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Lipo Charging

  1. Nov 28, 2016 #1
    I have a ton of 3.7 volt lipo's at my house, each of which can be charged @ 5C, and since each have a capacity of 270 mAH, than I should easily be able to charge them @ 1amp. I put 8 in parralel with eachother so it is a 1s, 8p battery pack. each of the lipos is the same (in tems of specs, ik every lipo is slightly different).

    There are two main ways to charge a lipo, thats cccv (constant current then constant voltage) and CV (just constant voltage. I am trying to charge them with a constant voltage @ 8 amps. I have a battery protection module that is rated for 10 amps, so that the voltage does not reach higher than 4.25v as a saftey measure.

    When ever I hook my battery pack up to a constant voltage, or for that matter any batter that can charge at a high C rating (ex. 10c), the current draw is at most .5 C max. I was wondering how do I charge my lipos with a digital power supply @ a constant voltage @5C of there capacity (the batterys can again handle it).

    Thank you So So Much!
     
  2. jcsd
  3. Nov 28, 2016 #2

    rbelli1

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    Gold Member

    Are these bare cells or protected cells? Bare cells can NOT be charged constant voltage. You WILL blow something up. Protected cells may or may not be able to be charged at a constant voltage. You will need to consult the data sheet.

    Protected cells will include a PTC thermal over-current device. These have a minimum sustaining current that could believably be 135mA. That seems a bit high but not completely unreasonable.

    Is 5C charging the maximum rate or the recommended rate? If possible charge at the lowest rate as close to the recommended rate as possible. This is often 0.5C. A variable rate charge cycle would be good as you can charge at 5C when needed and at a lower rate when the need is less pressing. They will have a longer life and be less explody.

    BoB
     
  4. Nov 28, 2016 #3
    Thanks for the response man, I hooked them up to a protection circuit (they were bare way back when) which the protection circuit is only a over and under voltage controller, so it will cut off the current once the charge in the batterys reaches a certain point. 5C was the max amount recommended on the data sheet, and I do know that charging it at a lower c such as .5 c will help them last longer, but I cannot wait that amount of time, and are willing to pay any price. As for the cut off circuit, its cut off current is 10 amps, but even if I hook them up without it for just a few seconds, the amperage is still less than 1/2c, I dont understand why. I would think that they should be pulling 1 amp each when hooked up to 4.2 volts, is that a incorrect assumption?
     
  5. Nov 28, 2016 #4

    rbelli1

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    What are the characteristics of this supply? If it is regulating to 10A then you indeed have a CCCV arrangement and you should get a measured 10A. If it is just sensing >10A and cutting out you may have a brief much larger than 10A pulse and then a very small current for a period of time and repeat. This cycling may be from the supply, the battery protection, or both. Do you have an oscilloscope handy?

    BoB
     
  6. Nov 29, 2016 #5
    It is a 12 v power supply connected to a buck converter that produces 4.2 volts dc. It can handle up to 20 amps. I hook it up directly to the lipo and measure the current and it is always .5c no matter what
     
  7. Nov 30, 2016 #6

    NascentOxygen

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    I think you should not regard this as a 1s 8p pack. Unless they have been properly matched, one or two cells can be expected to hog the current during charge.

    When you say it is charging at only 0.5C what value are you using as C? I don't think you should regard C_of_the_assembly = 8×C_of_the_individual_cells because the cells are not matched.
    Is this one protection circuit for the pack, or 8 protection circuits so each cell has its own? Because they have not been matched, each cell needs its own protection board otherwise it is not protected against excessive current.
     
  8. Nov 30, 2016 #7
    ok, that seems to be the problem then, I consider the C to be the C of the individual times 8, so i guess it is one or two pulling amps and the rest almost nothing. How is it that i fix them, I am assuming by matching them but how would i go about doing that?
     
  9. Nov 30, 2016 #8

    NascentOxygen

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    I don't know precisely how factories match LiPo cells, perhaps they graph their characteristics then group those that look similar. Model car suppliers sell packs of matched cells without protection that can withstand rapid very high C charging.

    Protection boards for individual cells come with 4 solder pads and cost about $1 each.
     
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