What is the normal resting voltage of a fully charged AGM 12V battery?

Matty
Matty Registered Users Posts: 36 ✭✭
in Solar Beginners Corner #1
I have a number of 12V AGM batteries that I have fully charged and the resting voltage (measured several days after charging was completed with no discharge) ranged from 12.96 to 13.3V. I used a 9 stage 15A smart charger to charge the batteries. The batteries are all less than 18 months old, seldom discharged and not to less than 12V. Is this variation normal? Is 13.3V on the high side?
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  • Estragon
    Estragon Registered Users Posts: 4,503 ✭✭✭✭✭
    #2
    The resting voltage of a fully charged lead acid battery is generally ~12.7 12.8v. 

    With those readings after several days resting, I'd suspect measurement issues, but you might try applying a small load for a short time.  If you're reading a residual "surface charge", you should see the voltage drop fairly quickly to ~12.8, the stay there for some time.
    Off-grid.  
    Main system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited May 2019 #3
    It depends on the battery.  For example, some Optima AGMs rest at up to 13.2V.

    In general, it is a bad idea to use any generic voltages when dealing with lead acid batteries.  

    I am available for custom hardware/firmware development

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  • Estragon
    Estragon Registered Users Posts: 4,503 ✭✭✭✭✭
    #4
    True, should always check the specific battery maker for voltages.  Regardless, the range of resting voltages for fully charged batteries suggests to me there may be a measurement issue. 

    Are the batteries charged individually, or a number in parallel at the same time (and if so, how many at once)?
    Off-grid.  
    Main system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #5
    You should look up what the Manufacturer has for self depletion (degradation?) and resting voltage and for how long....  All AGMs may seem similar and yet can be very different depending on the storage conditions they are living in...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • Matty
    Matty Registered Users Posts: 36 ✭✭
    #6
    Estragon said:
    True, should always check the specific battery maker for voltages.  Regardless, the range of resting voltages for fully charged batteries suggests to me there may be a measurement issue. 

    Are the batteries charged individually, or a number in parallel at the same time (and if so, how many at once)?
    I have 6 x 130 Ah batteries. I had 2 x 3 of these batteries wired in parallel on charge rotation when used recently on a camping trip. At the end of the trip, I charged each battery individually before putting in parallel in 3 pairs for periodic maintenance charging. I have tested the voltage of each of the 3 banks and the voltages are 13.11, 13.19 and 13.2V. I disconnected the parallel cables and the voltage of each individual battery is identical to that of the bank it belongs to so it appears that the voltage has equalised across each battery in the bank. I have another 60Ah battery fully charged of the same brand which has a resting voltage of 12.96V and another 120Ah battery of a different brand which has a resting voltage of 13.3V. These 2 smaller batteries have not been wired in parallel to any other battery.   
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  • Estragon
    Estragon Registered Users Posts: 4,503 ✭✭✭✭✭
    #7
    I'd look up spec resting voltage for the various batteries.  The 3 pairs are close enough IMHO.  I don't know if the same brand necessarily means same resting voltage.
    Off-grid.  
    Main system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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