[othello]

Car batteries are built for delivering short high Amp bursts (ie starting your motor) and beeing recharged constantly. Applying a constant load (recharging your RC batteries) for a rather long period of time and maybe depleting your battery over half its capacity will degrade its capacity and cycle life in a short amount of time. Running your engine while recharging is the better option (when you don't want to degrade your cars battery).

As _paralyzed_ has already mentioned
-> A marine or RV battery is your best bet