Energy storage device

The device has nickel-cadmium accumulator cells (11) i.e. electrical double layer capacitors, for charging and discharging electrical energy and arranged in a module housing (12). A temperature measuring unit (13) comprises temperature sensors (14) for each cell or group of cells. The sensors are designed as temperature-dependent electric resistors i.e. positive temperature co-efficient (PTC) resistor, and interconnected to an electrical measurement circuit. An evaluation unit (17) evaluates distribution of operating temperature of the cells from measured values of the resistors.