deep cycling. SLI batteries should not be used in a PV system since their characteristics are not optimized for use in a renewable energy system because lifetime in a PV system is so low.
Traction or motive power batteries. Traction or motive batteries are used to provide electric power for small transport vehicles such as golf carts. Compared to SLI batteries, they are designed to have a greater ability to be deep-cycled while still maintaining a long lifetime. Although this feature makes them more suited to a PV system than one which uses SLI batteries, motive power batteries should not be used in any PV systems since their self discharge rate is very high due to the use of lead antimony electrodes. A high self discharge rate will effectively cause high power losses from the battery and make the overall PV system inefficient unless the batteries experience large DOD on a daily basis. The ability of these batteries to withstand deep cycling is also far below that of a true deep-cycle battery. Therefore, these batteries are not suited to PV systems.
RV or marine batteries. These batteries are typically a compromise between SLI batteries, traction batteries and true deep-cycle batteries. Although they are not recommended, both motive and marine batteries are used in some small PV systems. The lifetime of such batteries will be restricted to a few years at best, so that the economics of battery replacement mean that such batteries are typically not a long-term cost effective option.
Stationary batteries. Stationary batteries are often used for emergency power or uninterruptable power supply applications. They are shallow-cycle batteries intended to remain close to fully charged for the majority of their lifetime with only occasional deep discharges. They may be used in PV systems if the battery bank is sized so that it never falls below a DOD of between 10% and 25%.
Deep-cycle Batteries. Deep-cycle batteries should be able to maintain a cycle life of several thousand cycles under high DOD (80% or more). Wide differences in cycle performance may be experienced with two types of deep cycle batteries and therefore the cycle life and DOD of various deep-cycle batteries should be compared.
5.8 Potential Problems with Lead Acid Batteries
A lead acid battery consists of electrodes of lead oxide and lead are immersed in a solution of weak sulfuric acid. Potential problems encountered inlead acid batteries include:
Gassing: Evolution of hydrogen and oxygen gas. Gassing of the battery leads to safety problems and to water loss from the electrolyte. The water loss increases the maintenance requirements of the battery since the water must periodically be checked and replaced.
Damage to the electrodes. The lead at the negative electrode is soft and easily damaged, particularly in applications in which the battery may experience continuous or vigorous movement.
Stratification of the electrolyte. Sulfuric acid is a heavy, viscous liquid. As the battery discharges, the concentration of the sulfuric acid in the elecotrolyte is reduced, while during charging the sulfiric acid concentratin increases. This cyclicing of sulfuric acid concentration may lead to stratification of the electrolyte, where the heavier sulfuric acid remains at the bottom of the battery, while the less concentrated solution, water, remains near the top. The close proximity of the electrode plates within the battery means that physical shaking does not mix the sulfuric acid and water. However, controlled gassing of the electrolyte encourages water and sulfuric acid to mix, but must be carefully controlled to avoid problems of safety and water loss. Periodic but infrequent gassing of the battery to prevent or reverse electrolyte tratification is required in most lead acid batteries in a process referred to as "boost" charging.
Sulfation of the battery. At low states of charge, large lead sulfate crystals may grow on the lead electrode as opposed to the finely grained material which is normally produced on the electrodes. Lead sulphate is an insulating material.
Spillage of the sulfuric acid. If sulfuric acid leaks from the battery housing it poses a serious safety risk. Gelling or immobilizing the liquid sulfuric acid reduces the possibility of sulfuric acid spills.
Freezing of the battery at low discharge levels. If the battery is at a low discharge level following the conversion of the whole electrolyte to water, then the freezing point of the electrolyte also drops.
Loss of active material from the electrodes. The loss of active material from the electrodes can occur via several processes. One process that can cause a permanent loss of capacity is the flaking off of the active material due to volumetric changes between xxx and lead sulphate. In addition, xxx. Improper charging conditions and gassing can cause shedding of active material from the electrodes, leading to a permanent loss in capacity.
