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.
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 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.
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.
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.
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 voltage.