I've never seen a UPS with truly proprietary batteries (not saying they don't exist, but I haven't ever seen one). Most of the time I've opened up dead UPSes,

I've never seen a UPS with truly proprietary batteries (not saying they don't exist, but I haven't ever seen one). Most of the time I've opened up dead UPSes, they're just commodity lead-acid SLA batteries slapped together into a 24V or 48V pack with a minimal wiring harness. Quite a few seem to use 12V 7AH SLA batteries frequently used in emergency lighting, which cost about $15-20 per online, if you're willing to fuss with getting them bundled up and shoved back in tightly. Many of the bigger rackmount UPSes have a big plug on the back to attach extra battery cabinets. I wouldn't get a big UPS without this feature. They'll let you hang whatever batteries you want off of the charger/inverter, as long as you get the voltage and charge rates right. (Obviously we are in warranty-voiding territory here.) I have a couple of old Tripp-Lites, bought basically at scrap value because the internal batteries were dead, and they use Anderson SB175 forklift-charger connectors on the back. It works fine with just external batteries, giving you the advantage of being able to buy whatever SLAs happen to be cheap on the local market. (Though I'd definitely want to make sure all 4 were purchased at the same time since you're wiring them up together in a set.)