Hi I want to connect to regular APC CS500 UPS a 40 Ah battery to reach 3 hours of backup time on 150W load(13A on battery). But I have found, that small UPS-s are switching off after 15 minutes, regardless of battery size. I have one APC and one GE UPS and both behavior is similar. What makes matter worst is, that all is controlled by special APC chip, which I cant modify. Only thing I noticed, that on freerun(0.5A on battery) it works longer. Does someone know, how UPS measures its load, and how to simulate UPS, that there is no load. Thanks T.