The problem is not its size but the lack of easily replacing it at the charging station with a fully charged one. Also you could make it modular and increase its size. Maybe that kind of design would help replacing it faster.

You could also make it attack its owner and drain his life force to recharge itself quickly.

Having such an easily replaceable battery would be nice, but it's not infinitely nice such that any tradeoff is necessarily worth it.

You could not do that without also raising the center of gravity of the car. That thing is massively heavy, putting it under the car is the only place that makes sense for it.