Although it is impossible to measure the internal characteristics of a battery because it is a closed system, we can use the observable parameters at the battery’s terminals to estimate internal measurements, such as the stored charge and total energy capacity.
The watt-hour capacity of a battery, expressed as Wh or W·h, is a measure of the amount of energy being supplied to the load. It is a measure of the total electrical power the battery can supply for an hour or the total duration for which it can supply a specific load.
The milliampere-hour capacity of a battery describes the duration for which a battery can supply one milliampere of current and the maximum amount of current it can supply for one hour.
For example, a 10 mAh of capacity means that the battery can supply a load of 10 milliamperes for one hour before losing all its charge, or it can indicate that the battery can supply a 1 milliampere current to a load for 10 hours. The electric charge for smaller batteries is generally measured in milliamp-hours, abbreviated mAh or mA·h.
To convert watt hours to amp hours, simply divide watt hours by volts:
Amp hours (Ah) = Watt hours (Wh) ÷ Volts (V)
Let's say you have a 2000Wh LiFePO4 battery. You know it's a 12 volt battery, and you want to calculate its amp hours.
To do so, you'd divide watt hours by volts:
2000Wh ÷ 12V = 166.67Ah
In this example, your battery has a 166.67Ah capacity.
Note: Most people use a battery's nominal voltage when calculating amp hours or watt hours, but it's worth reminding you that a battery's actual voltage is slightly different and varies based on its remaining capacity. For instance, a 12V LiFePO4 battery actually has a voltage of 13.6V when fully charged.