A simple charger equivalent to an AC/DC wall adapter. It applies 300mA to the battery at all times, which will damage the battery if left connected too long. A battery charger, or recharger, is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it. The charging protocol (how much voltage or current for how long, and what to do when charging is complete, ..., for instance) depends on the size and type of the battery being charged.