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Batteries in parallel for higher mAh

Discussion in 'Power Electronics' started by dbyrd26, Nov 13, 2014.

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  1. dbyrd26

    dbyrd26

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    Sep 16, 2013
    I know that when you connect 2 identical batteries in parallel you get the same voltage but twice the capacity. So, my questions are 1. What happens if you connect 2 batteries that are not the same voltage? And 2. Should one use a diode when connecting batteries in parallel?
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    If your batteries are not at the same voltage, then the higher voltage battery will discharge into the lower voltage battery.
    If they are rechargeable batteries, then this will balance itself out, just so long as the difference is not so great that one damages the other. It's always a good idea to make sure they are at the same level prior to connecting them.

    A diode will prevent one from feeding into the other, but has a draw-back. It will not allow the batteries to be charged in-circuit, and they cause a voltage drop... which could result in up to 0.7V to be lost, and if the battery voltage is low to begin with, this will cripple your power supply. (ie... two AAs in parallel, with a diode on each will only give you about 0.8-0.9V even though the batteries are 1.5V)

    And, when connecting them in parallel, the 'Current' is additive, the voltage stays the same, so you would get the sum of both batteries mAh, but you would also have a new 'battery' that is capable to putting out more Amps at a time. This is why some Diesel trucks use 2 batteries in parallel, so they can push well over 100A into the starter to get the engine moving.

    Oh... if you do plan to use two different batteries, that have a different voltage, then a diode will, like stated above protect one from back-feeding into the other. This will essentially allow the lower voltage battery to be a 'back-up' for when the higher voltage battery finally dies enough that it's voltage drops down.
     
  3. dbyrd26

    dbyrd26

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    Sep 16, 2013
    Ok. This is very helpful thank you. I have 2 lipo batteries, 1 is 7.2 volts and the other is 3.7 would this cause issues for me?
     
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    Yes.
    The 3.7V lipo would not be used until the 7.2V was below half it's voltage, which would not be very good at all for the 7.2V battery.
     
  5. dbyrd26

    dbyrd26

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    Sep 16, 2013
    Got ya. Thank you!
     
  6. BobK

    BobK

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    If you really need to add the capacity of these two batteries together, you would need to put them in series and then use a DC to DC converter to bring the voltage down the desired level.

    Bob
     
    KrisBlueNZ likes this.
  7. dbyrd26

    dbyrd26

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    Sep 16, 2013
    I didn't think that capacity increased in series
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    That's right .... so you would just need a higher capacity battery

    its a really bad idea to parallel uneven voltages, it can cause the lower voltage battery to cook and maybe burst into flames
     
    Arouse1973 likes this.
  9. Anon_LG

    Anon_LG

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    Jun 24, 2014
    Agreed, Lipo batteries are EXTREMELY dangerous if misused, I could tell you a story (true story) but it might put you off.
     
  10. dbyrd26

    dbyrd26

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    Sep 16, 2013
    Well, I'm not going to connect these in parallel haha. I've decided against it, and I have seen some pretty crazy stuff happen with lipo batteries. I'm working on a project right now and am trying to make a decision on what batteries to use... I have 2 8650 cells but they were quite a bit larger than I expected. I also have a single flat lipo that I'm thinking about trying. But the reason I asked about these 2 is because I happened to come across them at work and thought the size would be good.
     
  11. Kiwi

    Kiwi

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    Jan 28, 2013
    "This is why some Diesel trucks use 2 batteries in parallel, so they can push well over 100A into the starter to get the engine moving."

    You are a bit light on the batteries fitted to some large American diesel powered 12v trucks. They can have up to four 1000CCA batteries fitted in parallel, giving a possible 4000A.
     
    Gryd3 likes this.
  12. Gryd3

    Gryd3

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    Jun 25, 2014
    With Bob's trick it does...
    The DC-DC buck converter will output the same power that is put into it.

    So a 7.2V lipo rated at 2400mAh, can output 3.7V at a little under 4800mAh.
    Power = Voltage * Current

    (Unless I fubar'd something here.)

    You will need keep on eye on the voltage level of the batteries independently if you simply hook them up in series and go. The lower capacity battery will die first.
     
  13. BobK

    BobK

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    Jan 5, 2010
    It does if you then reduce the voltage via a DC to DC converter.

    Say you put 3 nominally 3.7V LiIon batteries with 1000mAH capacity in series.

    If you then run them through a buck converter and get 3.7V at 1000mA, the batteries will last 3 hours because they will only be draining (at a rate of 1A) for 1/3 of the time. This is how a buck converter works. It switches the power on and off to keep the current flowing through an inductor. If you reduce the voltage to 1/3 they only need to be on 1/3 of the time in order to keep up that current. Actually it is a little more than that. Typically they are only 80 to 90% efficient.

    Bob
     
  14. Kiwi

    Kiwi

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    Jan 28, 2013
    Bob, instead of using three batteries in series with a buck converter, would it not be better to use three identical batteries in parallel?
     
  15. Gryd3

    Gryd3

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    Jun 25, 2014
    It would be yes,
    But Bob's method above was a 'last-resort' for using the two specific batteries the OP currently had which were rated at 7.2v and 3.6V.
     
  16. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes absolutely correct, DO NOT DO THIS.
    Adam
     
  17. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I could also tell you I have had one explode behind me when some one connected a 7.2V battery to a 3.6V battery. Ripped a hole in his shirt. They were non rechargeable which I know is different but goes to show how dangerous they are.
    Adam
     
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