There is a "how to" article in the September 2001 issue of 4 Wheel Drive & Sport Utility on setting up dual batteries. For saving space they have an idea I haven't seen anyone mention. The article says that for your second battery you can use two Optima 6 volt batteries. I haven't investigated the situation, but maybe you could fit two 6 volt batteries under the hood easier than an extra 12 volt battery.