I have used external batteries on several old, out-of-warranty UPS models over 15 years, ranging from 500VA to 1500VA (about 400 watts to 1000 watts). I prefer sine wave UPSs for their universality with small transformers and solenoid valves, but most computers and peripheral equipment use switching power supplies that work fine with modified sine wave (a kind of square wave) UPSs - I have one of those, too.
My TV and DISH receiver have a 500VA sine wave UPS, as any power glitch causes the DISH receiver to spend 5-minutes re-syncing to the satellite. That's annoying. The brick-sized AGM batteries I currently have on that UPS would run the system for 15 minutes or so.
I have sometimes added a tiny thermostat to the main internal heat sink to operate an add-on fan (inside or outside, depending on space) for longer run times. (Look for ~95°F KSD9700 N.O. thermostats, but some are NOT electrically isolated so they can't touch anything.) However, in my circumstances I have rarely used a UPS at it's full rated capacity. Most modern electronics consume little power - just a few amps usually. If you are going to run a UPS at 25% (or so) of its rated VA (or watts) you probably don't need an additional fan… although it doesn't hurt.
The most important thing is to use AGM batteries if you can possibly afford to, as they have a slightly higher charge voltage by about 0.3 volts compared to flooded (wet) batteries which (per the manufacture's email) should float charge at 13.5V. AGM batteries prefer about 13.8V. AGMs are not as tolerant of overcharging as flooded batteries. If you use flooded batteries the UPS will continue trickle charging after the batteries are actually at 100%, resulting in 'gassing off' distilled water from the cells. Flooded cells tolerate overcharging well but then you MUST add distilled water every 2-3 months to prevent the plates from being exposed - that's death to a flooded battery.
Since UPS manufacturers don't document how to adjust their charging circuits or offer a AGM/Flooded selector switch (pity), it is possible (for a 24V system) to add a pair of opposent-facing power diodes to drop the UPS's charging voltage to the flooded batteries by 0.6V (0.3 for each battery). I have built an interposing "charge matcher" to do this, but it gets much too complex to explain here and I don't recommend it. It was an interesting project, but you would be better off buying $100-$300 of AGM batteries instead.
I would suggest a digital voltmeter for each battery, and a digital amp meter from the UPS to the battery(s). The amp meter should ideally read both positive (charging) and negative (discharging) current to a resolution of at least 0.1 amps. These will allow you to monitor your battery(s) charging rates and health. I used LED meters from Amazon and they have worked fine so far.
ALWAYS fuse the batteries appropriately in case of a short circuit - large batteries can start fires. Mine have 50A and 100A low-voltage auto-marine fuses respectively. In a 24V system, placing the fuse in the link between the two batteries is a good idea.
ALWAYS use a battery box with flooded cells. A heat detector on the battery boxes and smoke detectors nearby are a good idea.
WARNING! Flooded batteries can release hydrogen and a nearby spark or heat source can cause an explosion in a cell which can spray battery acid over the room. Use extreme care!
WARNING! A UPS is designed to provide 120VAC even when NOT PLUGGED INTO the wall outlet! That voltage can kill you! Be careful!
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HOW TO MEASURE 'TERMINAL CHARGE VOLTAGE' FOR A UPS
How do you know what voltage a given UPS will charge its battery(s) to? It's actually fairly easy to determine. You will need a large capacitor, at least 4700uF. Mine happened to be 36,000uF and was the size of a soup can. You will also need one or two batteries - whatever your UPS requires. They need not be new or large, but should be capable of providing, say, 10 amps for a few seconds. And a digital voltmeter.
Turn off your UPS with the front panel switch, unplug it, and remove the battery(s) if there are any. On a workbench, arrange the equipment as follows: UPS, capacitor & voltmeter, battery(s). Connect the voltmeter to the capacitor with alligator clip leads. Pay careful attention to the capacitor's polarity! Using a handy ~1K resistor, pre-charge the capacitor from the battery(s) - important for really huge capacitors - and then connect it to the battery(s) with alligator clips which can handle 10 amps (18 gauge wire should do). Now use alligator clip leads to attach the capacitor to the internal UPS battery connector or wires, positive to red, negative to black! Plug in the UPS and turn it on at the front panel.
The UPS will wake up, test the battery(s) by drawing a few amps from them while monitoring the voltage, and then begin normal operation by trying to charge the batteries. The wake-up process takes under 20 seconds in my experience, but give it a minute or so. Now disconnect the battery(s). The UPS will quickly charge the capacitor to what I term the 'Terminal Charge Voltage' and the charging current will drop to zero. The voltmeter will show you what voltage the UPS will try to charge the batteries to. Since it expects AGM batteries, that should be 13.8V for one battery or 27.6V for two. A tenth of a volt per battery either way is likely okay - my hobbyist experience with AGM battery life is limited. But if it's too high it will shorten the life of the AGM battery(s).
Addendum: The best battery pack -to- UPS connector is the SB50. Gray is for 24V systems, yellow is for 12V systems. Each color is keyed differently. A given color can use different wire gauges, like 4-6 or 8-10 gauge, so choose the SB50 terminals that you wish (the housing is the same). Once soldered onto your wire the terminals just push into the connector. There is an internal flat spring in the plastic housing that can be depressed with a small screwdriver to remove each terminal. SB50s are sexless, so you will receive two identical pieces. There are larger versions of the SB50 available, like the Anderson SB120 and SB175. The number is the amp rating.
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The Outboard Charger Method
If you have a good used UPS that creates power but over- or undercharges its batteries, you can use an external low amperage charger intended for your battery type and smart enough to not overcharge it. To prevent your UPS from trying to charge the battery(s), install a hefty diode (on an appropriate heat sink) that will allow power to flow from the battery(s) to the UPS, but not to them from the UPS. Thus only the outboard charger can charge the battery.
You may also install a bypass relay that is line powered, so that during a power failure the NC contacts bypass the diode and allow full power to the UPS. The contacts must be rated for the maximum current you expect to draw, perhaps 30 amps or more. The relay's coil must be rated for "continuous duty". Without the bypass relay, the silicon diode and heat sink will heat up due to the 0.6V drop across it and the amps through it. V x I = watts (i.e.: 0.6V x 30A = 18 watts). Without the relay, you will need a cooling fan on the heat sink as it can get quite hot over time. Note that some smart UPSs might complain that the battery is weak due to the 0.6V drop when they periodically check the battery, so test your design on a lab bench to see if it is going to work for you.
Design and test with care as your UPS will operate when you are not present.