Anyone else using old UPS batteries for a small 12v setup?

Ran this exact experiment about three years ago when I were setting up my shepherd's hut. Grabbed four 12V 7Ah sealed lead-acid units pulled from scrapped APC UPS boxes — the sort of thing that turns up on eBay or Gumtree for almost nothing.

The honest verdict: it works, but with significant caveats.

The main issue is you genuinely have no idea what cycle history those cells have seen. UPS batteries sit on float charge for years and the capacity degrades silently. I tested mine with a proper load tester and found one was down to roughly 40% of rated capacity. Binned that one immediately.

What I'd suggest before committing to a UPS battery bank:

• Do a proper discharge test — don't just trust the rated Ah
• Check internal resistance if you can (a Victron BMV or even a cheap capacity tester will help)
• Wire in parallel rather than series where possible to avoid a dud cell dragging down the whole bank
• Keep expectations realistic — these are genuinely short-term solutions

My four batteries gave me maybe 18 months of light LED lighting and phone charging before I migrated across to a pair of Fogstar Drift 100Ah lithiums, which was a significant upgrade in every measurable way.

That said, for someone wanting to experiment cheaply, or power a garden office temporarily whilst waiting for budget, there's nothing wrong with UPS batteries as a learning exercise. You'll understand battery management, charging stages, and load calculation far better having worked with something you're not precious about.

What sort of loads are you trying to run? That'd help everyone give more specific advice about whether UPS cells are even worth considering for your use case.

@ExPostie82 Done something similar on the narrowboat — inherited a pile of old UPS bricks from a mate who works IT support. The temptation is real when they're basically free.

Honest experience though: the capacity testing was brutal. Most measured under 40% of rated Ah after a proper discharge cycle. On a boat where space and weight actually matter, lugging around six units to get the equivalent of two decent cells felt daft pretty quickly.

Replaced the lot with a single Fogstar Drift 12V 100Ah lithium and never looked back.

Worth doing for a shed or hut where weight's irrelevant and you're not depending on it through a cold February — but I'd go in with eyes open about how much genuine capacity you're actually working with.

@ExPostie82 @Jonno — worth flagging that those 7Ah UPS cells are almost universally rated for float/standby duty only, meaning they degrade rapidly under cyclic use. I tested four pulls from an old Eaton rack UPS in my garden office build; after capacity-testing with a proper discharge rig, three measured under 4Ah actual usable capacity despite showing full voltage on a multimeter.

Key thing I'd add: always test internal resistance before committing them to any real load. A cheap YR1035 tester off eBay tells you immediately whether a cell is genuinely viable. Anything above roughly 30mΩ on a 7Ah unit I'd bin without hesitation.

If you're keeping UPS chemistry long-term, keep absorption voltage tight — around 14.4V — and avoid deep discharge below 50% SoC. They'll punish you otherwise.

@DodgyRoamer raises the most important point here. Those APC cells are typically designed for maybe 200-300 shallow cycles at best before capacity tanks — I've pulled apart enough of them to know. Running any meaningful load cycles will age them brutally fast.

That said, if you're genuinely scrapping them for free and expectations are realistic, there's something there. I ran a similar bodge on the boat for LED lighting and a 12v fridge fan — nothing demanding. Checked each cell with a proper load tester before bothering, chucked about 60% immediately.

The ones that passed gave me maybe 18 months of light duty before I swapped to a proper Fogstar 12V LiFePO4. Night and day difference in usable capacity.

Bottom line: treat UPS SLA as a short-term experiment, not a foundation. Budget for the upgrade from day one.