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Thread: E/X-Mag Battery Housing for Replaceable Cells

  1. #1

    E/X-Mag Battery Housing for Replaceable Cells

    All,
    I am tired of these proprietary battery packs, one cell goes bad, and the whole assembly is shot. So I thought I would try my hand at 3D printing a prototype e-mag battery housing that would allow the use of replaceable cells. Unfortunately, I sold my 3D printer last week for a 140% profit. ABS wouldn't be the ideal material anyway.

    Pros:
    Use high quality AAA cells (eneloops).
    Replace failed cells individually.
    Maintain/refresh cells using a modern smart charger.
    Compatible with existing charger.
    Cheaper to replace batteries.

    Cons:
    Assembly is more complicated.
    Ano.
    May not handle shock as well as the original shrinkwrapped pack.
    Slightly taller to accommodate contacts.

    I started working on a SketchUp CAD of the concept, which I think will work. (It's 1" wide vs 0.975" for the original pack, and maybe 0.25-0.5" taller.) It would use two FR4 terminal boards with http://www.amazon.com/gp/product/B00LPOW3NM contacts. The idea is that the terminal boards would fit inside the center section of the new housing, to avoid the need to mill the end pieces. Here are a couple of drawings. Note that I made the terminal board last, which is the reason for the overshoot on the outside terminal pads. The center section would need to be modified to accommodate that overhang. A backer board would need to be used for each terminal board, once the contacts were installed. One wire would run through the pack to the top and connect to an original pack's terminal plate.

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    If there are any machinists out there (*cough* Xmagterror and Luke), would you please be so kind as to comment on the feasibility of such a design? Any idea what it might cost to produce? Feel free to send me a PM.

    Is there anyone else out there who would be interested in a run of these?

    Thanks,
    AE
    Last edited by ArmyEngineer; 08-23-2015 at 12:46 PM.

  2. #2
    Join Date
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    Location
    Halifax, N.S., Canada
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    In theory changing the bad cell out for a new one works fine. In practice, you end up with a battery pack that is only as good as one of the other weaker cells. The reason one of the cells in a pack fails, is often because the voltage was allowed to go below the minimum safe level. If the voltage goes low enough, all sorts of weird chemical things happen. By replacing that bad cell, you get the pack back, but the rest of the cells are most likely compromised as well. Since the pack is a series combination of cells, if any one of the other cells has a higher internal resistance as a result of a charge or discharge issue, then the whole pack will have a lower overall capacity. The pack won't take a charge properly and it will run out of charge even though it appears to have lots of voltage capability.

    You can't change just one cell in a pack for the same reason you don't mix brands or sizes of batteries. They have slightly different parameters and they don't play well together.

    If you want to reclaim a bad cell, put a controlled high voltage charge on the bad cell. I have used a 2V charge on the 1.2V NiMh cells with success. I monitor the heat and current input to the cell. As long as it is taking the charge and not getting too hot, I let it go. As soon as I detect an increase in the heat, I stop. Quite often, I can salvage a bad cell like this. It will go from dead back to useable and can be replaced back into the pack it came from.

    Your idea is still valid. I do like having a pack that uses regular NiMh cells. I was contemplating making one myself. You just have to operate it like one large pack and not as individual cells, although you can charge the cells individually if you want. Just make sure they are all topped up fully or balanced each time they are used.
    Except for the Automag in front, its usually the man behind the equipment that counts.

  3. #3
    Thanks for taking the time to respond athomas.

    You make a valid point, when a cell fails and the pack is placed under load, there can be collateral damage. Still, a more modular approach has the obvious advantage of easy characterization, conditioning, and recovery of the component cells. With the packs, if a cell goes bad, it's too much work to pull the whole thing apart, troubleshoot, replace, and reassemble the pack. In a 14 cell pack, the odds of having at least one under-performing cell are not insignificant. I use a BC-700 around the house, and have noticed unexpectedly high infant mortality is several major brands. (Don't ever buy Energizer rechargeables. So disappointing.) Having the ability to monitor cell health, set charge rate, and more importantly, the ability to use high quality cells like eneloops would in my mind make the effort entirely worthwhile. Having to spend $85 every couple of years for a new AGD pack or ~$60 for a rebuild is a bummer, considering you can buy 16 eneloop AAAs for around $30.

    I know the use of LiFePO4 or LiPO cells has been mentioned a few times in the past. Do you think that would be a better approach, athomas? I feel like AAAs offer a more reliable, interchangeable solution. I looked at RCR123a cells, but we'd need 5-6 of those plus voltage regulation.

    I want to make this happen!

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