Abstract: An apparatus and method for providing supplementary power to an electrical system of a vehicle or other machinery is provided. The apparatus and methods specifically provide a rapid-delivery of high amounts of power. The significant power levels are sufficient, for example, to turn the starter of a heavy-duty commercial transport vehicle stranded by a disabled battery. The power is supplied by a power source having the combination of high-density capacitor and battery. The power delivery is controlled to avoid such standard problems as reverse or same polarity connections and overvoltage. Control is effected by a combination voltage sensing circuit and isolation circuit to prevent current between the power source and the electrical system unless prescribed conditions are satisfied.