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  1. #1
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    DIY battery pack w/ protected cells

    Just wanted to touch base with someone more knowledgeable about this than I am.

    I have built up two 2s1p and two 2s2p battery packs from protected panasonic ncr18650b 3400 mAh cells. I am fully aware of the dangers of DIY soldering 18650b cells, but accept the risk since I'm actually soldering to the protection circuit and bumpout, not the cell itself. Anyway, lets not get into that can of worms.

    My concern is that having two protection circuits may cause problems. In theory, if while discharging, the lower voltage cell will kick off early while the other cell may have some left in the tank. Conversely, while charging, the cell with the higher leftover capacity will cut off early, leaving the other cell without a full charge.

    Now in practical application, the batteries are never used until they go fully flat. I'm not sure if the light heads have a more conservative protection, but I wouldn't be surprised. When charging, I am fairly confident the chargers quit well before the protection circuit. I normally use the charger than came with my BT40s, but occasionally will use my hobby charger. If I charge with my light charger first, but then move the battery to the hobby charger, I can still fit 40 or 50 more mAh into the pack before reaching the 4.0 stop voltage on the hobby charger.

    Anyway, I was considering soldering in balance charging leads, so I could balance the packs every ten or so rides, but I'm wondering if this is really necessary, or simply overthinking things.

    Thoughts?

  2. #2
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    If you add a balance lead and use it with some frequency, your concerns about cell imbalance causing charge/discharge issues will not happen. I'm a firm believer that having balanced cells lengthens the useful life of a pack considerably. I balance every charge and have some batteries that are more than 7 years old that have only lost ~25% of their original capacity.
    GoPro adapters for bike lights http://www.pacifier.com/~kevinb/index.html

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