Abstract: A battery pack having an electrical circuit including a battery array of one or more batteries, a thermostat switch, and a positive temperature coefficient resistor (PTCR). Preferably, the thermostat and PTCR are disposed in a trianguloid channel formed by the nip of adjacent cylindrical batteries in a multiple battery pack so that the finished pack may have the same size and shape as a non-protected pack for the same application. Preferably, the batteries are rechargeable and the pack includes a charging plug connected across the battery array in series with the thermostat and PTCR. Preferably, the pack is wrapped in a film of a heat-shrinkable polymer to provide physical integrity.