Two extra combination deep cycle/starter batteries have been added in parallel that can power the booster cells independantly from or in conjunction with the vehicle's electrical system via a selector switch. The extra batteries, located in the trunk, are continuously recharged by solar panels with a combined output of 50 watts. With this set up, normally, powering the booster cell imposes zero load on the vehicle's engine. On cloudy days or trips that take longer than a few hours, I can engage the vehicle's primary electrical system, from the driver's seat, to power the booster cell.