Yeah definitely go easy on them if you are going to test them... They dont hold up well otherwise, I promise.
As for schematics, if they are APC units, they released the diagrams at some point for several models. I actually kept a few of them because it took a little digging... try a google search, if you don't have any luck, let me know what model you have and I can see if I have it...
The main 'modification' (if it can really be called that) was to reroute the battery connection out of the rear of the unit so that it can be connected to an external battery. For convenience, on some of them I introduced an extra connector. A 30A inline fuse along with a fan helps protect things. They have the ability to run much longer now than they originally did, and as you probably know, the heat sinking on the MOSFETs isn't really designed for extended run. The 24V units already have a fan built in, so all I needed to do was reroute the cabling.
5 (and 7) years is quite a feat from my experiences with these things... Must have got your hands on a good batch or something
For my 12V units, I'm using cheapy U1-3 lawn 'n garden variety FLAs for the batteries. While not exactly ideal for this application (they're cranking batteries), they give me between 2 and 4 years depending on the usage they see, and only hit me for about $25 a piece after core. Much cheaper insurance than replacing them with the real-deal replacement versions from APC, and they have a longer runtime than their internal counterparts if I really need it!
Steve