Stand-by power supply question

You are thinking correctly. Your UPS charging system may not keep the larger batteries in shape. If you use an auxiliary charger on the larger batteries make sure they are not conflicting with each other. You may need some isolation. Tom

Yes, You can do that. I have done the same thing here. It works very well how ever, there is a short fall on some UPS units where if a larger battery is on line it may not see the battery ramping up and current not retarding. Some units may see this as a damaged battery and fault out. Also, some units may run a little more heat due to the larger battery online when charging after commercial is returned, while the load is still attached..

I've done this with three different units to date and haven't yet see any issues.

Thanks Tom and Jamie,

Tom, when you say "isolation" what do you mean? My electronics knowledge is a little limited. I can wield a solder iron and build stuff but hopeless with design.

Jamie, do you have any brand names to share? I have an APC here.

Thanks guys.

I have 2 different APC's, they both work find with external batteries. the third is a cheaper flat one, I can't remember the name on it now but it only cost me 79 bucks, that also works with a larger battery because all these units had their original battery killed and now I use batteries I get pulled out of flood lights and so on, when the locale college refreshes their units. Its nice to know some one in places like that :)

Be careful; understand some UPS batteries can be mains live. Nothing wrong with that just the way they are designed. In other words there is no complete isolation between the battery circuit which is both charged by and when necessary operates the UPS for a certain period and the mains input/output. The OP's model may differ.

Make sure it doesn't overheat. The electronics in the UPS may not be designed to run for long periods.