I love hooking up large lead acid batteries to old UPSes, there's no reason to be limited by the original battery capacity.
However...I discovered why some APC UPSes are sealed and the battery cannot be accessed. I was converting one such UPS which had no monitor interface. Having already opened it up to splice in an external battery, I noticed an empty header for a serial port. Cool. Plugged that into my PC and flash/bang/smoke. Fortunately my PC had a good path to ground and was not damaged. But the UPS had some pretty large burn marks at various points in the PCB and was certainly dead.
It took me a bit to figure out what I did wrong. The reason it went bang is directly related to why there's no user access to the battery, and why there's no external monitoring port. They went cheap on the electronics and did not include an isolation transformer. Any points within the UPS which are "ground", including the battery negative terminal, are actually referenced to the full bridge rectifier, which provides a direct path to live. Ahh...I'm just very glad I didn't touch any of the battery terminals while I had the thing plugged in.
However...I discovered why some APC UPSes are sealed and the battery cannot be accessed. I was converting one such UPS which had no monitor interface. Having already opened it up to splice in an external battery, I noticed an empty header for a serial port. Cool. Plugged that into my PC and flash/bang/smoke. Fortunately my PC had a good path to ground and was not damaged. But the UPS had some pretty large burn marks at various points in the PCB and was certainly dead.
It took me a bit to figure out what I did wrong. The reason it went bang is directly related to why there's no user access to the battery, and why there's no external monitoring port. They went cheap on the electronics and did not include an isolation transformer. Any points within the UPS which are "ground", including the battery negative terminal, are actually referenced to the full bridge rectifier, which provides a direct path to live. Ahh...I'm just very glad I didn't touch any of the battery terminals while I had the thing plugged in.