How do you calculate battery Capacity?

Answer 1

A battery's capacity is measured in Amp Hours. This is how many Amps of current a battery can provide in a given number of hours. A battery can produce a lot of current for a short time, or a little current for a long time.

A curious property of chemical batteries is they are able to provide a higher total energy capacity if they are discharged at lower rates. For example, a particular battery may be able to provide 10 amps of current for 1 hour for a battery capacity of 10 Ahr. The same battery may be able to be discharged at 5 amps for a 2.5 hour discharge period for a capacity of 12.5 Ahr. And at a discharge rate of 1 amp the exact same battery may may last 15 hours for a capacity measurement of 15 Ahr. For that reason typically battery capacity is specified over a standard period of 10 or 20 hours time. When comparing batteries, make sure that the test period for the battery rating is the same. A battery with 100 Ahr over a 20 hour test period is not the same as a battery with 100 Ahr over a 10 hour test period.