14.8V Li-ion Battery Pack- Mtbr.com
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  1. #1
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    14.8V Li-ion Battery Pack

    I was building a battery pack for my halogen light.

    It will be 14.8V at 4800mAh.
    I have 8 Li-ion 18650 cells.
    3.7V each @ 2400mAh

    Depending on how I solider them together, it seems to affect the runtime dramatically.

    If I hook two cells up in parallel, do that for all the cells and hook the 4 packs in series, I get about 40 mins runtime.

    If I hook 4 cells in series, then another 4 in series, put them together parallel, I can 2 hours 30 mins.

    That is a huge difference. Can anyone explain why?
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    Last edited by alanchan; 06-25-2009 at 01:26 PM.

  2. #2
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    If my understanding is correct, and it may not be:

    I believe your first diagram is 4p2s wired. This will yield only 7.4v with 8800mah pack.
    Your second diagram is effectivley 2p4s wired. This will yield your desired 14.8v and 4400mah pack.

    Parallel wiring will multiply the mah of the cells and leave voltage unchanged. While series wiring will multiply the voltage and leave mah unchanged.

    The simplest is to look at two cells in series vs parallel.

    using two 3.7volt 2200mah cells:
    series wiring yields 7.4v and 2200mah
    parallel wiring yields 3.7v and 4400mah

  3. #3
    Austin, TX
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    Alanchan,

    I'm curious, what are the voltage and wattage ratings for your halogen bulb?

  4. #4
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    sorry, my diagram was incorrect, it has now been fixed. Thank you 'NEstinkyrider'.

    I did get 16v approx with both hookup. So they were ok, its just that diagram 1 hookup have the voltage dropping very fast.

    it was a 25W rated bulb. I measured the correct and it was around 2 amps.

  5. #5
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    My guess that problem is not in the configuration but somwhere in the connection/contact. 4S2P and 2P4S should have the same power capacity.

  6. #6
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    the 2P4S voltage drops really fast. From around 15.5v to 12v in 30 mins. Cutoff of the package should be at 2.7v/2.8v per cell. So I figure the protection is correct.

    the 4S2P voltage drops from 15.5v to 12v in 2:15 with the same load.

    I thought they will be the same too, 4S2P vs. 2P4S should make no differences in the capacity. But somehow gives different run time.

  7. #7
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    Are your cells protected?

    Nevermind, I just have read they are. I have an idea how it could make a difference, but I need to think more.

  8. #8
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    Cells are protected. But voltage do drop to desire voltage before cutoff. The thing is it drops a lot faster with 2P4S.

    Removing protection will help. But in theory, both are the same.

  9. #9
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    Check each cell, I have a hunch that you have one not too good cell and diagram 1 may highlight it.

    diagram 2 may mask it.

    Strange though as I would have expected the same results either way.

    For what it's worth diagram 2 is the usaul way of doing things.
    Last edited by yetibetty; 06-25-2009 at 02:53 PM.

  10. #10
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    is the second diagram correct??
    it seems you have the load between 2 +ve .....
    ...Scun.thorpe, UK

  11. #11
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    'HEY HEY ITS HENDO' ... I just fixed the diagram.

    First diagram is how I would like it to be done (4 series + 4 series) and put into 2 parallel. It seems to run a lot better.

    (2 parallel x 4) and hook them up in series runs a lot shorter (diagram 2)

  12. #12
    aka RossC
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    Quote Originally Posted by yetibetty
    Check each cell, I have a hunch that you have one not too good cell and diagram 1 may highlight it.

    diagram 2 may mask it.

    .
    +1. I don't think the configuration should really have much of an effect unless "one of these things is not like the other...."

    Now to figure out which one it is.

  13. #13
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    I haven't check each cell but I swap the config by desolidering it and resolidering and it gave me way longer runtime on the SAME CELLS!

    Basically changed from diagram 2 to 1.

  14. #14
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    As you have edited you diagram I will have to edit my answer and say number 2 is the usual way.

  15. #15
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    I did edit my diagram as it was a bit hard to understand. I posted the runtime along with it now. I thought #2 is the usual way but there is really no standard to it. When I first sat down to do the soldiering work, I just want to make 14.8V so I build diagram one. When I did my second pack, I did diagram two. Did a run test and found major difference. So I change it all diagram one again and it runs a lot longer.

  16. #16
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    Quote Originally Posted by alanchan
    I did edit my diagram as it was a bit hard to understand. I posted the runtime along with it now. I thought #2 is the usual way but there is really no standard to it. When I first sat down to do the soldiering work, I just want to make 14.8V so I build diagram one. When I did my second pack, I did diagram two. Did a run test and found major difference. So I change it all diagram one again and it runs a lot longer.
    When you have two cells wired in parallel and one is bad the good one will try to charge it wasting it's power. this won't happen when wired the other way you have done it.
    That's why I think you could have a bad cell or maybe a faulty pcb.

    hope the diagram helps the red X in this example is the bad cell.
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  17. #17
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    do the cells get hotter in the 40 min run.

    If the voltage of each cell was identical then the 2 circuits would behave the same. If the cells aren't matched then as yetti says the unmatched pairs will drain the higher voltage cell. This will generate some heat.

    When all 4 cells are in parallel you will get this same effect but if the average voltage across the two 4 cell strings is close it will be allot less. You may want to measure the voltage of each cell under load, then try to match up your cells so that the total voltage of the upper string and lower string are as close to the same as you can get them.
    TEX

  18. #18
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    I understand if I have one dead cell or bad cell, it will be a resistance to the entire circuit....no matter what layout I use.

    Are you saying layout 2 (having two cells in parallel first then series them) will create a larger effect if there is a dead cell? Where diagram one won't??


    I was not aware of any heat from any cells while discharging. Discharge rate is about 1.9 amps read from my Fluke meter. So runtime should be around 2:30 as its a 4800mAh pack. That runtime is achieve with diagram 1 wiring.

  19. #19
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    I'm not sure what the implications of a "dead cell" are. If the cell goes open then I'm not sure which is better. On the top circuit you would be left with one strand of 4 cells. The bottom one you would have 7 cells but all the current would need to flow through that one cell matched with the open cell, so the results may be similar.

    If the dead cell is a short then in either circuit you won't get your full voltage.

    The effect I am more concerned with isn't one cell being dead, but one or more cells having a lower voltage then the other cells, in which case diagram one should help. Again if you can match the voltage of your 2 strings of cells it will help even more.

    Quote Originally Posted by alanchan
    Are you saying layout 2 (having two cells in parallel first then series them) will create a larger effect if there is a dead cell? Where diagram one won't??
    .
    TEX

  20. #20
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    I think you are right Tex. As the battery pack is being charge together, it might not be perfectly balance. In another words, the charger will stop charging at a certain voltage but not all cells have the same voltage.

    In that case, diagram is better as you mention because I still have another series of 14.8V and can use up that entire 2400mAh at least. But wouldn't the other series (with one lower voltage cell) be a resistance to it? Or not?

    Where diagram two, if I have one (lower voltage cell), it will cause the two cells in parallel to be low as well because those two will try to balance each other out. So I might end off with 3.7+3.7+3+3.7 And since there is only one series in the end, overall voltage will drop in results to cutoff.

    Please let me know if I am correct.

    When I mention dead cells, I mean lower voltage cells which will be high resistance and provides no power. So more like an open.

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