Anyone else using old UPS batteries for emergency backup rather than proper leisure cells?

I've been running a pair of 12V 9Ah sealed lead-acid units salvaged from a scrapped APC UPS at work — wired in series to give 24V, then paralleled with a second identical pair for roughly 18Ah usable at 24V (being conservative, these are old). Feeds a Victron Phoenix 24/500 inverter and keeps my router, a couple of LED lamps, and a small USB hub running during power cuts. Total cost: essentially zero beyond some Anderson connectors and cable.

The obvious downside is cycle life and energy density are rubbish compared to even a budget Fogstar 12V LiFePO4 cell. I'm seeing maybe 50–60% capacity remaining on these after a couple of years of occasional use, and the resting voltage sags noticeably under even modest load. Charge them with an old CTEK MXS 5.0 on its AGM setting, which seems to keep them reasonably happy.

Curious whether anyone else is doing this kind of "free tier" emergency backup rather than investing in proper leisure or LiFePO4 kit — and at what point the faff of managing degraded cells stops being worth it versus just buying a 100Ah Fogstar Drift or similar?

@Turbo the maths is a bit off there — two 12V 9Ah in series gives you 24V 9Ah, not 18Ah. Paralleling a second identical series pair then brings you back to 24V 18Ah total, but SLA you're realistically only pulling 50% DoD safely, so ~9Ah usable. Fine for short-term emergency backup but I wouldn't rely on it long-term.

Bigger issue is UPS batteries are optimised for float charging and brief high-current discharge — not the cycling you get in off-grid use. They'll degrade fast under regular draw.

Had a similar setup on the narrowboat years ago before I switched to proper leisure AGMs. The UPS cells gave up within a season of regular use. If budget's tight, even a second-hand Fogstar or decent leisure SLA will outlast salvaged UPS units considerably.

@OffGridJack is right on the capacity point, but just to add — @Turbo, when paralleling those series strings, make sure both strings are at identical voltage before connecting them together, otherwise you'll get a rush of current between the packs that can cause real problems with already-aged cells. UPS batteries are typically only rated for maybe 3-5 minute discharge cycles, so their actual usable capacity for sustained loads will be significantly less than the rated 9Ah suggests. Worth doing a proper discharge test at your actual load current to see what you're genuinely working with. They can still be useful for short-duration backup, just don't rely on the nameplate figures.

@Turbo also worth knowing those UPS cells are typically rated for maybe 200–300 cycles at best before they're cooked, compared to a proper leisure battery — you're basically running on borrowed time with borrowed batteries.

Good points all round from @OffGridJack, @RussMitchell75 and @StormyDrifter. One thing nobody's mentioned yet — UPS batteries are designed to float at full charge for months then discharge rapidly during a brief outage, so the internal resistance tends to be quite high compared to proper leisure cells. You'll notice significant voltage sag under even moderate sustained loads, which can cause inverters and 12/24V appliances to cut out earlier than the state of charge would suggest. Worth doing a proper load test before you rely on them for anything critical. Also, if those cells have already done time in a UPS environment, they've likely aged unevenly — I'd individually capacity test each one before wiring them together, otherwise a weak cell will drag the whole bank down faster than you'd expect.

@FellFrank go on then, don't leave us in suspense — though I'll wager it's temperature sensitivity, because those APC cells absolutely sulk the moment it drops below 10°C, which on a narrowboat in January means your "emergency backup" is basically decorative.

@Turbo honestly for 24V I'd just grab a Fogstar Drift 100Ah and call it done — you'd spend less time babysitting it than rewiring salvaged UPS bricks, and you won't be stranded mid-canal because a 2009 server room battery decided it had better plans.

@FellFrank @RelaySolar — I'll throw another angle in while we're all guessing: internal resistance creep.

Been down this exact rabbit hole with my van conversion. Grabbed a couple of UPS units from a skip outside an office block in Cornwall, thought I'd cracked free power. Ran them through a Victron BMV-712 and watched the internal resistance climb week on week even under light use. What looked like 18Ah on paper was delivering maybe 11Ah by month three.

The sneaky bit is the voltage curve stays deceptively flat until you're genuinely almost empty — so your BMS or charge controller thinks everything's fine right up until it isn't.

For genuine emergency backup I'd rather have a single knackered Fogstar Drift cell than fresh UPS bricks. At least you know what you're actually working with.

Solid thread this — and @Turbo, for a first post you've stumbled into exactly the right kind of debate! Welcome aboard.

One thing worth adding to what @CoastalCamper and @FellFrank have touched on: self-discharge rate. UPS cells are optimised to sit on float charge indefinitely, so when you actually need them in a genuine emergency — maybe after weeks without mains — they can be surprisingly flat.

On the boat I always keep mine on a Victron IP65 trickle charger precisely because of this. Worth budgeting a tenner for a proper float charger if you haven't already.