Experiments In Nothing: UPS Failure!

I have a 1400 watt rack mounted UPS that powers my computers. The computers and UPS are in a rack. Yesterday I discovered all my computers rebooted in the early morning. The UPS appeared to be running fine by way of the indicator lights on the front. I gave it the big test by pulling the power cord from the wall. The entire rack went dark. The UPS was running in bypass apparently. I brought the UPS back on line and now I had a red malfunction light on the front panel. This is a refurbished UPS which is one of two that I have. They are about a year old. I swapped the failed unit for the other one which is only running one PC downstairs. I am now in process of checking the batteries. I have noticed the battery charge status leds have been flashing. I have a technical bulletin about that indication. The document talks about a calibration process that may remove that warning. Before I do that I need to test the batteries to make sure they are still in spec. There are four batteries in the UPS. They are 12 volt Power Sonic 7.2AH batteries. They are connected in series and parallel providing 24 volts at 14 Ah. I have about 650 watts on this UPS so that only gives me about 10 to 15 minutes run time before they crap out.

I test the batteries using a computerized test fixture. This is a device by West Mountain Radio. Plugs into the USB port on the PC and by running the provided software you can do a load test and see if the battery meets specification. You can see the resulting chart after the test on the first battery. You set the load current as indicated by the specification. Most batteries of this type are tested over a 10 or 20 hour rate. Power Sonic lists the spec for both 10 and 20 hours so I used the 10 hour rate. For a 7.2 AH battery that is 0.700 Amps. They also specify the "dead" voltage which is typically 10.5 volts for a 12 volt battery at this load current. As you can see from the chart I got 8 Ah out of the battery by the time it hit 10.5 volts. So battery 1 is good. This battery goes on the charger and I connect up the second. At ten hours a test this will take a little time.