"hot rodders" put the battery in the trunk so it's out of sight - or maybe so it's closer to the gas tank when they get rear ended
racers put the battery in the trunk because that's the standard procedure at the track, and allows for a handy emergency kill switch near the emergency gas shutoff
at most your battery will weigh about 50 lbs, in a vehicle that weighs thousands of pounds, carrying passengers that weigh hundreds, and a payload of a thousand or more, the weight of the battery is a total non-issue regardless of where it is
Bill
