Capacity of battery is the quantity of energy that can be stored and it is generally calculated in ampere-hour (Ah). This result is obtained by multiplying the current in ampere by the time in hours of discharge until tension of 1.75 V per element. For automotive batteries it is used a time interval of 20 hours; for this reason we usually express the capacity of battery as: XX Ah (where XX is capacity, for instance 45 Ah) in 20 hours or simply C20.
