An uninterruptible power supply operates between a main supply input, a battery and a load, and comprises an input converter and an output converter. In a first mode of operation, AC mains power received at the supply input is converted by said input converter and output converter in series to provide an AC power supply to the load. In a second mode of operation, stored energy is released from the battery via the output converter to maintain the AC power supply to the load. A further converter can be used during the first mode of operation to charge the battery from a charging supply input. The charging current bypasses the input converter, allowing charging rate to be increased, without increasing a power rating of the input converter. The further converter may be bidirectional and be used also in the second mode.