Batteries in series.

tabbycat
tabbycat Solar Expert Posts: 55 ✭✭✭
in Off Grid Solar & Battery Systems #1
I have two 12 volt batteries in series. The voltmeter across both batteries reads 27 volts. When I check each battery one battery reads 14 volts and the other reads 13 volts. Shouldn't they both be the same voltage? Thanks. Steve S.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
    Re: Batteries in series.

    Yes, they most certainly should be the same voltage. Are they the same battery design/model and age? They should be, otherwise the battery of lowest capacity will come up above it's normal fully charged voltage, tricking the controller into "thinking" the battery pack is fully charged, thus leaving the one with larger capacity undercharged and subject to sulfation. Likewise, if one battery has sulfated, or is reaching the end of it's life, it's ability to accept charge will have dropped off, and again the system will react as above. If the batteries have not yet been damaged, and if they are the same design/model etc, you can try to EQ them, assuming they are of the flooded type, and see if this controlled overcharge will bring up the voltage of the one now showing lower voltage. Again, if the flooded type, check the SG of each cell to see if you might have a bad cell or cells. Good luck.
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  • boB
    boB Solar Expert Posts: 1,032 ✭✭✭✭
    #3
    Re: Batteries in series.
    tabbycat wrote: »
    I have two 12 volt batteries in series. The voltmeter across both batteries reads 27 volts. When I check each battery one battery reads 14 volts and the other reads 13 volts. Shouldn't they both be the same voltage? Thanks. Steve S.

    Yeah, this is the reason that there needs to be a decent but inexpensive battery balancer available so they don't start to divert from each other, if one of the batteries aren't in really bad shape that is. I find that once they start to divert from each other, it can just get worse and worse.

    This is also a big reason why a series battery string should not be tapped for some lower voltage load, like 12V from a 24 or 48V string.

    What you might want to try is to temporarily connect those 2 12V batteries in parallel and give them a good charge as a 12V battery.
    That ~may~ help if one battery isn't too far gone. I suspect that should help to get more life out of the pair.

    My 2 cents worth for the day...
    boB
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  • KeithWHare
    KeithWHare Solar Expert Posts: 140 ✭✭✭
    #4
    Re: Batteries in series.
    tabbycat wrote: »
    I have two 12 volt batteries in series. The voltmeter across both batteries reads 27 volts. When I check each battery one battery reads 14 volts and the other reads 13 volts. Shouldn't they both be the same voltage? Thanks. Steve S.

    What is the precision on your voltmeter? If it only does whole volts, you might really have a reading of 13.6 volts that is rounded up to 14 volts, and 13.4 volts that is rounded down to 13 volts.

    Keith
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #5
    Re: Batteries in series.
    KeithWHare wrote: »
    What is the precision on your voltmeter? If it only does whole volts, you might really have a reading of 13.6 volts that is rounded up to 14 volts, and 13.4 volts that is rounded down to 13 volts.

    Keith

    I sure wouldn't be happy with those voltages either.
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  • tabbycat
    tabbycat Solar Expert Posts: 55 ✭✭✭
    #6
    Re: Batteries in series.

    I'm using a digital multimeter to check the battery voltages. The batteries are identical but one is new and the other is two years old. Just as an experiment I will switch the position of the batteries to see if the voltages are battery dependent or location dependent. The older battery is the one with the higher voltage now and is connected to the negative terminal of the controller. Thanks. Steve S. P.S. I once discovered that a jumper cable between the batteries was making a bad connection and causing the voltage drop.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
    Re: Batteries in series.

    it usually is not a good idea to mix batteries of different age, type, capacity, etc. as this sets up differences in the charging. you also did not say if the battery levels or specific gravities are fine either. you might try boB's suggestion and charge as 12v and not in parallel, but individually.
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