Considerations when buying a UPS Replacement Battery

If you get a replacement battery from anyone other than the manufacturer of the UPS you will invalidate any warranty or connected-device guarantee/insurance that it is covered by and may actually invalidate warranties on the connected devices too (though "how would they know?" comes into play there).

If the UPS is out of warranty and offers no other guarantees/insurances then this is not an issue - though if the equipment/data that the UPS is serving is in any way remotely critical I would consider replacing the whole UPS at that point.

If buying a replacement battery from a supplier other then the one that made the UPS, check the small print of any guarantee very carefully. The original manufacturer's small print will no doubt be weasely enough, but the one from the aftermarket supplier may amount to nothing more than "use at your own risk".

Yes, there are aftermarket vendors for these batteries. A replacement battery from APC will come with a 1 year warranty and will cost more than after market batteries. In general, you can purchase extended warranties for replacement batteries, but I've never bothered with it. If you buy from an aftermarket vendor make sure you are getting new batteries and not refurbished batteries.

While not directly answering your question.... I have had excellent luck going to RefurbUPS for my replacement batteries and simply using their recommendations. I have used them to get replacements for products in the Back-UPS and Smart-UPS lines, and have not seen any noticeable performance loss. I have not, however, performed any kind of performance analysis to determine if the replacements perform differently than the originals.