A battery at a voltage potential of zero is practically a dead short circuit and tons of current will flow through it. I hope you realize that the power produced will be substantial and the battery will get hot instantly and could lead to serious injuries.
A dead lead acid battery is still at 11.8 to 12 volts potential you cannot get one to zero volts.
The whole premise is that you start with a charged field box battery use it at the field and charge it back up connected through a fuse to your cars 12 volt system. It is safe it will NOT cause the battery to explode and I have yet to have a fuse blow. Let's inject some reality here!
John