Depending on which one of the above problems is of most concern for a particular application, appropriate modifications to the basic battery configuration improve battery performance. For renewable energy applications, the above problems will impact the depth of discharge, the battery lifetime and the maintenance requirements. The changes to the battery typically involve modification in one of the three basic areas:
- changes to the electrode composition and geometry
- changes to the electrolyte solution
- modifications to the battery housing or terminals to prevent or reduce the escape of generated hydrogen gas.
5.8.1 Corrosion of terminals
Corrosion consists of a set or reduction/oxidation regions in which both the reactions take place at the same electrode. For a battery system, corrosion leads to several detrimental effects. One effect is that it converts a metallic electrode to a metal oxide.
All chemical reactions proceed in both the forward and reverse direction. In order for the reverse reaction to proceed, the reactants must gain enough energy to overcome the electrochemical difference between the reactants and the products and also the overvotlage. Usually in battery systems the probability of the reverse reaction occurring is small, since there are few molecules with a large enough energy. Although small, however, there are some particles that do have sufficient energy. In a charged battery, a process exists by which the battery can be discharged even in the absence of a load connected to the battery. The amount a battery discharges upon standing is known as self-discharge. Self-discharge increases as temperature increases because a greater fraction of products will have enough energy for the reaction to proceed in the reverse direction.
An ideal set of chemical reactions for a battery would be one in which there is a large chemical potential which releases a large number of electrons, has a low overvotlage, spontaneously proceeds in only one direction and is the only chemical reaction which can occur. However, in practice there are several effects that degrade battery performance, due to unwanted chemical reactions, to effects such as the change in phase of volume of the reactants or products and also to the physical movement of reactants and products within the battery.
5.8.3 hange in form of materials
While undergoing chemical reactions, many materials undergo a change either in phase, or if they stay in the same phase, the volume, density of the material may be altered by the chemical reaction. Finally, the materials used in the battery, primarily the anode and cathode, may change their crystallinity or surface structure, which will in turn affect the reactions in the battery. Many components in redox reactions undergo a change in phase during either oxidation or reduction. For example, in the lead acid battery, sulfate ions changes from being in solid form (as lead sulfate) to being in solutions (as sulfuric acid). If the lead sulfate recrystallizes anywhere but the anode or cathode, then this material is lost to the battery system. During charging, only materials connected to the anode and cathode can participate in electron exchange, and therefore if the material is not touching the anode or cathode, then it can no longer be recharged. The formation of a gaseous phase in a battery also presents special problems. First of all, the gaseous phase will usually have a larger volume that the initial reactants, thus giving rise to a change in pressure in the battery. Secondly, if the intended products are in the gaseous change, they must be confined to the anode and cathode, or they will not be able to be charged.
A change in volume will also usually be detrimental in battery operation.
5.8.4 Modifications to the electrolyte
A standard "flooded" lead acid battery has the electrodes immersed in liquid sulfuric acid. Several modifications to the electrolyte are used to improve battery performance in one of several areas. The key parameters of the electrolyte which control the performance of the battery are the volume and concentration of the electrolyte and forming a 'captive' electrolyte.
5.8.5 Electrolyte Volume and Concentration
Changes in the volume of the electrolyte can be used to improve the robustness of a battery. Increasing the volume of an electrolyte makes the battery less sensitive to water losses, and hence makes regular maintenance less critical. Adding to the volume of the battery will also increase its weigth and reduce the energy density of the battery.
5.8.6 Captive Electrolyte Lead Acid Batteries
In 'captive' electrolyte batteries, the sulfuric acid is immobilises by either 'gelling' the sulfuric acid or by using and 'absorptive glass mat'. Both have lower gassing compared to a flooded lead acid battery and are consequently often found in "maintenance-free" sealed lead acid batteries.
