How do you test these batteries?

[Anonymous user]

My UPS's all seem to use the standard 12 volt size that are two batteries in series.  I recently tested my XS 1500 with a MacPro on it. The second I unplugged the UPS the computer went down/off.  So I go to replace the batteries.

The pair of new batteries tested at 12.6 volts each, but the old batteries that wouldn't power up even a second with a disconnect, both measured 13.5 volts.  This is crazy to my way of thinking. How can they have more voltage than the new ones and still not work?  If it's the current we're concerned about and not voltage, how do you measure that? It would have to be under a load wouldn't it?  How do you test to make sure it's the battery and not just a UPS gone bad?  This has always eluded my logic between the batteries  being good or bad and the UPS being good or bad.  There must be some easy way to test them. Can you suggest a good method?

[BillP]

connect bulb 60w or 120w in parallel with battery and test, if voltage low quickly , the battery is bad

[BillP · Accepted solution]

Yes it does seem counter intuitive.  It has to do with the internal resistance of the cells and connections within the battery.  An old and worn out battery will have high internal resistance so that even a relatively low external load will cause a considerable amount of voltage to be dropped across the battery itself and the load will get little to no current.  With a load connected, the bad battery's voltage will drop from the 13.5 you measured.  The good battery, the one measuring 12.5 volts but with low internal resistance, will allow much more voltage to be provided to the load and, therefore, higher current to flow.  The new battery, when charged properly to float level, will eventually reach the 13.5 or 13.6 you measured across the bad battery.  Hope this helps.