The airplane was substantially damaged when it was involved in a ground collision with two other parked and standing airplanes. Both other airplanes were substantially damaged. The pilot stated that the battery was not strong enough to turn the engine over, so he pulled his car up and attached the jumper cable to the outer jump device. He started the engine, reduced the power to idle, and applied the parking brake. He then waited thirty seconds to a minute to ensure the airplane was not capable of moving. He left the cockpit, removed the jumper cable, and climbed on the wing to re-enter the cockpit. As he was climbing on the wing, the parking brake let loose, and the sudden movement caused him to lose his balance and fall off of the wing. Before he could catch up to the aircraft, it clipped one aircraft near the propeller and ran into the second aircraft at less than five miles per hour. An inspection of the airplane revealed no pre-impact anomalies.