Charging at the track

i think i understand what all of you guys that are helping me out are saying, that if i get a great quality marine or RV battery and replace it with my car battery, and then when i go to the track to charge my lipos, i should have the car turned off. what voltage is the bear minimal to start the car successfully, i am going to charge up my slash packs a few times and several 4s lipos for my xb808e, i guess these (deep cycle)batteries can handle it no problem.

thank you for your input.