I've got 2 APCs, the older one doesn't seem to charge batteries anymore, so retired it. Been through a few batteries, and most anything that will fit in the case, will work just fine, if the rating is reasonably close. You can use a single 12volt, in place of the two series 6 volts (like I'm currently using). I you go with a lower rating, you'll have a shorter backup time. Have read about a few people using boat batteries (won't fit in the case though), but the APC will get hot after a while, if the power it runs off the battery a long time. Potential fire hazard...