A battery made up of nickel-cadmium cells includes an indicator for displaying a state of charge comprising sensors for performing continuous measurements of voltage and current and of temperature of the battery, a memory storing at least a history of charge/discharge, a predefined reference voltage, current and temperature, and the measurements and history determined, and a control unit for determining an operation mode of the battery and determining a correction to be applied to the voltage and current measured, calculating a theoretically available capacity of the battery depending on the operating mode and the correction, determining a need for recalibration of the theoretically available capacity of the battery enabling a capacity actually available of the battery to be calculated, and based on the capacity actually available, defining the state of charge of the battery.