Battery chargers essentially function by feeding an electric current into batteries for some time until the cells within retain some of the electricity running through them. Chargers supply a constant voltage/current until switched off. More sophisticated chargers utilize a smaller, subtler “trickle” charge for longer durations.

Overcharging is a common problem, as this can result in overheating and pressure buildup that causes the batteries to break, leak chemicals or gas and in extreme cases, explode. Developments in chargers have led to the creation of timer chargers that switch off automatically after a set time. Premium chargers use electronic circuits to gauge how much charge is within the batteries by measuring variances in battery voltages and cell temperatures. Such chargers turn the current off or switch to a low trickle charge as the battery is nearly fully charged.

Different chargers are built to charge at different currents/voltages at different rates. It is, therefore, a rule of thumb that the chargers suitable for one battery may not be so for another. It is important to match the charger of the right voltage and current to the batteries.

A common charger would be the overnight charger. Being a slow charger, it applies a constant charge approximately one-tenth of the rated capacity and has no full-charge detection. On overcharge, such chargers become warm to touch. Such charges are often found in cheap consumer chargers for common AA and AAA batteries.

A rapid charger, on the other hand, indicates that the charger is ready and changes to a trickle (maintenance) charge when batteries are fully charged. Such chargers have in built temperature sensors and are found in consumer products.