The VOTRONIC StandBy Charger is used to automatically recharge and maintain the charge of the starter battery or the auxiliary battery if the mains charger, the solar charge controller or the generator only has one charging output. The device can be easily retrofitted by simply connecting it between the on-board and starter battery or on-board and auxiliary battery without having to change the existing wiring. If, for example, the mains charger is in operation and charging the on-board battery, the second battery (starter battery or auxiliary battery) is also charged with a small portion of the charging current, depending on the state of charge. This charge retention works automatically and can be recognized by an increased voltage level of the second battery. The second battery cannot be overcharged as the charging voltage is reduced by at least 0.6V and the charging current is limited to 3A (24V version: 2A).