The amount of electricity delivered by a fully charged battery

The capacity of a battery is defined as the amount of electricity delivered by a fully charged battery during continuous discharge at a constant current until the final voltage value is reached.

The capacity of a accumulator indicates how long the battery can power a connected load. Typically, battery capacity is measured in ampere-hours, and for small batteries – in milliampere-hours.

The energy of a storage battery (the energy that can be stored in a fully charged battery) or EMF (electromotive force) is not equal to the capacity of a battery. The higher the battery voltage, the greater the stored energy. Electrical energy is equal to the product of the voltage, the current, and the duration of current flow. Consequently, the battery energy is equal to the product of its capacity and its nominal voltage.

According to GOST 959-71, the nominal capacity (C_nom) of starter batteries is guaranteed during continuous 20-hour discharge at a current equal to 5% of the battery capacity down to a voltage of 1.75 V per lagging cell, at an average electrolyte temperature of 25 °C and an initial density of 1.285 g/cm³.

The nominal battery capacity is the expected amount of electricity that a battery should theoretically deliver when fully charged. This value is indicated on the battery label or encoded in its type designation.

The state of charge (charge level) is the amount of electricity actually stored in the battery.

The battery voltage is the potential difference between the terminals of the battery.

Residual capacity

During operation, capacity remains stable for some time and then begins to gradually decrease (residual capacity).

The residual capacity of a battery depends on the ambient temperature.

The correction factor for changes in residual battery capacity depending on the battery temperature is shown in Table 1.

Table 1

Series and parallel connection of batteries

The capacity of a battery pack in a series connection of identical cells is equal to the capacity of a single cell, while the EMF of such a pack is equal to the sum of the EMFs of the cells included in it. Battery internal resistance will be equal to the sum of the internal resistances of the cells.

When connecting batteries in parallel, the pack capacity is equal to the sum of the capacities of all the cells, while the EMF is equal to the electromotive force of a single cell. Batteries are usually connected in parallel to increase the maximum current output.

Internal resistance of a battery
Capacitor bank capacity