A defibrillator is disclosed. The defibrillator includes a secondary battery having a volume less than 5 cc. One or more capacitors are configured to store electrical energy from the battery in an amount sufficient to provide a patient with one or more defibrillation shocks. A processing unit is configured to control the storage of electrical energy in the one or more capacitors and to control the discharge of the electrical energy from the one or more capacitors. The battery can have a total volume of less than 4 cc or less than 3 cc.