A battery module comprises charging for connecting the battery module to a power source, a battery cell arrangement, and a DC-to-DC converter. A first terminal of the battery cell arrangement is connected to a first charging terminal. An input terminal of the converter is connected to the second terminal of the battery cell arrangement and another input terminal of the converter is connected to the second charging terminal. An output terminal of the converter is connected to the first terminal of the battery cell arrangement. The converter converts an input voltage (U3) received at its input terminals to an output voltage (U2) at the output terminal of the converter. The output voltage is higher than the input voltage. The battery module may for example be employed as a backup power source in a mobile communication base station. A switch arrangement may for example allow the converter to be bypassed.