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Thread: Repacking NiCd battery pack with Li-ion 18650 considerations

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    Default Repacking NiCd battery pack with Li-ion 18650 considerations

    Got a 7.2 volt battery pack stuffed full of dying NiCd 1200mAh cells, along with a drop in charger to suit (handles either NiCd and NiMH packs)
    Planning to repack it with a couple of 18650 Li-ion cells, and hoping to continue using the NiCd/NiMH drop in charger to charge the pack.
    Presume that I should use 'protected' 18650s in this application? Or do I need to re-jig the drop in charger with a Li-ion charger module such as a TP5100?



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    The device being powered must be able to tolerate the 8.4v over-voltage condition (full lipos) for the duration of time it takes those cells to deplete to the nominal 7.2 volts.
    A 'protected' lipo cell is only protected from over charging/discharging (terminal voltage) ~ the IC (dw01 or similar) does not control the charging profile/current rate.
    The charging/current profiles for NiCd/NiMH differ from what's required to charge lipo cells (must be current limited for low cell terminal voltage), and are unsuitable.
    2 lipo cells in series (2S) should ideally be changed by a 3 wire charger (tap inbetween the 2 cells)

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    All good there - brand new NiCds open circuit fully charged voltages often get to 1.50 volts, so a 7.2v nominal pack can reach 9 volts. The connected device is happy up to 12 volts, so that's no problem.
    Sounds like I will need to rebuild the charger to suit the Li-ion. The TP5100 module will make that easy, which will also take care of any over-charging issues. Just need to make sure the device definitely shuts down at a suitable voltage threshold to protect the Li-ion cells from over discharge.
    Thanks wotnot, time to bust out the soldering iron!

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    Protected cells will disconnect at minimum cell terminal voltage ~ they protect themselves, device needs to do nothing =)

    TP5100 will eventually cause a slight cell imbalance, but it's usually not too bad ~ bigclive just did a vid about this actually...

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    I never use 18650 in DIY projects, period
    Why? Because I never know if it is about explode unlike Li-Ion pouch batteries where they just puff up and you know early enough that they are crook. They have no cap that can suddenly become a projectile.

    I have been going the opposite direction lately and replace Li-Ion batteries with Eneloops or use at least only LiFePO4 cells in my projects.

    The amount of house fires whilst charging Li-Ion batteries is becoming alarming and although we are warned never to charge Li batteries unattended and always switch off the Chinese charger when finished, we are all human and can forget things.

    The black AA Eneloops are rated around 2500mAh(which I got for $4.70 each in bulk) but I have been getting 2800mAh out of them, which is a higher energy to volume ratio than similar sized Li-Ion.
    I often use only 2 Eneloops and little adjustable step up boards to get 5V-9V for projects.

    In RC vehicles where I still need to use LiPos, I take them out and lay them on the concrete floor, together with my tool batteries, in my garage(that never houses a car) when I am away a bit longer.

    I mentioned above Chinese charger but the only time I actually had a fire was with a German one, charging an E-bike.
    The charger caught fire but luckily the bike's battery survived.
    Nowadays Wifi smoke alarms are available, which I have in the garage of course that triggers all my smoke alarms in the house at once.
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    Quote Originally Posted by mitaux8030 View Post
    Got a 7.2 volt battery pack stuffed full of dying NiCd 1200mAh cells, along with a drop in charger to suit (handles either NiCd and NiMH packs)
    Planning to repack it with a couple of 18650 Li-ion cells, and hoping to continue using the NiCd/NiMH drop in charger to charge the pack.
    Presume that I should use 'protected' 18650s in this application? Or do I need to re-jig the drop in charger with a Li-ion charger module such as a TP5100?
    I have modified Motorola SABER batteries with two protected cylindrical Li-ion cells. Those radios are good up to 9V without any problem. There is a weight savings of about 6 oz and a definite improvement in runtime operation.

    I suggest buying a wall charger for 8.4V Li-ion from e bay and modify your drop in charger to accept that, or install a coaxial jack in the batterry to accept the wall charger. Actually I find it convenient to use the wall charger alone when travelling.

    Sent from my SM-T350 using Tapatalk
    "Have Spectrum Analyzer, - Will travel".

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