An electrical power supply system having: a DC distribution bus; a rechargeable battery module configured to deliver DC power to the DC distribution bus in a discharge mode of operation, and to absorb DC power from the DC distribution bus in a recharge mode of operation; a DC/DC converter comprising an inductor and plural switches, the DC/DC converter being operatively connected between the DC distribution bus and the rechargeable battery module; and heat transfer means configured to transfer heat between the DC/DC converter and the rechargeable battery module; wherein the rechargeable battery module also has an idling mode of operation in which it is configured to neither deliver DC power to nor absorb DC power from the DC distribution bus; characterized in that the DC/DC converter is arranged such that, in the idling mode of operation, the DC/DC converter is configured to repeatedly switch between: (i) a ramping-up configuration of the switches in which a current is withdrawn from the DC distribution bus, flows through the inductor and is returned to the DC distribution bus, and (ii) a freewheeling configuration of the switches in which the current from the ramping-up configuration is isolated from the DC distribution bus to flow in a continuous loop within the converter; and wherein the electrical power supply system further has a controller which is configured to control the repeated switching between the ramping-up configuration and the freewheeling configuration such that, in the idling mode of operation, heat resulting from the repeated switching and the current flow produced thereby in the DC/DC converter is transferred by the heat transfer means from the DC/DC converter to heat the rechargeable battery module. a DC distribution bus; a rechargeable battery module configured to deliver DC power to the DC distribution bus in a discharge mode of operation, and to absorb DC power from the DC distribution bus in a recharge mode of operation; a DC/DC converter comprising an inductor and plural switches, the DC/DC converter being operatively connected between the DC distribution bus and the rechargeable battery module; and heat transfer means configured to transfer heat between the DC/DC converter and the rechargeable battery module; wherein the rechargeable battery module also has an idling mode of operation in which it is configured to neither deliver DC power to nor absorb DC power from the DC distribution bus; characterized in that the DC/DC converter is arranged such that, in the idling mode of operation, the DC/DC converter is configured to repeatedly switch between: (i) a ramping-up configuration of the switches in which a current is withdrawn from the DC distribution bus, flows through the inductor and is returned to the DC distribution bus, and (ii) a freewheeling configuration of the switches in which the current from the ramping-up configuration is isolated from the DC distribution bus to flow in a continuous loop within the converter; and (i) a ramping-up configuration of the switches in which a current is withdrawn from the DC distribution bus, flows through the inductor and is returned to the DC distribution bus, and (ii) a freewheeling configuration of the switches in which the current from the ramping-up configuration is isolated from the DC distribution bus to flow in a continuous loop within the converter; and wherein the electrical power supply system further has a controller which is configured to control the repeated switching between the ramping-up configuration and the freewheeling configuration such that, in the idling mode of operation, heat resulting from the repeated switching and the current flow produced thereby in the DC/DC converter is transferred by the heat transfer means from the DC/DC converter to heat the rechargeable battery module.