Can I trust a UPS self-test failure?

[Anonymous user]

I would greatly appreciate answers to two technical questions.  I have an old APC Back-UPS Pro BP650PNP and the battery in question was installed brand new in the BP650 only about 18 months ago:

(1)  What, exactly, does the self-test do?  Simply check the battery output voltage?  Open-circuit, under normal load, or both?  If so, what values are required to pass?

(2)  Does the following make sense:  The RBC4  12v  12AH battery failed the self test.  Immediately after removing the battery from the UPS, the open-ckt voltage was 13.12 v.  The next day, the open-ckt voltage was 12.88 v.  Immediately after attaching a 16-ohm load (wire-wound, ceramic-coated resistor, cold), the voltage was 12.44 v.  An hour later (resistor now hot, 16.6 ohms), voltage was 12.53 v under load and  12.74 v open-ckt.  The next day the open-ckt voltage was 12.80 v.

Can this battery really be bad?  Or did the BP650 give me bad info?  In addition to two failures of regular scheduled tests, I also initiated several "manual" tests with the same result: "Battery needs replacing".

[BillP · Accepted solution]

Hi cslemaker,

The unit's selftest is meant to qualify the rate of discharge of the internal battery in direct relation to the percentage of attached load during testing. When discharging, the battery voltage must remain within the designated Load percentage algorithm pre-set in to the unit's EEPROM, to pass the selftest. The battery voltages you have provided however, all appear to be far less than expected for this unit. Instead of the open-ckt voltage of 13.12vdc which is being observed, we should be seeing between 13.6 - 14vdc for proper charging of the internal battery. This BP650PNP product is likely 10+ yrs old, as this model has been discontinued for sometime. I believe what we're seeing here is a gradual failure of the rectifier/inverter circuit, which has become unable to apply the correct charging current to the internal battery.

[Anonymous user]

Hi Secret Squirrel,

Thank you very much for your reply.  Incidentally, I'm new to this forum and am not yet familiar with its protocols.  I marked your response "helpful" only because I'm not qualified to say if the answer is "correct" or not.  I do have one tid-bit of information to add, however.  I have installed a new battery and it passes the self-test OK so far.  I did check its open-ckt voltage before installation but, unfortunately, I didn't write down the value.  It was somewhat low, though, as I remember.  That this new battery tests OK whereas the old one did not suggests that there IS some difference between the batteries.  Might the old battery somehow have been degraded by a year of improper charge maintenance by the UPS?  Incidentally, my BP650 was purchased Aug 29, 1997, so it will be 16 this month!  Thanks again.