Hi Steven,
many of us, including myself, use a small jump start batterysystem available from any motor spares type shop eg Super Cheap. They come complete with an inbuilt clever charger and mains pack for recharging from the mains. Apart from the convenience of these devices, there is no difference to running your gear directly from a car battery. The cheap ones contain a 17AHr battery which gives you adequate power for 2 nights viewing if not more. A further advantage is that they contain fully sealed batteries and are hence there is not such a concern if they topple over. They usually have a cigarette lighter receptacle on the side into which you can plug your scope. I have added a second socket to mine so that I can run my dew-heater as well. No problems.
Certainly a lot more convenient than a thumping great car battery.
Robert
PS Usually cost between $60-$80 although you can get more expensive ones which contain a compressor for your air-bed or a flat tyre or alternatively a bigger battery. Certainly could be useful if ever you get stuck with a flat car battery. Usually have a peak capacity of 900Amp.