Abstract: At approximately 5:21 a.m. on July 5, 2002, Employees #1 and #2 were starting the ignition of public transportation diesel buses that were stored in a large warehouse in San Francisco, CA. In addition to starting the buses in the morning, service employees also performed maintenance on these buses. Sometimes employees required jump starters because these buses developed dead batteries during overnight storage. Employees #1 and #2 were injured when they were using a six-month-old, portable jump starter. The portable jump starter was a Goodall Model 13-024 Portable Boost-All 12/24 volt jump starting, battery powered charger, Serial Number 84479. The Boost-All had been charging all night, without the benefit of opening the unit to ventilate the battery-holding chamber. An explosion occurred within the battery charger when the leads from the Boost-All were directly connected to each of the terminal posts of bus 6277's drained batteries. This was its first use that day. Both injured employees were taken to San Francisco General Hospital. Employee #1 suffered a laceration on his upper lip that required stitches. He walked towards the bus when the explosion occurred. Employee #2 suffered hearing loss in both ears, with significant loss in his left ear. He must wear a hearing aid in his left ear and suffered tinnitus.