A battery's capacity is a measure of its storage ability. It is measured in ampere hours (Ah) and indicates the available quantity of current that a battery can supply in a certain time when discharging with a constant current until a specified voltage is reached (final discharging voltage). A nominal capacity is always given for prescribed conditions (normally 20° Celsius and K20 for starter batteries). A battery with a 20-hour capacity of 60 Ah can deliver a current of 3 amperes (A) over 20 hours (h): 60 Ah/20 h = 3 A. The capacity is not constant but depends on temperature and the level of current. An increasing discharge current or lower temperatures reduce the battery's capacity.
In practice, the cell voltage of an individual cell is not sufficient. Therefore several cells are linked in series, with the negative terminal of one cell connected to the positive terminal of another cell. If 6 cells are connected in series, a total voltage of 12 V (nominal voltage) is achieved.
The process of discharging and charging a battery is known as the cycle. Supply batteries with their thicker grids and plates are best suited to this purpose. Due to their construction with thin plates and separator material, starter batteries are not really suitable for repeated deep discharging. If a starter battery is used for cyclical applications (e.g. for powering electric motors) then the service life of the battery is cut significantly.
The cold-test current denotes the battery's ability to output current at cold temperatures. According to the EN standard, during the cold start test at a temperature of -18°, a 12V-battery should still generate 7.5 V 10 seconds after discharging has begun.
A cold start involves two conditions which are unfavorable for the battery:
The charge state of the battery in cold weather is especially important: the more it is discharged, the thinner the acid becomes. This means that the freezing point can radically alter (a battery with frozen electrolyte can no longer be used to start the vehicle):