This happened the night before to my employee's LS 400. I thought a bomb had blown up next to me. When she started the car. BOOM! I thought it was a misfire but, she only cranked the engine for like 2 secs max. Glad no one was hurt including me. I think it was because the battery ran dry but not sure. However, another employee had a old Nissan Sentra and wouldn't start. It turned out all of the cells were bone dry. It didn't explode and started after refilling with water. I thought these battery exploding events mostly happened during the hot Summers but, it is Winter over here and still exploded.