Abstract: A battery pack (4) comprises a stack of lithium solid state cells (1) and a DC/DC converter (2) in a housing (3), for use in supplying current to an electronic apparatus, e.g. a portable telephone. This provides a stable voltage for operating the electronic apparatus as the cells become progressively more discharged without having to include a DC/DC converter in the apparatus itself. The battery pack (4) can be arranged to supply current directly from the cells (1) as well as via the DC/DC converter (2), and the DC/DC converter (2) can be made voltage controllable to supply different voltages depending upon requirements. Conductive screening around the battery pack can be provided to prevent electromagnetic interference from the DC/DC converter (2).