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 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.
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.
Installation
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.
Safety
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.
Maintenance
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.
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.