Abstract: The invention relates to a defibrillator having a capacitor (4) which is chargeable via a DC-DC converter (3) by an accumulator (i.e., rechargeable battery) (1). Between the accumulator (1) and the capacitor (4) there is situated a control device (2) which, as the internal resistance (Ri) of the accumulator (1) increases, draws a lower current from the latter, so that the output voltage of the accumulator (1) is kept approximately constant. Before the defibrillator is automatically deactivated and energy already stored in the capacitor is internally destroyed, the controller interrupts the charging of the capacitor when the residual energy remaining in the accumulator is less than an amount required to charge the capacitor to the desired energy.