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Thread: Replace NIMH batteries with Li Ion 18650s in a battery pack

  1. #1
    Unenlightened
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    Question Replace NIMH batteries with Li Ion 18650s in a battery pack

    Hoping this is the correct forum, seemed to be from searching.

    I'm looking to replace the cells in the battery pack for my Trend Airshield Pro respirator.


    I found this how to where someone replaced the cells in his battery with new NIMH cells, but I want to upgrade to lithium ions for higher capacity.

    I have three spare Olight 18650 3.6v 3600mAh batteries, and they seem like the perfect choice for this project because of the various protection circuits they're advertised to have.

    I've never replaced cells in a pack like this before, so I have several questions:

    1. Can I use the existing charger with these new cells? If not, can I use the existing barrel plug with a compatible AC charger?
    2. Do I need to add a Battery Management System or anything else to the pack besides new cells?
    3. I assume I need to use a spot welder and not try soldering to connect the cells together, right?


    If anyone is in the DC area and is able to help out by letting me use your welder or with other advice, please let me know as well.

    Thanks for the help!

  2. #2
    *Flashaholic* Lynx_Arc's Avatar
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    Default Re: Replace NIMH batteries with Li Ion 18650s in a battery pack

    1) If the charger is smart it probably won't charge the 18650 at all, just give warning or error or bad cell indication. Lithium ion cells should not be charged by any other than a charger designed to charge them as the possibility of fires and exploding cells arises greatly by improper charging.
    2) I depends on the device, if the device itself stops working before the batteries can drop too low (unsafe voltage) then you are probably fine but too often devices can discharge batteries too low. A protection circuit may be needed
    3) Depending on the expected current draw you may be able to suffice using a battery holder instead and just solder to the wires on it or add your wires. These battery holders vary in construction some may not be good for current draws over about an amp or so.

    There is one other "solution to consider and that is to see if a power bank would work with the device or not and wire up a USB cable to the device but you have not said what voltage the device requires.
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  3. #3
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    Default Re: Replace NIMH batteries with Li Ion 18650s in a battery pack

    I looked at the thread you linked. It showed if I am correct, three NIMH cells in SERIES to power your device. NIMH cells are 1.2 volts. You end up with 3.6 volt pack.

    Notice that each 18650 is 3.6 volts already. So you can build the pack with three 18650'a in PARALLEL which will keep the new pack at 3.6 volts or fully charged it should read 4.2 volts.

    The NIMH that you are replacing are put together (bat1)positive to negative, (bat2)positive to negative and (bat3)positive to negative. Just like in a flashlight.
    What you will do with the new pack is simple wire it (bat1)positive to positive to (bat2)positive AND negative to (bat3)negative to negative.
    Soldier or weld the batteries together and put heat shrink tubing large enough to go around the three batteries. Then put plastic over the ends so you won't short the pack out and it should work.

  4. #4
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    Default Re: Replace NIMH batteries with Li Ion 18650s in a battery pack

    Took a couple photos of the charger and the battery pack after I opened it up.

    Looks like image sharing isn't cooperating, but here's the pics:

    https://pasteboard.co/JQxjb5b.jpg
    https://pasteboard.co/JQxiVxP.jpg
    https://pasteboard.co/JQxir3B.jpg

    Would there be another charger I should use instead that would work with these cells installed in this pack?

    I knew I was going to need to run the 18650s in parallel vs the NIMH in series. The 18650s I have seemed perfect for that since I wouldn't need to mess with the voltage beyond using these cells, but I'm guessing it would still be advisable to use these with a BMS chip of some sort.

    Again, I'm new to this and it's outside my wheel house, so I'm trying to play it safe and not do anything that'll start a fire when it's charging or an explosion when it's mounted to my head.

  5. #5
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    Default Re: Replace NIMH batteries with Li Ion 18650s in a battery pack

    There is one other "solution to consider and that is to see if a power bank would work with the device or not and wire up a USB cable to the device but you have not said what voltage the device requires.
    This is a solution I'm very open to, but I'm not sure how I would go about approaching it. Would I need to leave the existing battery pack in the respirator and just plug the battery pack into the charging plug in the helmet?

  6. #6
    *Flashaholic* Lynx_Arc's Avatar
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    Default Re: Replace NIMH batteries with Li Ion 18650s in a battery pack

    Quote Originally Posted by DedEye View Post
    This is a solution I'm very open to, but I'm not sure how I would go about approaching it. Would I need to leave the existing battery pack in the respirator and just plug the battery pack into the charging plug in the helmet?
    It depends on how it is wired up. You may need to disconnect the battery pack in it and connect a cable to the power bank.
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