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.