This is a modification I normally do to all my consumer grade UPS.
Used a pair of 50A binding post terminals commonly found in power inverters/DC power supplies, and a pair of SMH 50 amp connectors, in order to use a cheap ~$20 USD U1R 230CCA lawn battery, but not limited to, an U1R sized AGM battery will be optimal for a few more bucks, also the base of the battery can be an empty DVD case.
- Cheap battery replacement alternative.
Extended UPS runtime.
Easy battery hotswap/maintenance.
Can operate with internal or external battery.
- Slow battery charge time in some small units.
Possible charging circuit overheating on small units.
Additional space used by outboard battery.
Note: I've used a very short 12 AWG mil-spec silver plated teflon insulated wire, since the black lead can't reach the binding post terminal, otherwise a standard #10 AWG should be used.