Rechargeable Lead Acid Charging
If you are thinking normal "car" batteries, then you shouldn't discharge them below half their capacity.  That may still be the cheapest way to get portable power, though.

Voltage is not a good indicator for state of charge for lead-acid batteries.  You'd have to track total current.

Other battery technologies would be more suitable, or get something fully packaged so that you don't care what is inside.  Any decent UPS, for example, will not allow discharge deep enough to cause significant damage to its battery.