A defibrillator, equipped with a battery power source, is described which is arranged to operate in any one of both a first mode and a second mode, the battery power source comprising at least two voltage sources. The voltage sources are arranged to be connected in parallel to each other when the defibrillator is operating in a first mode, and in series with each other when the defibrillator is operating in a second mode. The invention can be implemented by a battery pack for a defibrillator. This arrangement allows both voltage sources to be drawn down at the same rate which lengthens the overall life of the batteries. A more efficient use of battery power is thereby obtained. The invention ultimately extends the projected life of the batteries and when applied to an automatic external defibrillator increases the shelf life of the defibrillator.