Design and function: Capacity

The capacity of a fully charged battery is its ability to give a constant current during a certain time and is stated in the unit ampere-hours (Ah). The time for discharging varies depending on the battery's purpose.

In principle, the capacity is controlled by the plates' area as well as thickness. In a battery there is a big area on the plates, on which chemical reactions can take place, and also where a great amount of electrons (current) can be generated between the plates. This gives them a higher cold-start current value (CCA = Cold Cranking Amperes).

A start battery has big area but thin plates.

For a deep cycle battery, e. g., battery for leisure use, it is important that the chemical process can continue for a long time with lower current use, that is why the plates have to be thick. This gives them many ampere-hours or "spare minutes". Since the plates are thick, they need longer time to charge so that the chemical reaction also takes place deep inside the plates.

For start batteries, 20 hours' capacity (K20) is mentioned. This refers to how much current the battery can deliver during 20 hours in an ambient temperature of +25°C (+77°F) without the terminal voltage going below 10.5 V.

Example:  A battery (of open type) with a stated capacity of 70 Ah should be able to provide a constant current of a max. 3.5 A (3.5 A x 20 h = 70 Ah) for 20 hours.

The capacity of a battery is not only dependent on the size and construction of the battery, but also varies considerably for batteries of the same type with the size of the discharge current. The lower the discharge current the higher the capacity and vice versa. The capacity of the battery is also affected by the temperature and the age of the battery. The nominal value is given at +25°C (+77°F). The capacity of the battery reduces considerably at low temperatures.

The table shows the difference in capacity in a battery at +25°C (+77°F) and -18°C (-0.4°F).