For those of you who know how to mess around safely with car batteries and other high-amperage/low-voltage applications, here’s an interesting one (please note the warning below!:
APC makes an extended-run UPS called the Smart-UPS XL, which supports external battery packs. The ones I have (Smart-UPS XL 1000) are a 24V system, which means that the internal batteries and the external battery packs are all 24V. The external battery packs and replacement batteries are expensive.
So when my batteries gave up the ghost, I wondered what would happen if I replaced the 20 amp-hour gel-cell batteries with 100 amp-hour RV batteries, which were cheaper in spite of having five times the capacity.
Using a battery connector from a spare/dead battery pack (you can also scavenge them from most dead APC batteries), I wired up two new RV batteries in series and plugged them into the expansion pack connector. Result? Greatly extended run time, as expected.
The batteries have been in use for five years now and have been slowly losing capacity. It’s about time to replace them. Five years is what I got from the APC batteries when the units were new. (By the way, this really does mean that an APC UPS will last more than ten years if you replace the batteries.)
Now, RV batteries are deep-cycle versions of car batteries, which means that they outgas hydrogen and need topping off, so use them in a well-ventilated area. Also, since they’re full of sulfuric acid, they are a little harsh on the surrounding area, as you know if you’ve ever looked at the area around the battery in an old car. Thirdly, gel cells use a slightly higher charging voltage than flooded-cell batteries, so water evaporates out of the battery faster than it should, and you need to top the cells off several times a year with distilled water.
All of these problems would be solved by using gel-cell batteries instead of the cheaper flooded-cell batteries. Good maintenance-free batteries would help but would not solve the problem.
I have also tried this trick on two Pacific Power Vanguard 1200 UPS systems, systems that I got for almost nothing years ago. Converting these was trickier because I had to open up the units and add external battery cables. The APC Smart-UPS XL is the only line that I know of with a convenient external battery connector. The Vanguard overcharges more than the APC, and was generally much less satisfactory.
I’d stick to Smart-UPS XL units for this trick if I were you, but if you ignore this advice, one thing to look for is a UPS with a cooling fan inside. I think that some of the short-duration UPS systems expect to run out of battery power before they have time to overheat, and thus would be lousy candidates for having their battery life extended.
Tip for anyone who would rather do things more conventionally: replacement batteries are very heavy and expensive to ship. Ask at your local car-supply stores and battery shops if they can get what you want. They get daily delivery by truck anyway, and the shipping should be free (ask). This will save you beaucoup bucks.
Tip #2: The world is full of low-quality gel-cell batteries that are shipped halfway around the world just to fail immediately in your UPS. If you buy them locally, you’ll probably get a warranty that means something. Ask. If you have to ship them back, the shipping alone will kill you. I know that APC batteries are good, and I think that ABC batteries are also good.