The capacity specifies how much electricity a rechargeable battery stores, and is usually printed on the outside of the battery. For small rechargeable batteries, this is usually given in milli-ampere-hours (mAh), and in ampere-hours (Ah) for large batteries.

The charge current is the amount of power the charger applies to a battery while charging. A label on the charger usually shows the "secondary" voltage and current in either milli-ampere (mA) or ampere (A).

Not all of the power applied to a rechargeable battery is absorbed by the battery; some of the power is lost e.g. as heat. This energy loss results in a longer charging time, and 30% to 40% are commonly observed in practice.

Some chargers, e.g. those with "electronic charge control", may adjust the charge current while they are charging, or might even discharge a battery before fully charging it. These extra functions can lead to significant increases in charge time.