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Most battery backups use off the shelf batteries with a sticker over the original information. Look up the battery and find a compatible replacement. If you can't find the model number on the battery look at the specs for the battery and cross reference it that way. There are tons of different places that sell the batteries so just start searching.

Also most LFP batteries that match the original leaf acid batteries in size and voltage will not be able to output enough power unless the loading is light. To handle a full load on something similar to a 1500 va ups you need about 80 amps of available power most lfp batteries you get that will fit in a ups are rated at 1c so a 10ah battery can only supply 10 amps of power. That will require a custom battery build that will also need a bms and likely require a modification to the ups to be used and likely will not fit inside the case.

I've bought some brand called MightyMax on the rainforest site for replacement UPS batteries They work good. My friend who works IT swears by them to recondition old APC Schneider units.

Generally cheap sealed AGM lead acids are a lot safer than cheap lithium, the chemistry is simpler, harder to fuck up and the internal safeties are mechanical, not BMS based.

I've used local supplier for years who has spesifically selection for UPS batteries. Even APC ones tend to be pretty standard, just rip the APC stickers off and get the actual battery model number and ask from your local shop for replacement. I got a pair for new-for-me UPS a few weeks ago. Official APC kit would've been several hundred euros, the ones I got were ~50 with postage. They might not last quite as long as 'brand name' ones and power output is a slightly lower even on spec sheet, but that unit is running at around 15% load anyways, so in my case it doesn't really matter.

Depends on the model, but many, especially cheaper ones, have standard lead batteries which you can buy at motor or electronics shops.

A few years ago I decided to try the lithium-ion UPSs. I'm on my 3rd year with them, and I will never go back to lead acid. I've got one "classic" UPS that is still in good shape as long as you don't try to run more than 100w from it, but when it goes, it goes.

These use the LiFePO4 batteries, and I get close to twice the runtime as my old ones, and they don't drop from 80% to 10% like lead acids do. The battery chemistry is also good for about 10 years of daily cycling, so assuming the electronics hold out, they seem like they'll last.

The only hiccup with the model I got is it doesn't have a serial monitor connection, but you can probably fine plenty that have it.

We have a local 'battery' store where you bring in your appliance with dead batteries, say a cordless drill, and they will refurbish it with a replacement battery. Sometimes the refurb process is more expensive than just outright buying a new appliance, so you have to use your judgement.

honestly, I have always had pretty decent experiences with non-oem lead-acid batteries. my local battery place has a decent supply and longevity is roughly the same as the oem ones (3-4 years). I have never had any issues (type or frequency) that were not also an issue with oem batteries.

almost no UPS mfcr makes their own batteries, so if you strip off the labeling from the oem ones, you may even find an exact replacement.

edit: another advantage of a local place is the core-return rebate and disposal of your old batteries.

Batteries plus had a Duracell version of the one my APC unit needed, in stock in store so I could get it same day. If you’re in the US, might try them.

I have been happy with https://batterysharks.com/ . Just keep in mind batteries are heavy so shipping costs are high.

I usually check ebay when i need electronics batteries, but you have a chance of a seller selling knock off batteries that as you said, could be sketchy. I've personally never had an issue, but YMMV.

https://www.ebay.com/sch/i.html?_nkw=ups+battery