The battery charger charges batteries that are used on board vehicles to generate power or supplies them with a retention voltage so that they do not discharge. It is automatically charging the batteries and maintaining them with float voltage after charging has finished. Therefore the battery charger can be plugged in during longer periods of time without damaging the batteries. A control lamp on the device enables constant monitoring in the battery charger. The device has several protective systems such as overvoltage protection, overheating protection and overload protection. The cooling system uses convection cooling to surrounding air.