Anyone else using an old UPS as part of their backup system?

Picked up a pair of APC Smart-UPS 1500s from a local IT recycler for £40 the lot. Swapped the internal sealed lead-acid batteries out for some decent AGM replacements and they've been running solid for about eight months now as part of my emergency backup setup at home. The built-in inverter/charger is surprisingly capable and the management software gives you proper runtime estimates, charge data, all that.

Main question is around charging — the UPS charges at whatever its internal profile dictates, which is fine for the AGM replacements but I'm wondering if anyone's tried retrofitting lithium cells into one of these units. Fogstar Drift cells are tempting given the price per kWh, but I'd imagine the charge profile would need sorting. The BMS would handle overcharge protection but the absorption/float voltages would still be wrong out of the box.

Has anyone hacked around with the charge settings on APC or Eaton units, or is it more trouble than it's worth and better to just keep AGM in these things?

@Spud74 done exactly this with a couple of Smart-UPS 750s I grabbed off eBay — brilliant bits of kit for the price. One thing worth noting: the internal charger profiles on those APC units are fixed for SLA chemistry, so if you eventually want to push towards lithium (even a small Fogstar Drift cell), you'll hit problems with charge voltage mismatch. I've got mine sitting downstream of my Victron Multiplus rather than as primary charge management, which sidesteps that entirely. Also worth grabbing PowerChute software while you still can — lets you monitor runtime estimates and tweak shutdown thresholds. The build quality on those older Smart-UPS units genuinely embarrasses most modern consumer stuff. Torroidal transformers, proper bypass switching, decent AVR. £40 for the pair is a steal.

Great find @Spud74! I've got a Smart-UPS 1000 doing similar duty here. One thing worth mentioning — the AVR (automatic voltage regulation) in these units is genuinely useful if you're running them alongside a small genny, as it smooths out the rough edges nicely before anything sensitive sees the power.

Worth keeping an eye on the battery temperature if yours are in an enclosed space — the charging profile on the older APC firmware can be a bit aggressive and I've found AGMs run warmer than the original SLA units did. Nothing dangerous, just worth knowing.

Also check whether yours have the serial/USB management port working — you can pull real-time load and runtime data from them using apcupsd on a Raspberry Pi, which makes monitoring your whole setup much tidier.

Ran one for two years in my cabin before the Victron took over — it's basically a free inverter/charger if you squint at it sideways.

@WayneTaylor that's exactly how I looked at it — free inverter/charger, can't argue with that logic 😄

Been running a Smart-UPS 2200 here for about 18 months alongside my Victron setup. Use it as a buffer for the office gear specifically — NAS, router, switch. Means the Victron doesn't need to kick in for every little blip.

One thing I'd add — the NMC software is worth setting up if you haven't already. Can pull runtime estimates, set low battery cutoffs, log everything. Runs fine on an old Raspberry Pi. Gives you proper visibility rather than just hoping for the best.

Only downside is the runtime on these old units is pretty limited, but for ride-through power and keeping sensitive kit happy they're spot on.