Can a messed up battery constant cause premature self-test failure ?

[Anonymous user]

The question really is, what variables does an APC smart-ups processor look at when determining when it is time to replace batteries ? Is the ups able to determine the internal resistance (impedance) of the battery string and monitor for increases in impedance caused by aging ? Or does it only look at the discharging voltage/current when on battery during the self test? Does it use the value for the battery constant in any way ? IE if the batttery constant is messed up could the batteries be flagged bad prematurely during a self-test ?

I have been replacing a lot of batteries lately that are only 3 years old, and doing post-mortem testing on individual cells in the strings with my B & K tester shows the impedance of the battery has not risen that significantly - Eg I just pulled a tray of eight 12V/7.2 AH cells out of an SUA3000 , and each of the cells measured between 0.140 and 0.150 ohms versus 0.100 ohms for a brand new cell.

Are there any actual APC ups engineering staff listening to this forum that can enlighten me / us ?

Thanks,
Dave
Univ of BC
Canada

[BillP]

i dont know the details but the the self-test process compares the battery voltage during a discharge to an internal bad-battery template – basically, it places a load on the battery and measures the voltage drop. If the drop is too large for the given load, then the voltage will be beneath the failure level and the self-test will fail, thus illuminating the replace battery LED.

[upsguy_apc · Accepted solution]

The problem with measuring resistance in an battery is that you have to simultaniously put a load on it while reading this measurement for acurate readings on battery health. Batteries internal impedances are read better read with a meter that can accuratly measure into micro ohms while applying a load(alber cell corder). So from what you just said a new battery has an internal impedance of 100K uohms and a flagged battery has around 140K-150K uohms. Thats a 50% increase in internal impedance. You will not see a very high impedance before the battery can no longer properly hold a charge and supply current during an outage. You would never see 1 or 2 actual ohms on a battery, or should I say a properly working battery.

To increase you life on your batteries I would reduce the self test frequency from 7 days to 60 or 90 days. The less you discharge your batteries the longer they will last. Also watch your server room temperature and where the UPS is placed. A standard rule of thumb for batteries is that there life is reduced by half for every 10 degree's f over 77.