I have a few uninterruptible power supplies that can be used to power a computer (like this one). They have an internal 12v lead-acid battery inside to supply power when needed. I also have a few larger 12v lead-acid batteries for cars and lawn mowers that I need to keep charged. Can I use the UPS to keep those other batteries charged by hooking my own battery up to the leads? It seems like it should work, but I don’t know if there’s some wiring weirdness in a typical UPS which would cause problems.
you can buy battery trickle maintainers that can be had for $5 to $10 that are made to do that and have long cords and strong battery clips.
The tinkerer and cheapskate in me refuses to buy something unless it’s a last resort 
Yes the UPS works well as a trickle charger.
One issue is that if you connect the fully charged battery to a lower charge battery, you can get a high current flow. This could exceed the capability of thin wires…
If you want to connect two batteries, use really heavy duty cable, like automobile starter leads to connect them. Then you can add the trickle charger lead onto the pair after that.
I haven’t opened up many UPS’s lately so I can’t say what they are commonly putting in them these days. I think most of them have current limiting built into them, so putting a larger battery in them than what they are designed for will only result in them taking a lot longer to charge it. I can picture the cheaper ones not having any current limiting in them, though. Again, depending on how they are designed, even that might not be a problem, but if you have a relatively dead car battery and a UPS that lacks decent current limiting, you could end up drawing more current from the UPS charger than it was designed to output which could damage the UPS.
Yes it works fine and did for years. I used a automotive battery , actually a deep cycle marine battery to replace the UPS one. I did that perhaps for some tinkering and to get more run time out of it during a power failure. It worked well.
As for the wire upgrades, I never did, just got (made) crimp on connectors that plugged into what would normally plug into the battery.
A few times during a extended power failure, I recall hooking the battery up to a standard car charger as well as the UPS, as the battery ran low I would start up the generator and rapid charge (as much as I could), the marine battery, once charged would shut down the generator and again run off the battery. This was to conserve fuel, and the periodic generator starting kept the fridge cold.