Gelling. In a "gelled" lead acid battery, the electrolyte may be immobilized by gelling the sulfuric acid using silica gel. The gelled electrolyte has an advantage in that gassing is reduced, and consequently the batteries are low-maintenance. In addition, stratification of the electrolyte does not occur with gelled batteries and therefore boost charging is not required, and because the electrolyte is gelled, the chances of spilling sulfuric acid are also reduced. However, in order to further reduce gassing, these "gell-cell" batteries also typically use lead calcium plates, making them unsuited to deep discharge applications. A further drawback is that the charging conditions of a gelled lead acid battery must be more carefully controlled to prevent overcharging and damage to the battery.
Absorbtive Glass Matting. A second technology which can be used to immobilize the sulfuric acid is "absorptive glass mat" or AGM batteries. In an AGM battery, the sulfuric acid is absorbed in a fiberglass mat which is placed between the electrodes plates. AGM batteries have numerous advantages including the ability to be deeply discharged without affecting lifetime, allowing high rates of charge/discharges and an extended temperature range for operation. The key disadvantage with these batteries is their need for more carefully controlled charging regimes and their higher initial cost.
6 Other Battery Types
6.1 Alkaline Battery
The alkaline battery consists of a redox reaction in which the anode is Zn (which also usually the casing) with an electrolyte consisting of a paste of NaOH or KOH. The half reactions are:
In this battery, if one mole of zinc is present, and two moles of MnO2, then the Zn is the limiting component for the reaction.
6.2 Nickel-Cadmium Batteries
While lead-acid batteries are undoubtedly the most commonly used batteries in photovoltaic systems, in some photovoltaic applications, nickel-cadmium may be cost effective on a life-cycle/cost basis. Nickel-cadmium batteries consist of a positive electrode of nickel (or hydroxide) and a negative electrode of cadmium hydroxide. They are commonly used in a sealed configuration in small household appliances, but larger vented or sealed batteries are also availible for PV applications. Nickel-cadmium batteries have several advantages as listed below.
Long lifetime and long storage life. In nickel-cadmium batteries, the positive and negative electrodes undergo oxidation and reduction reactions. Material does not enter the electrolyte and then re-plate to the electrodes as it would in lead-acid batteries. This means that the active material does not shed from the plates, and that a process analagous to sulfation of a lead-acid battery does not occur. As these processes reduce the lifetime of lead-acid batteries, nickel-cadmium batteries have a higher lifetime. Furthermore, the electrolyte in nickel-cadmium is less corrosive to battery parts than in a lead-acid battery which also increases lifetime.
Can be fully discharged. Nickel-cadmium batteries can be fully discharged without damage to the battery.
Can be overcharged. Nickel-cadmium batteries are less sensitive to overcharging, thereby reducing the requirements during the charging regime. Due to the ability to completely discharge, the tolerance to overcharging and the charging regimes for these batteries, in some cases the battery regulator may be eliminated.
Reduced sensitivity to temperature. Since the electrolyte composition does not change during charging or discharging, nickel-cadmium batteries are not more susceptible to freezing at low levels of charge, in the same way that lead-acid batteries are. Consequently, nickel-cadmium batteries are less sensitive to colder temperature, tolerating temperatures of -50 C. In addition, the lifetime of
nickel-cadmium batteries is not as strongly affected by high temperature operations as lead-acid.
Low maintenance requirements. As nickel-cadmium batteries emit fewer corrosive elements and have lower gassing, they require less frequent maintenance.
However, they also have a number of disadvantages. Some of the disadvantages include;
Expense. Nickel-cadmium batteries are typically at least twice as expensive than lead-acid batteries. However, some of this cost may be offset by the ability to fully discharge, eliminating the need for oversizing the battery, and by the possible elimination of the regulator. Consequently, in applications which are not critical, nickel-cadmium batteries can be used, assuming that they will be nearly fully discharged each night. If, however during a charging cycle there is a cloudy day, then no power would be available. Nickel-cadmium batteries, therefore can only be used in non-critical loads.
