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Thread: Reviving "Old" Ni-MH Cells?

  1. #1
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    Question Reviving "Old" Ni-MH Cells?

    I have a couple of Ni-MH Energizer rechargeable cells. Had these for maybe 15 years, but have hardly been used. Recharged maybe 6 times or so (only)!

    Noticed that these cells are not recharging anywhere near their rated capacity of 2500 mAh. Maybe charging to around 1800 mAh only.

    From various online sources I understand that chargers with refresh function might revive these cells to (near) their rated capacity. Unfortunately the chargers I have tried (OPUS BC3100, BC2000) have not been successful in improving these cells capacity (much).

    I understand that a more drastic approach might be to "zap" these cells briefly using a current limited/regulated 12 volt power supply. The 12 volt power supply should limit current to no more than about 0.5 amps (I think) and the power supply should only be connected to the Ni-MH cells for only a second or so.

    The idea is, apparently, to "realign" or "rejuvenate" something within the cells to enable the cells to accept more charge after "zapping" them with 12 volts.

    Can anyone one confirm that this procedure actually works?

    Failing this, are there any good chargers that will (successfully) rejuvenate old Ni-MH cells? If so, which chargers please?

    Thank you!

  2. #2
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    Default Re: Reviving "Old" Ni-MH Cells?

    those old 2500's were well known to die young.
    you can run them through a few cycles to attempt to "wake them up" but they are likely done at this point.
    use in non critical stuff till they are useless if you want to wring some more use from them.
    order up some eneloops for the mission critical jobs where a battery death is a show stopper.
    my favorite charger for nimh is the maha/powerex c9000.
    for the longest time they were the gold standard to most flashaholics.

  3. #3

    Default Re: Reviving "Old" Ni-MH Cells?

    Do not zap these batteries. Aside the case they are really dead, your real means of rescuing them is by using a CC (Constant Current) charger both for charge and discharge with a low current like 200 or 300mA. PWM (Pusle Wave Modulation) chargers use quite a high pulsed current, even if averaged to values like 200mA, thus these cells will not get a chance to get discharged and get exercised. I was in this situation with batteries return 0 mAh capacity when using maha MC9000 - due to its pulsed 1A discharge - I tried a true CC charger (liitokala Li500) and managed to get the whole capacity out of them after 3-4 cycles. Now these batteries also return good capacity in Maha after rescued by Liitokala. So, I doubt you'll have much of luck reviving these cells if you use a pulsed charger. Of course, the case can also be what snakebite says about dying young (those 2500mAh ones).
    Last edited by apagogeas; 07-04-2018 at 08:48 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by apagogeas View Post
    [..] PWM (Pusle Wave Modulation) chargers use quite a high pulsed current, even if averaged to values like 200mA, thus these cells will not get a chance to get discharged and get exercised.
    What precisely do you mean when you say they won't get "discharged and exercised"? Maybe your remark has more to do with lowest current that the charger can supply rather than the method (PWM vs CC).
    Last edited by Gauss163; 07-04-2018 at 09:16 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Agree with the above posts. Also, your cells are likely suffering from very high internal resistance, as well as capacity loss. For use in lights at moderate or high brightness, your cells are no longer any good. But you can still use them in low-drain applications, such as solar lawn lights or things that only use a few dozen millamps. They won't hold a charge for a long time (no more than a few weeks), so usage in things like a remote control or clock is probably not that great unless you enjoy charging them.

  6. #6

    Default Re: Reviving "Old" Ni-MH Cells?

