Your best options are:

1) Car battery. Monitor the car battery voltage and start it up occasionally to charge it up.
2) Deep cycle marine battery.

I know a few guys that use option 1. You'll probably figure out how many batts you can charge, or how long you can charge before you have to start it up and run it for a few minutes.

For option 2, figure out how many Watt-hrs you need at the field for battery charging (nom batt pack voltage x total amp-hrs you want to charge), and try to get a battery with at least that Watt-hrs rating you need. Watt-hrs rating for a marine battery will be approximately 12V x amp-hr rating.

Example:

You want to recharge six 3S 2200mAh battery packs: Watt/hrs required = 6 x 11.1V x 2.2Ah = 146.5 Watt-hrs
So you want to get a battery with at least 150 Watt-hrs of capacity = 150Watt-hrs/12V = 12.5 Amp-hrs

The charger will require some power, and there will be some power loss due to charger inefficiency, so you may want to add 10-20% to that Amp-hr rating to be sure you can charge that many batts.

Hope that helps!