Gonna DIY a Magicshine Battery- Mtbr.com
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  1. #1
    ballbuster
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    Gonna DIY a Magicshine Battery

    I'm thinking of doing a 6 cell battery for my MS light, 18650 cells all side by side for a flat pack. It would fit perfectly into my top tube frame bag.

    I'm wondering if a 2 cell 7.4v battery protection PCB is a good idea. As in, to keep the cells in balance. I know most cells have individual protection PCBs on them already, but I was concerned with what happens when you double up protection boards, or if loading on more cells on a 2 cell board is a good or bad idea, even if I'm not drawing more current than the board can handle.

    Opinions are welcome.

  2. #2
    Do It Yourself
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    I don't think you don't need more than one pcb with a 2s3p pack. The 4 cell 2s2p only has one protection circuit. Adding more cells to the parallel sets doesn't change anything. Electrically it's still the same as two cells. Balance shouldn't be much of a problem with matched cells from the same lot and made with name brand cells from a reputable source.
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  3. #3
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    Quote Originally Posted by pimpbot
    I'm wondering if a 2 cell 7.4v battery protection PCB is a good idea.
    No. Those PCBs see only 2 cells. If you already have protected cells you don't need an additional PCB. Also, those cheap 2S PCBs don't balance the cells, so you will have to charge them individually.
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  4. #4
    Do It Yourself
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    Quote Originally Posted by zemike
    No. Those PCBs see only 2 cells. If you already have protected cells you don't need an additional PCB. Also, those cheap 2S PCBs don't balance the cells, so you will have to charge them individually.
    Yes. A set of three cells in parallel is effectively one cell with triple the capacity. You only need a 7.4v pcb. There isn't any thing electrically different from a 2s pack to a 2s2p pack to a 2s3p pack. They are all just 7.4v with three connections: positive, center, and negative.

    The balance issue has been blow out of proportion. Crappy cells with soft shorts are a balance problem and a balance charger won't fix it. Balance isn't a significant issue with packs made from matched cells from a reputable source. None of the major brand light companies offer balance chargers with there Li-Ion systems. Why? Because it's not an issue.
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  5. #5
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    Depends on how you connect them, right? If you don't connect them "in the middle" you'll have three 7.4V cells connected in parallel.

  6. #6
    Do It Yourself
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    Quote Originally Posted by xyz-saft
    Depends on how you connect them, right? If you don't connect them "in the middle" you'll have three 7.4V cells connected in parallel.
    There are many ways to physically connect the cells but electrically there is only one way to do 2s3p.
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  7. #7
    www.hahntronix.com
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    Most plain cells do NOT come with a protection circuit, for example if you are using old laptop battery cells, they have no protection PCB built into the cell.

    You can buy protected cells for pretty cheap from Chinese distributors such as DX and KD.

    I've never tried hooking two protected cells in parallel. Depending on the protection circuit used it "should" work. If I were you, I'd try the first tests of hooking protected cells in parallel outside, someplace non-flammable . I would think that if you are using protected cells, you should not need another PCB in your system. Anybody ever tried cheap DX protected cells in parallel?

    You could use protected cells in a cell holder, such as: http://search.digikey.com/scripts/Dk...K-18650-PC6-ND.

    With the protected cells from DX, there is often a slight variation in length. If your cells seem exceptionally tight in the above holders, you may need to bend the tabs in the holders so they are a bit flatter.

    Mark
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  8. #8
    Do It Yourself
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    Again, I would highly recommend building a proper battery pack with quality unprotected cells and an appropriately sized pcb for protection. Solder up the bundle and seal it in heat shrink. This is how the pros do it. Protected 18650s are for flashlights.
    Long Live Long Rides

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