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  1. #1
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    5x18650 limit charge to 4.0V for Troutie's 7up

    How much capacity do I loose if I charge a liion to 4.0v instead of 4.2V

    Due to the max input voltage of 20.0V for the Maxflex 5, we should not be using 5series li-ion charged to 21V. (4.2v x5)

    Some of the smart chargers can adjust their max charge voltage for li-ion cells.

    So I am thinking about building a 5 series liion, but capping the charge at 4.0V per cell. Obviously this does not make sense if I lose more than 20% of the capacity per cell as I'd be adding weight and maintaining the same capacity as 4series. I've been looking at the li ion discharge curve, and it looks like I may only lose between 5-10% capacity.

    I am going to charge a li-ion to 4.0V, then measure the incremental charge that takes the cell to 4.1 then to 4.2V. ( Most of my chargers are capped at 4.1V to play it safe)

    Will post the results later . hopefully our 5s packs are not useless yet.

  2. #2
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    Quote Originally Posted by heatstroke
    How much capacity do I loose if I charge a liion to 4.0v instead of 4.2V

    Due to the max input voltage of 20.0V for the Maxflex 5, we should not be using 5series li-ion charged to 21V. (4.2v x5)

    Some of the smart chargers can adjust their max charge voltage for li-ion cells.

    So I am thinking about building a 5 series liion, but cappign the charge at 4.0V per cell. Obviously this does not maks sense if I lose more than 20% of the capacity per cell as I'd be adding weight and maintaining the same capacity as 4series. I've been looking at the li ion discharge curve, and it looks like I may only lose between 5-10% capacity.

    I am going to charge a li-ion to 4.0V, then measure the incremental charge that takes the cell to 4.1 and 4.2V. ( Most of my chargers are capped at 4.1V to play it safe)

    Will post the results later . hopefully our 5s packs are not useless yet.
    Been having a good look at this myself. It appears that the issue is the maximum rating on two FETs. Being a design eng myself it is not good to exceed maximum voltages - however the difference between 20V and 21V is almost irrelevant. The issue here would be any noise superimposed over the top (eg ignition spikes etc).

    Personally I'll run with fully charging 5 x 18650s, but would not use 5 x LiPos.

    HTH

  3. #3
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    is there not a fair bit of sag when you connect the 7UP which may help .
    certainly when connected to a 4 cell pack it drops at least 1 volt if not 1.5

    I have used a Makita 18 volt drill battery but spun the drill a bit to bring it down to 20 volts first

  4. #4
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    I set the end voltage on my balance charger to 20v after George put me off doing what I had been doing for ages, using the full 21v.

    The difference in run time is about 10 mins on full (cheap Chinese cells)

  5. #5
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    Quote Originally Posted by yetibetty
    I set the end voltage on my balance charger to 20v after George put me off doing what I had been doing for ages, using the full 21v.

    The difference in run time is about 10 mins on full (cheap Chinese cells)


    So an 18.5v lipo off fleabay and then set an IMAX balance charger (from DX) to 20v max will work on the 7up?
    blah blah blah

  6. #6
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    Provided the charger has the ability to set the end voltage and is accurate, you should be fine.

  7. #7
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    In theory yes. But you need to check that you are not limiting the capacity of the pack down to the equalivent of a 4s pack. Will check when back from work...

  8. #8
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    I think that undercharging like that can eat more than 20% of the LiPo capacity, because of the CC/CV charging algorithm.

    It is better to charge the pack completely and then discharge it down to 20V in the charger.

  9. #9
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    Quote Originally Posted by heatstroke
    How much capacity do I loose if I charge a liion to 4.0v instead of 4.2V

    Due to the max input voltage of 20.0V for the Maxflex 5, we should not be using 5series li-ion charged to 21V. (4.2v x5)

    Some of the smart chargers can adjust their max charge voltage for li-ion cells.

    So I am thinking about building a 5 series liion, but capping the charge at 4.0V per cell. Obviously this does not make sense if I lose more than 20% of the capacity per cell as I'd be adding weight and maintaining the same capacity as 4series. I've been looking at the li ion discharge curve, and it looks like I may only lose between 5-10% capacity.

    I am going to charge a li-ion to 4.0V, then measure the incremental charge that takes the cell to 4.1 then to 4.2V. ( Most of my chargers are capped at 4.1V to play it safe)

    Will post the results later . hopefully our 5s packs are not useless yet.
    There's nothing wrong with charging to 4V per cell. The batteries will actually last a lot longer if you do that. You will lose 20-25% of the stated capacity of the pack.

  10. #10
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    I don't see the large benefit of the 5 cell pack at 4V, you might as well just go for the fully charged 4 cell and cycle the batteries properly, because I do think even li-ion packs will lose the top capacity if you cycle them only in the mid or low range (as in always keeping a low charge, then charging just a little, no full cycles). This is why it is recommended to cycle your laptop battery occasionally as those devices are constantly on and off the charger for short periods.

    If you have a 5 cell pack around, then go with 4v and use it, otherwise, if you're building a new pack, just use 4 cell, save the money on the extra cell, or even build a 4s2p pack, it's only the weight of 3 extra cells if you're really looking for run time.

    My 2 cents : )

  11. #11
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    Quote Originally Posted by DIYtailight
    I don't see the large benefit of the 5 cell pack at 4V, you might as well just go for the fully charged 4 cell and cycle the batteries properly, because I do think even li-ion packs will lose the top capacity if you cycle them only in the mid or low range (as in always keeping a low charge, then charging just a little, no full cycles). This is why it is recommended to cycle your laptop battery occasionally as those devices are constantly on and off the charger for short periods.
    Cycling li-ion batteries in the middle range, staying away from the high and low extremes will give you anywhere from 2x to 3x more cycle life. The reason some laptops recommended cycling them in the early days of li-ion is because that's how they calibrated their fuel gauge circuits. Almost all high quality laptops do not support or recommend cycling anymore due to the fact that it shortens the life of the pack.

    The main reason Li-ion packs have fairly short lifetimes in laptops is because it's almost always pretty warm inside a laptop, and the batteries are held at 4.2v per cell whenever the laptop is sitting around plugged into power. Warm temps and being held at a high voltage cause li-ion cells to degrade rapidly.

  12. #12
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    ah okay, my info was old then, good stuff to know

  13. #13
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    How about running the supply through a couple of schottky diodes in series to drop the voltage?
    ...Scun.thorpe, UK

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