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Thread: Charging parallel 12V lead acid batteries of different amphours?

  1. #1
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    Question Charging parallel 12V lead acid batteries of different amphours?

    Setting out on my first solar and at all involveld electrical project I bought a 7.2aH batter. To you all this must sound like a joke but to start with this is all I need. However the idea of more capacity and ability to draw more amps keeps crossing my mind. So my question is simple. With my current setup...

    PV panel ---> Charge controller ---> Loads
    __________________ |
    __________________ |
    _____________ 7.2aH Lead acid

    ...could I simply add a larger capacity battery in parallel? The second battery would most likely be in the range of 14-30aH.

    From my knowledge of how electrical circuits work I see now problem with doing so but I can't help but think there is some more fundemental problem with it.

    Any help would be great, thank you

  2. #2
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    Anybody have an answer for this simple question? I've searched the web but can't find an answer since most people seem to buy batteries of the same size but I all ready have a small one and want to upgrade.

    Thank you

  3. #3
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    Mixing batteries that are not from the same lot, is problematic. Because of subtle differences in chemistry and age, mixing new / old/ different will give "less than expected" results in the long term.
    What happens is the best of all the batteries, hogs all the work, till it becomes as aged as the worst one in the batch. Then the age profile of the bank, follows that worst one till death.

    So if you must introduce a new battery, make sure to charge it first, so it's at the same voltage as the existing bank, or you will have one bank rapidly charging & the other discharging. That can sometimes be very exciting !CarBatteryExplosion_7736E0DA-07B1-57ED-A7D81B501A6FB218.jpeg
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    Thanks, I can see how this could be a problem, but my battery is as good as new (had it about a month and only been using it to test loads for short periods of time) However I now calculate that it leaves me with no reserve for a day lacking sun.

    This must seem very small to you but my setup is 70W panel, 7.2aH battery, 15 minutes a day of load at about 80W. I don't know the sun rating for my area but on a 'bad day' I get about 1 amp from the panel so about 15W for 3-4 hours. So I should have enough to top up the 20Wh of used energy even at 50% efficiency. I'm sure you can see the need for a second battery and I was thinking at least double the capacity so about 15aH, giving me 2 days backup.

    A seperate question: when my charger is charging and my load is drawing (through the charge controller), does my battery still get discharged at the full rate or does the panel's power ease the power being drawn directly from the battery? Just wondered as on a good day my panel's power is a significant proportion of what the load needs.

    Thanks again

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    yep, If the sun is shining, and the charger working, you are running 80% efficient, and not cycling the battery. That's why I have my water pump, set to run around solar noon, and it feeds right off the array.

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    Ideal, for once the numbers seem to work in our favour! Love the solar hinged box too

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    I've been thinking of doing the same thing. But the circumstances of each voltage lead acid is not the same so I fear the short-circuit. I do not know whether to add current limiting will solve the problem. Maybe I will watch post here. thank you

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    I'm not quite sure what you mean, are you referring to one of the batteries discharging through another?

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    yes, I think a very small voltage difference though will cause the current flowing from one battery to another battery and I think it might not be a problem if all batteries have the same voltage. The problem is, will each battery always have the same voltage when it is in a circuit under charging or discharging?

    Thank you

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    Quote Originally Posted by ventisca View Post
    yes, I think a very small voltage difference though will cause the current flowing from one battery to another battery and I think it might not be a problem if all batteries have the same voltage. The problem is, will each battery always have the same voltage when it is in a circuit under charging or discharging?

    Thank you
    When you connect two batteries in parallel, then for discharging or charging the voltage at the point where the wires from the two batteries come together must be the same. If the resistance of the wires and the battery connections is equal on both sides, the only way that there can be different voltages at the two batteries's terminals is if there is a different amount of current flowing.
    And the only way that the resting voltage of the two batteries can be different but the voltage when they are connected together can be the same is if current flows between them and the internal resistances of the two batteries use that current to cause a difference in voltage from the resting voltage.

    If you parallel two batteries of the same design and chemistry, with the same state of charge, then they will have the same voltage and no current will flow from battery to battery. If they have different states of charge, then current will flow from one battery to the other discharging one and charging the other. Since charging is an inefficient process, this effectively wastes some of the energy in the higher SOC battery.

    If the two batteries have slightly different chemistry (the worst case), then current will flow until one of the batteries has discharged and the other charged until their voltages are equal and their SOC is different. Also inefficient, and likely to cause loss of capacity in the less charged battery.

    Unfortunately, the effects of unequal charging and discharging will generally end up causing one battery to be overworked and fail prematurely, leaving the other battery to be overloaded and fail next. With proper care this can usually be avoided with two strings of batteries in parallel, but it becomes more difficult as the number of strings in parallel increases.
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