An energy storage system is disclosed. The energy storage system comprises an energy storage device configured to operate above ambient temperature, and a thermal insulator at least partially surrounding the energy storage device, wherein heat losses from one or more other devices are received within the thermal insulator to provide heat energy to the energy storage device. Utilising heat losses from one or more other devices, such as associated electronic components, enables the energy storage device to be maintained at its elevated operational temperature for longer providing extended battery life. In the application of wireline logging, this results in more data log available per trip in a well.