balancing two sla batteries

Discussion in 'General Electronics Chat' started by bug13, Jan 22, 2014.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I just brought two 12V sla batteries for my project, and I am going to use them in series to power my 24V motors.

    So how can I balance my two batteries if i want to charge them in series with a 24V 1.6A charger?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    I cannot think of a way to charge two SLAB in series and maintain a balance.
    I try to charge them separately where possible. When the batteries are new there should not be a problem charging them in series.
     
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  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ideally, you should only series batteries with equal capacity. If you bought two used batteries; good luck. If they are new, from the same manufacturer, then it will work.

    When placing them in service, initially, it is best to fully charge them individually using a 12V charger.
     
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  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    My batteries are new and they are the same, but they read 200-300mV different when in series, should it be worried?

    Or better off to charge them in parallel or separately?
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    There are several ways to do it, from simple to not so simple. Simple might be a 13 volt 1.3 watt zener (BZX85B13) and say about a 47 ohm 1/2 watt resistor. One across each battery so that the zener turns on when the battery voltage reaches 13 volts and steals some current away from the battery that reaches that voltage first. I've seen this done with a little lamp like a number 48 which gives a nice visual effect.:cool: The zener turns off as the battery returns to it's resting voltage of 12.12 volts. The more complex I built for my golf cart was a switched capacitor type. The circuit placed a large cap across 1 battery then moved it to the next. The result is an extra charge to the lowest battery. Depending on your charger you may have to "play" with the resistor so the charger still turns off right.
     
    Last edited: Jan 22, 2014
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  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Isn't each 12V battery just 6 2V cells in series? What is the difference in charging 6 cells in series and charging 12 cells in series other than the charger voltage? That assumes, as stated earlier, they are the same (age, capacity, manufacturer)

    How would you balance the individual cells within each battery? Large chargers have an "equalize" feature. Check this out....
    http://batteryuniversity.com/learn/article/equalizing_charge
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    As stated before, charge them fully with a 12V charger. After that you can connect them in series and charge with a 24V charger. Monitor the individual battery voltages periodically and check for imbalances.
     
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  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You need a smart charger that monitors each battery independently and turns on a shunt around the one that is fully charged while the second one finishes charging.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It's a known fact that the six cells are all the same age, same design, same capacity, etc so series charging them is done..... but as the battery ages, the difference in the cells does cause problems. With two different batteries, you can't assume they will track.
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Ok, it's actually more complicated than I thought, I might charge them in parallel to 13.8V, could that be a better option?

    I don't have those 13.8V zener diode, and i surely don't want to invest on a smart charger.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It is not complicated, listen to MikeML. I've used all kinds of series combinations, [ 6V+6V+2V ] X 2; 2-6 V 200Ahr in motor home, 2- 12V in 24V mower.
     
  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I have charged my batteries over night, they are just 6AH, they are new and same model.

    But if I put them in series, I still read 100-200mV different, so it's all right to have 100-200mV different?

    Or just something wrong with my new batteries?
     
  13. inwo

    Well-Known Member

    Nov 7, 2013
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    It'll be fine!

    Ive tried equalizing batteries before putting them in series.

    It doesn't have to be perfect.

    If it did, a 12 volt battery would soon get out of whack.

    Remember that its 6-2 volt cells in series.

    If one gets a little bit ahead on a charge cycle the other one will catch up after a little more charging.

    It's never perfect. Just get them as close as you can.
     
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