    As others have said the 2500mah AAs (other than eneloop varieties) of the past once they start to go bad they just get worse. Even if you revive the capacity the high discharge rate and internal resistance make them only useful for devices that drain them daily as I've had some so bad they are half depleted after sitting just a few days. Go get some eneloop type cells either the pros or originals with the pros having higher capacity and less longevity and robustness vs the originals with lower capacity and very robust.
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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by Gauss163 View Post
    What precisely do you mean when you say they won't get "discharged and exercised"? Maybe your remark has more to do with lowest current that the charger can supply rather than the method (PWM vs CC).
    Maha discharges at 1A. If you set it to 200mA discharge, maha simply shortens the time 1A discharge is applied which will result in an average of 200mA discharge. For example, to get 200mA discharge using 1A pulsed current, you have to apply a 0.2 seconds discharge every 1 second (which means 0.8 seconds without any discharge) - this is what we call PWM (pulsed Wave Modulation). On old batteries, due to elevated internal resistance, this 1A pulsed charge will rapidly drop the voltage under load, therefore maha will consider this battery empty even if it is actually fully charged, all due to higher internal resistance. Therefore, that battery will not get the chance to get discharged and dissolve any passivation layers that may have formed over time. This is the problem I mentioned at my previous post and the reason I got 0mAh capacity with maha. So, if you use a true CC charger and set a low actual discharge current, you actually reduce the votage sag and don't trip the cut-off point of voltage under load. That is basically the reasoning behind usage of CC chargers to rescue old batteries. I have succesfully revived 6 AAA batteries which maha rejected right away and they do perform very good after treated by Liitokala which is a CC charger.
    Last edited by apagogeas; 07-07-2018 at 08:49 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by apagogeas View Post
    Maha discharges at 1A. If you set it to 200mA discharge, maha simply shortens the time 1A discharge is applied which will result in an average of 200mA discharge. For example, to get 200mA discharge using 1A pulsed current, you have to apply a 0.2 seconds discharge every 1 second and 0.8 seconds without any discharge - this is what we call PWM (pulsed Wave Modulation).
    Yes, of course I know what PWM means.

    Quote Originally Posted by apagogeas View Post
    On old batteries, due to elevated internal resistance, this 1A pulsed charge will rapidly drop the voltage under load, therefore maha will consider this battery empty even if it is actually fully charged, all due to higher internal resistance. Therefore, that battery will not get the chance to get discharged and dissolve any passivation layers that may have formed over time. This is the problem I mentioned at my previous post and the reason I got 0mAh capacity with maha.
    If what you claim is true then that is due to a poorly implemented (dis)charge algorithm in your Maha charger. Not every PWM based charger behaves so poorly on cells with high IR.

    Quote Originally Posted by apagogeas View Post
    So, if you use a true CC charger and set a low actual discharge current, you actually reduce the votage sag and don't trip the cut-off point of voltage under load. That is basically the reasoning behind usage of CC chargers to rescue old batteries. I have succesfully revived 6 AAA batteries which maha rejected right away and they do perform very good after treated by Liitokala which is a CC charger.
    Competently implemented PWM chargers can do likewise. The root of the problem is not PWM vs CC but rather, excellent vs. poor (or ancient) design.
    Last edited by Gauss163; 07-07-2018 at 09:00 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by apagogeas View Post
    Maha discharges at 1A. If you set it to 200mA discharge, maha simply shortens the time 1A discharge is applied which will result in an average of 200mA discharge. For example, to get 200mA discharge using 1A pulsed current, you have to apply a 0.2 seconds discharge every 1 second (which means 0.8 seconds without any discharge) - this is what we call PWM (pulsed Wave Modulation). On old batteries, due to elevated internal resistance, this 1A pulsed charge will rapidly drop the voltage under load, therefore maha will consider this battery empty even if it is actually fully charged, all due to higher internal resistance. Therefore, that battery will not get the chance to get discharged and dissolve any passivation layers that may have formed over time. This is the problem I mentioned at my previous post and the reason I got 0mAh capacity with maha. So, if you use a true CC charger and set a low actual discharge current, you actually reduce the votage sag and don't trip the cut-off point of voltage under load. That is basically the reasoning behind usage of CC chargers to rescue old batteries. I have succesfully revived 6 AAA batteries which maha rejected right away and they do perform very good after treated by Liitokala which is a CC charger.

    I use an old dumb-charger that does a slow, constant, timed charge to NiMH cells. It's great for charging really old batteries, that smart-chargers either refuse to charge or will terminate too early. I'm talking IR more than 3000mOhms, so they're really low-discharge cells now.


    Quote Originally Posted by Gauss163 View Post
    Competently implemented PWM chargers can do likewise. The root of the problem is not PWM vs CC but rather, excellent vs. poor (or ancient) design.
    What is a good PWM charger design for charging really high-resistance cells? I'm guessing fast PWM? Or something like a long pulse of charge followed by a brief discharge pulse? Can PWM do as well as constant-current, because I always thought that constant-current was a positive trait of good chargers.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Hello Gaus163,

    I don't understand the evolution of PWM dischargers so help me understand.

    Given a discharge current of 1 amp.
    Given a pulse of this 1 amp current to simulate lower discharge currents.
    Given a high internal resistance cell that drops below the discharger cut off voltage during a pulse of 1 amp.

    Are you suggesting that there are dischargers that somehow are able to continue to discharge the cell even though the maximum discharge voltage has been reached?

