A device (8) for determining the energy state of an energy storing device (7) of a data carrier (4) in which a voltage stabilizer (9) is used to derive a stabilized DC voltage for the supply of the data carrier from an unstabilized DC voltage available at the energy storing device. The device (8) has an evaluation circuit that includes an auxiliary capacitor (13), a measurement circuit (15, 16, 21) for measuring the charging times of the auxiliary capacitor, and an evaluation logic (18). A quantity describing the energy state of the energy storing device is provided at the output of the evaluation circuit.