Lower efficiencies. Nickel-cadmium batteries have both lower coulombic efficiencies, between 75% to 85%, and lower overall efficiencies, between 60% to 75%.
Memory effect. Some nickel-cadmium batteries can require full discharge to prevent "memory" development, and subsequent inability (in a normal discharge cycle) to discharge below the level it has been subjected to in the past. Elimination of this effect requires a slow, full discharge/charge cycle.
An additional feature of nickel-cadmium batteries is the relatively constant voltage curve on charging and discharging. While this is an advantage in discharging in that the voltage stays relatively constant between 10% and 80% discharge, it is a disadvantage in charging in that the voltage is a poor indicator of battery state of charge and therefore determining SOC is more difficult.
6.3 Vanadium Redox Flow Battery
Redox flow batteries use a reductio-oxidation between two valence states in solution rather than changing the composition, and hence the valence states of solid material on an electrode. A flow battery consists of two volumes of solution separated by a selective membrane which allows some ions to pass but not others. The two solutions are pumped to the permeable membrane, which allows xxxx.
Flow batteries have several potential advantages over solid batteries. A key advantage, which is particularly important in transport applications, is that the battery may be re-charged simply by pumping out the uncharged solution and replacing the solution with charged solution. This eliminates potentially long recharging times, such as are encountered in electric vehicles. Replacement of the solution allows the electric car to be recharged in the same fashion in which a car is filled with fuel. Another advantage is that the capacity of the battery is determined by the volume of solution, while the power of the battery is determined by the membrane contact area between the two solutions.
The vanadium-Vanadium redox flow battery, developed at the University of New South Wales, is a particularly promising flow battery. It consists of two states of Vanadium. It has high efficiencies, with coulombic efficiencies of 97% and energy efficiencies of 87%. In addition, since both solutions (anode and cathode) in the battery use vanadium, cross contamination between the two solutions may discharge the battery, but will not cause damage the battery.
7 Functions and Uses of Storage in PV Systems
Storage is used in PV systems to increase the amount of time that the PV system can be used to power a load. Batteries are the most common type of storage in a PV systems. However, in specific types of systems or applications, other storage components can also be used. For example, in water pumping systems, the amount of battery storage can be greatly reduced or eliminated if extra water is pumped and stored in a water tank for use in cloudy periods.
In stand alone electricity generating systems, some form of storage is needed unless the load is exactly matched to the time during which the sun is shining. (Such an exact match is rare and limited to a few types of systems - for example powering a fan for cooling or in some cases water pumping for irrigation). In stand alone systems, storage is needed not only to power loads at night, but also allow a load to operate during cloudy weather. The number of days of storage needed depends on the weather pattern at a particular location, with cloudier locations needing more storage. In systems with a large amount of storage, and additional utility of the storage system is that is can buffer the system against periods of low insolation, such as in winter. For example, in telecommunications systems that require high reliability, a large battery bank can allow high reliability without requiring the PV array to be sized to meet the worst possible insolation conditions. In general, the larger the amount of storage included, the less sensitive the system will be to periods of low insolation, and the more reliable the power availability will be. The figure below shows how the power availability increase with increasing storage.
Battery state of charge over a year showing the battery discharge overnight, during cloudy weather and seasonal variations.
In systems connected to the utility grid electricity supply, storage is typically not needed. PV power is used when the sun is shining, and at night or during periods of cloudy weather, the grid provides the electricity. However, even in grid-connected systems, storage can be included, not to increase the reliability of having power as in a stand-alone system but rather to increase the value of the PV-generated electricity. In the load seen by many utility companies, an air conditioning load occurring on summer afternoons increases the overall load that the utility must supply. These peaks in the load are significantly more expensive to supply power for. Since the power output from PV is typically largest during summer months, the output from the PV system can well-matched to the peak load of the generated electricity is stored for a few hours. The use of storage for this application is called peak shifting and is shown in the figure below.
