I have a deep discharge small lead-acid battery bank comprising only 2 batteries in series, whose terminal voltage reads 26.5V. My past method of determining the need to change batteries is based on it's terminal voltage and overall installation age. This particular bank is about 2yr 10months old (so still should have a 1.5-2yrs of service life, from past experience), and terminal voltage seems in healthy range too. This is not a SMF type, but has had been regularly been topped-up with distilled water.I know that terminal voltage is easy to test but not the best.Do I need to look at things like source impedence or load capacity?

The suspicion on battery is being raised by the UPS service technician. The UPS (APC make), had the fault indication when I called him. He checked UPS, and is asking me to change batteries. I'd prefer to be sure that it is indeed the batteries, since he's done couple of flip-flops with his diagnosis.

Edit (Feb 11, 2013) Found some excellent reading material here, although it is clear that understanding the health of lead-acid battery is not a simple matter of testing only terminal voltage. Low terminal voltage, after what might be a long duration charge, can indicate a poor health of battery, but not much more.

  1. Battery university
  2. Someone's thesis
  • \$\begingroup\$ What was the fault indicated by the UPS? Does the UPS hold a load? If so, for how long? \$\endgroup\$
    – HikeOnPast
    Commented Feb 9, 2013 at 4:42
  • \$\begingroup\$ In some short power outages of about 30 mins, it is able to hold out a combined load of about 100-120Watts, without issues. Lucky for me, there have been no long outages of late. \$\endgroup\$
    – bdutta74
    Commented Feb 9, 2013 at 4:46
  • 3
    \$\begingroup\$ It may be an applicable time to simulate a power outage and see straight on if the batteries can supply the hold up time that you need. If not then you know that it's time to replace them. Nothing beats guessing better than a real test. \$\endgroup\$ Commented Feb 9, 2013 at 14:10
  • \$\begingroup\$ Thanks @MichaelKaras. Sometimes simplest of solutions escape ones mind! Indeed, I can simulate and see how well the batteries are holding, i.e. for how long. \$\endgroup\$
    – bdutta74
    Commented Feb 9, 2013 at 16:48
  • \$\begingroup\$ Folks voting to close this question, kindly leave a comment explaining why the vote ! The comment helps avoid asking similar questions in future. \$\endgroup\$
    – bdutta74
    Commented Feb 9, 2013 at 16:51

2 Answers 2


The best way is to periodically (eg every few months or so) do a capacity test by running the UPS (from fully charged, with power off) with a known heavy load and recording the time it takes to drop out on low Volts.

The important thing is that you can graph the results over time (eg years) and note how it is deteriorating. This will give you notice well in advance of any failure due to low capacity.

If you can't do a full capacity test, simply measure the instantaneous voltage drop with a known large load, and again graph the results.

Terminal voltage by itself is not at all useful.


There is no one way to determine the remaining capacity of your battery other then testing it as described above. I would go about 115% of your worse case load for 115% of your required backup time, This will give you a bit of safety room.

Try this link for a lot of good information: http://www.power-thru.com/documents/The%20Truth%20About%20Batteries%20-%20POWERTHRU%20White%20Paper.pdf

There are no shortages of battery testers, but a closer look reveals that most lack accuracy. Currently capacity, the leading indicator of battery state which is difficult to obtain on the fly.

Stating that a battery tester measuring internal resistance will also provide capacity estimation is misleading. Resistance-based instruments can identify a dying or dead battery then again so can the user.


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