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.
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Thanks lemonie You've answered my concern. I have a 12V car battery and was considering using it instead of the small internal UPS batteries. The running time would be greatly extended of course, but you do make a good point about possibly overloading the internal charging circuit. On the other hand, a UPS only runs off the battery when there is a power failure. Otherwise it runs the regular way. Therefore, by using an external charger a bloke would defeat the automatic switching to battery power when the house power would fail. In this case, the UPS would run off the battery continuously, and the battery would have to charged at the same rate for as long as the house current is on, naturally. Plus one would have to disconnect the annoying buzzer which sounds when the UPS is running off the battery.
. If you're just wanting to add the car battery in parallel with the UPS batteries for extra run-time, it more than likely won't work. As L points out, the charging circuit in the UPS isn't going to be able to handle the extra load. Mixing batteries in not a good idea, either.
. As far as the inverter is concerned, 12 VDC is 12VDC (or whatever voltage it runs on). It will be perfectly happy with any battery with the proper voltage.
. If the charging circuit has some headroom, you might be able to get away with adding a battery of the same type, but the manufacturer probably didn't spend the money to put in a charger that is any more powerful than it needs to be.
Running time of the UPS? Take it apart and find the existing battery - are they the same voltage? If so that's good. I'd think you just have to wire the terminals of the batteries together like-to-like.
This could give you a problem charging though, because the car battery would put extra load on the UPS that it wasn't built to take - so I wouldn't advise trying to charge it off the UPS.