You're not talking small here. Seems like SLA might be a good choice. An 18 Amp-Hr 12V SLA weighs 13-lbs
and costs $50. You can get a rough estimate of the runtime by dividing by current, 18 A-H/2 A = 9 hrs,
18 A-H/10 A = less than 2 hours. So for selection, you need to also figure desired runtime into your calculations.
Also, the voltage will probably sink faster for higher currents, due to heating losses, etc.