Magicshine battery failure- Mtbr.com
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  1. #1
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    Magicshine battery failure

    Recently my Magicshine has been acting up... battery light turning red prematurely and sometimes just cutting out completely. I finally got around to figuring out what was going on when I discovered if I squeezed the battery lengthwise I could replicate the problems... so it was time to remove the the wrapper see what was going on.

    Wow, not too impressed with the quality of the spot welds on the strips used to wired the battery together! Fortunately I have a good temperature controlled soldering station that made for a very quick soldering job with little risk of overheating the cells. Also replaced that shaky looking red wire. The pack is now reassembled and wrapped in duct tape, probably not the greatest from a waterproofing point of view, but since I can't seem to find Plasti-dip here in Toronto it will have to do.
    Attached Thumbnails Attached Thumbnails Magicshine battery failure-magicshine-battery-failure.jpg  

    Last edited by kwarwick; 03-21-2010 at 03:24 PM.

  2. #2
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    Did you check the balance of the cells before you connected everything back up? Since the 2 cells on the right in the picture were likely randomly connecting and disconnecting, they may be out of balance from the pair of cells on the left.

    The sets of cells that are connected in parallel will rebalance naturally because by definition they have to be at the same voltage due to being connected in parallel. The two banks of cells connected in series may be out of balance from each other.

    You can check that with a volt meter without disconnecting anything. Just charge the pack up until the light on the charger turns green. Take if off the charger and measure the voltage of each bank. They should be close to 4.2 volts and equal. If they aren't within 0.1 volts of each other then you need to rebalance by either discharging the bank with a higher voltage or charging the bank with a lower voltage. Once you get it back in balance, they should naturally stay balanced as long as all the cells are in good shape.

    To charge a single bank, you need a charger designed for a single cell that is regulated to 4.2 volts. You can discharge a bank with a bulb for a 3-cell flash light and a couple of wires. Just monitor voltage carefully as you discharge so you get it equal to the other bank. It'll rebound a little when you disconnect the bulb.

  3. #3
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    Didn't think to do that, but the whole pack was charging up to 7.4v before I took the pack apart. Actually I noticed that the stock Magicshine charger was overcharging the cells as I was getting 7.45-47 at charger termination so I stopped using that charger. I now use a different charger that stops at a more reasonable 7.35v. At this point I won't be terribly surprised if this battery pack has a short life based on the previous over-charging.

    Karl

  4. #4
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    7.4 volts is the average voltage for a 2 cell pack. Li-ion cells should be charged up to 4.2 volts per cell or 8.4 volts for a 2-cell pack. All of the MS chargers I've checked have been pretty good about being close to the 8.4 volt limit. If you're only charging the pack to 7.35 volts, you're not going to be getting much capacity out of the pack. About 50% of the normal capacity.

  5. #5
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    Sorry, brain fart there. I meant 8.4v... the Magicshine charger was taking it closer to 8.45-8.47. The charger I'm now using terminates charging at 8.35v.

  6. #6
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    Quote Originally Posted by kwarwick
    Sorry, brain fart there. I meant 8.4v... the Magicshine charger was taking it closer to 8.45-8.47. The charger I'm now using terminates charging at 8.35v.
    8.47 is fine. That's only 4.24 volts per cell and you don't really have trouble until you get closer to 4.35 volts per cell which is when the protection PCB kicks in on most multi-cell packs. Even if the pack is out of balance by 0.1 volts, you'd still be ok with a 8.47 volt max charge voltage. The batteries themselves aren't really damaged or become unsafe until you go even higher still.

    Of course, using the charger limited to 8.35v per cell is fine and will give a slightly more cycles out of the pack. If you really want to increase the lifespan of your pack, you can charge them to 3.95 volts per cell. That is what they do in military and other applications where they want to optimize pack lifetime. You only get about 80% of the full spec capacity out of the pack, but it more than doubles the cycle life of the pack.

    Since I rarely need more than 80% of the capacity of my packs, that's what I charge them to for most of my rides. I charge them higher when I need the full capacity.

  7. #7
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    I should add that protected single cells are fairly different in that most of the single cell protection PCBs have much lower limits. 4.25 volts is pretty typical and it's not uncommon to see ones that kick out at 4.2 volts or lower in real life. Because of that, the voltage limit on single cell chargers has to be much tighter and lot of them are not good enough.

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