I've been running an APC ES750 on a lawn mower battery for over a year without any problems. It's far cheaper than replacing the internal battery, in fact it was free because I already had the lawn mower battery and hardware. The UPS's charging system doesn't charge the battery, but it's not a problem during the mowing season because I use the battery in my mower, so that keeps it charged. During winter, I keep the battery in the house so it doesn't freeze and I put it on a proper charger/conditioner every month or so to keep it in good shape. The APC Powerchute software recently started complaining about the battery needing to be replaced. Not sure why, other than it was down to 53% charge. I charged it back up and put it back on the UPS and all is well. Powerchute still has the notice and the light on the UPS is flashing, but it's all working fine.