Questions: 2nd battery for my sweet, sweet minivan?

I used a jump starter for a few years. Something like this: http://www.amazon.com/PowerStation-PSX-Multi-Purpose-Vehicle-Compressor/dp/B002YVU8WS
Mine had a built in inverter.

It plugged into the cigarette lighter for charging. Easy to put on the picnic table for lighting. Useful when not camping for boosting or using a power tool in the yard.