    Or are you suggesting that an advancement is that when the discharger pulses at 1 amp and hits the target minimum discharge voltage it regroups and changes the discharge pulse to a lower value?

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    Default Re: Reviving "Old" Ni-MH Cells?

    ^^^ The maha is using ancient technology - over a decade old. Nowadays using dirt cheap off-the-shelf components one can cheaply implement any type of charging scheme one desires - including fancy hybrid and multistage schemes and adaptive algorithms that are sensitive to the health of the cell. Whether those algorithms use PWM or CC at various stages does not matter much as long as they do the job well.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Tried doing "refresh" mode on these two Energizer 2500 mAh cells, using an OPUS BT-C2000. The refreshing took approximately two days using a discharge current of 100 mA and charge current of 200 mA.

    The result is that one cell has a capacity of 1240 mAh, and the other cell has a capacity of 1462 mAh.

    The quickest solution might be to discard these two cells as they are well below their rated capacity. But, would another charger (using different charging process) improve the capacity of these cells? Or try charging/discharging them another way using the BT-C2000?

    The only other chargers I have are the OPUS BT-C3100 and the La Crosse Technology BC-900.

    Try another charger??

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by Gauss163 View Post
    ...
    Competently implemented PWM chargers can do likewise. The root of the problem is not PWM vs CC but rather, excellent vs. poor (or ancient) design.
    Please state such a PWM charger that implements differently PWM on discharge and avoids rapid voltage sag on such older batteries. The problem is not CC vs PWM per se, the problem is due to existing PWM implementations we have high voltage sag, something we can avoid with low discharge current with CC. I am not aware of any such PWM charger.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by meeshu View Post
    Tried doing "refresh" mode on these two Energizer 2500 mAh cells, using an OPUS BT-C2000. The refreshing took approximately two days using a discharge current of 100 mA and charge current of 200 mA.

    The result is that one cell has a capacity of 1240 mAh, and the other cell has a capacity of 1462 mAh.

    The quickest solution might be to discard these two cells as they are well below their rated capacity. But, would another charger (using different charging process) improve the capacity of these cells? Or try charging/discharging them another way using the BT-C2000?
    I don't think you'll get much of an improvement using another charger. You've already improved the charging process by a factor of 25x over using 1000mA. If you really need less than 200mA to charge a AA NiMH cell properly, it's not worth keeping it.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by apagogeas View Post
    [...] existing PWM implementations we have high voltage sag, something we can avoid with low discharge current with CC.
    Yes, as I said above: ancient (maha) and/or recent technology not designed to handle this case.
    Last edited by Gauss163; 07-08-2018 at 08:01 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by Gauss163 View Post
    Yes, as I said above: ancient (maha) and/or recent technology not designed to handle this case.
    Name such a charger to have a look at it please. As I said, I am not aware of any such charger, they all seem to work in the same manner as maha.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by apagogeas View Post
    Name such a charger to have a look at it please. As I said, I am not aware of any such charger, they all seem to work in the same manner as maha.
    I don't keep up with consumer-level NiMh chargers these days since I use Li-ion almost exclusively. My point was merely to emphasize that it is possible nowadays to do much better than very old chargers (e.g. maha) for a much lower price using off-the-shelf components. One shouldn't project limitations of old technology into modern times.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Quote Originally Posted by Gauss163 View Post
    I don't keep up with consumer-level NiMh chargers these days since I use Li-ion almost exclusively. My point was merely to emphasize that it is possible nowadays to do much better than very old chargers (e.g. maha) for a much lower price using off-the-shelf components. One shouldn't project limitations of old technology into modern times.
    Could you expand on what you mean by this improved PWM charging tech? What exactly makes it better than traditional PWM charging? And is it as good as constant-current?

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    Default Re: Reviving "Old" Ni-MH Cells?

    My point is simply that nowadays using off-the-shelf components it is dirt cheap to add reviving algorithms (or most any other algorithm) to a charger because the cost of hardware is at least an order of magnitude less than it was in the days the maha was designed. So cheap that one could even employ dedicated hardware for such purposes. If you desire such capabilities then you should make that known to charger manufacturers (recent analyzing chargers have incorporated suggestions from users, e.g. Opus BT-C3100, SkyRC MC3000).
    Last edited by Gauss163; 07-08-2018 at 10:14 AM.

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    Default Re: Reviving "Old" Ni-MH Cells?

    Okay, without an answer as to why modern PWM algorithms are good, I'm just going to continue to believe that constant-current is still the best way to go.

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