I think a battery under back seats is not only a bad placement but also a dangerous one.

1) Hydrogen build-up in a closed space. Don't expect that all users are knowledgeable enough to buy appropriate batteries with vent tube. An explosion can easily kill passengers.

2) Neglected batteries under back seats will ooze out corrosive H2SO4 and potentially debilitate the structural body integrity of a car.

3) Fallen debris and sacked back seats may cause a short with the result as a fire or explosion.

Too dangerous, methink.

Eric