Knowledge building question about AGM lead acid battery voltages.

Discussion in 'General Electronics Chat' started by BrainFog, Apr 1, 2011.

  1. BrainFog

    Thread Starter Member

    Jan 24, 2011
    Recently A family member gave me 4 Yuasa 12 volt 7ah batteries as they would be useful to make an extra battery pack for a hub motor. He told me that they are new and unused, they still have the plastic caps over the connectors and were still in what looks like the box they were delivered in. However I do not know how long they were sitting around for.

    I put them on a float charger and they all reached the float voltage within a few minutes, then I disconnected them. After leaving them for a day I measured the voltages and was surprised by the results, which were: 12.69v 13.09v, 13.09v and 13.14v. Leading to my question which google has not been able to answer thanks to overwhelming links about recharging them. To my knowledge they should produce about 12.7v, what is causing three of them to produce a much higher voltage?
  2. SgtWookie


    Jul 17, 2007
    Did you check the temperature of the positive terminal? It could be that the lower voltage one got warmer than the other three during charging, or was in a warmer part of the box.

    Try leaving them on float charge for at least several hours. Try using the desulfator on them; if they've been sitting around in a partially charged state they may have started sulfation.
  3. nickelflipper

    Active Member

    Jun 2, 2010
    Lucky You! I think as a general rule the AGM batteries finish off at a higher voltage then flooded lead acid. My Deka group-31 AGM finishes off at 12.9V.

    It really helps to track down the data sheet from Yusa, on how they are to be charged and maintained. If my memory serves correctly, Deka doesn't recommend a desulphating charge for their AGM. Other manuf. may have a different slant. AGM batteries have a really low self discharge, after several months of sitting, the Deka will only lose a few tenths of a volt.
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    New batteries don't have full capacity yet, and the best way to condition them is to discharge and recharge them two or three times. Until they are 'broken in' don't put much faith in any readings you are getting.

    Use an automotive brake light bulb or just a power resistor and let them discharge to a reading of 12.2 or 12.3 volts, then recharge them. Repeat this two or three more times, but don't go below 12.3 volts. After the last recharge, disconnect them and let them sit idle and undisturbed for at least 24 hours. The voltage reading taken at that time should be an accurate level for the type of battery.

    And yes the voltage on AGM types will typically be a little higher than flooded cell types. Expect to see 12.8 to 13.0 volts
  5. #12


    Nov 30, 2010
    I just did this a month ago. My SLA was at 13.1 volts a week after I unplugged the charger, so, they do tend to hold a higher voltage than a car battery. It's the one at 12.69 volts that I would suspect, just because it's different from the others.