An electricity supply system comprises: a battery module, a control module, and a distribution box. The battery module includes at least two in-series modules, each comprising at least two battery groups connected in series. The control module is connected with the battery module and includes an IGBT module, a relay module, and a relay control module. The relay module includes a plurality of relays K. Each in-series module is connected to the relay module. The relay module is connected to the IGBT module. The relay control module is configured for controlling ON or OFF of each relay K so as to select an in-series module to work with the IGBT module. The distribution box is connected with the control module.