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Why is My 12 Volt Battery Reading as 13 Volts?

Inside diagram of Lead acid battery

All Lead acid batteries (Gel, AGM, Flooded, Drycell, etc) are made up of a series of 2.2 volt cells that are bridged together in series to reach their final desired voltage. For instance, a 6 volt battery will have 3 cells (3 x2.2= 6.6 volts), a 12 volt battery will have 6 cells (6 x2.2=13.2 volts) and so on.That 2.2 volts is the fully charged, straight off the charger number. The actual resting voltage, or the voltage a battery will settle at 12-24 hours after being removed from the charger, is closer to 2.1 volts per cell, or about 6.4 volts for a 6v battery, and 12.7 volts for a 12v battery. These numbers assume 100% healthy cells, and may vary a bit lower for older batteries.


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  • Excellent article. Thank you. The motorcraft XBT66-650 has a date code of Dec. 2007. This is by far the longest lasting battery we’ve ever had. The day following a cold snap it wouldn’t start. I charged it & its lasted another 2 mths. Then again last nite, it wouldn’t start the car. It read 11.86. I charged it, it now reads 12.54. And starts fine. The prorated warranty is for 100 mths. I’m surprised it lasted 84 mths. I will be getting another one. The cenntenials lasted 4 1/2 years, like clockwork. The Bosch batteries lasted 2 1/2 years like clockwork. For $20 more I will stick with the motorcraft

    Dd
    November 23, 2014 a 12:33 pm
    • I have had good luck with Motorcraft. The one in my old 91 Sable lasted 7 or 8 years. The ones in a 99 diesel Ford have been there since I got it. And that was in 2008, 8 years ago. Had a few close calls where I thought they would be changed… so far so good. I think the date code is 2004 or 05.

      JJ
      June 11, 2016 a 12:18 pm
    • I have 6 8volt batteries that are dead. My charger won‘t charge them because they are below 20 %. and they are 5 years old. Can they be saved?

      Tim
      January 21, 2013 a 11:22 am
      • If the 8 volt charger won‘t do it, try using a 6 volt charger to bring them up to the point where the 8 volt charger can ‘read them’ and turn on.

        BatteryStuff Tech
        January 21, 2013 a 12:20 pm
      • Very helpful information and well-written for a non-technical college grad. Thank you

        Joan
        April 23, 2012 a 9:02 am
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