7.1 Functions of Batteries
Batteries are a common feature in most types of PV systems that are not connected to the utility grid. In addition to providing storage, batteries can also be used for several other functions:
Storage. Batteries store energy being produced by a given generating source, and when this source is unavailable this energy can be be used by the load. The inclusion of storage in any energy generating system will increase the availability of the energy.
Start-up current. Batteries can provide higher currents to the load than the array alone can provide. This is especially useful if a particular load has a high current draw on start-up. Many motors initially have a high current requirement.
Power conditioning.Batteries can function as power conditioning. Two cases where this feature is used is in directly coupled systems, such as water pumping, and in uninteruptable power supplies.
In addition to the different mode of operation, batteries in photovoltaic systems also must meet several other criteria. As reliability and low maintenance are desirable in photovoltaic systems, the batteries must also have a long lifetime. Further, since batteries will often be a substantial fraction of the total cost of a PV system, cost is a significant factor in batteries for PV systems. In general, batteries manufactured for other applications are not well suited to photovoltaic energy applications. The key characteristics of a battery in a renewable energy system are:
- efficiency of the battery
- how battery capacity and lifetime is affected by deep cycling and extended states of low charge
- the initial and ongoing battery costs
- the maintenance requirements of the battery.
7.2 Electrolysis of Water
In battery solutions in which a component of the electrolyte is water (such as in lead acid batteries), the possibility of electrolysis water must be taken into account when charging a battery. The electrolysis of water, which is breaking water into oxygen and hydrogen.
According to the standard potentials, the voltage of this reaction is 1.23V. However, the activation overpotential of this reaction is large, and hence it does not proceed at a significant rate (and can therefore be neglected in battery charging or discharging) until voltages on the order of 2.2V are reached in the battery. During high charging rates, the charging voltage may exceed this voltage, and hence two reactions will proceed in such a battery: one the charging of the battery and the second the electrolysis of water. As the electrolysis of water gives of hydrogen and oxygen, both of which are gases, the battery is said to be gassing. The electrolysis of water has several impacts on the battery. Firstly, it leads to water loss in the battery, which must be replaced. Further, the evolution of hydrogen gas forms a potential safety hazard if released in an improperly ventilated area, or can overpressure the battery case. Both of these issues may be minimized or circumvented by preventing the gases, the battery is said to be gassing. The electrolysis of water has several impacts on the battery. Firstly, it leads to water loss in the battery, which must be replaced. Further, the evolution of hydrogen gas forms a potential safety hazard if released in an improperly ventilated area, or can overpressure the battery case. Both of these issues may be minimized or circumvented by preventing the gases, particularly the hydrogen from escaping from the battery. Batteries using this approach are called sealed or recombinant batteries. Despite the potential maintenance and safety problems associated with gassing, it may also have beneficial impacts. For example, in lead-acid batteries gassing can be used to mix the electrolyte, thus preventing regions of higher sulfuric acid concentration (which is denser) from sinking to the bottom (an effect called stratification).
The electrolysis of water is affected by the presence of small amounts of impurities in the lead acid batteries, and hence batteries with additives to the lead (for mechanical strength or other practical purposes) can experience significantly different gassing voltages. Further, since the activation energy is temperature dependent, the voltage at which gassing of a battery changes with the battery temperature and on the details of the battery components.
7.3 Uses of batteries in PV systems
While the primary function of a storage system is to provide power when sunlight is not available, hence increasing the fraction of time the photovoltaic system provides electricity, the addition of batteries has numerous other advantages which mean that the batteries can be used for multiple purposes. For small systems consisting of one or two photovoltaic modules, batteries can act as a load-matching system. Alternately, in photovoltaic systems which contain a load with a large initial current draw (such as experienced by an inductive load, typically represented by a motor), the batteries can be used to provide initial start-up current. In grid-connected systems, battery storage can be used for peak shifting, in which the power generated by the sun is stored for several hours in order to better match when the peak load occurs.