Reviving “Dead” NiMH Batteries

I prefer to use rechargeable batteries whenever possible. Recently I noticed that several of our AA cells were being reported as dead (“null” when inserted) by the charger. Some of them were rather new, so I was a bit miffed at the idea of them becoming unusable in such a short time.

I remembered a couple of colleagues in the lab at my former employer performing a trick to “revive” a dead NiCd battery pack — they charged a capacitor and then discharged it through the cell to blast away any dendrites that might have formed between the plates. Now these were NiMH, not NiCd, but I wondered whether there were any similar tricks to reawaken these.

It turns out the issue and solution are much simpler. The cells in question had depleted so far that they were below 0.9v, and the better chargers apparently view such cells as dead. There were all sorts of suggestions out there for hooking up larger batteries (or even arc welders!) to force the battery to a higher voltage that the charger would then view as “live”. I wondered about using one of the old style (a.k.a. “cook the battery”) chargers to boost the voltage instead (similar idea, but with much less risk), so I put the dead ones into the old charger I’d normally avoid – for just a 5 minute juicing.

That did the trick – the better (current sensing and limiting) chargers would now accept these cells and bring them back up to full charge without overheating them. This worked on all of the cells previously left for dead.

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16 Responses to Reviving “Dead” NiMH Batteries

  1. I was getting “null” on a couple of my batteries and this tip worked great to revive them. Thanks!

  2. seb says:

    also you can revive them with an other charged battery: join the batteries so the minus poles are connected, and use a metal appliance (I used a nut cracker) to make the connection of the both ends. 30 seconds-1 minutes should do the trick. This gives enough juice so the charger recognizes them. I did it on 10 batteries, only one failed to be revived.

    • Giancarlo says:

      yes works connecting a second battery. Easy
      thank you

    • Marcos says:

      So so easy… It took me longer to Google until I found your simple solution: I used a wine cork opener, connected the two batteries, waited for 40 seconds and it did the trick. Now my four AAA batteries are being recharged, not just two because of one of it being defunct. Big thank you.

    • Jonathan_UK says:

      I’m not really understanding the procedure :
      are you saying join the battery cells in parallel ?

      • Don says:

        As I’m not the one that reported that method, I’m just guessing here but I expect the answer is yes to connecting them in parallel (+ to +, – to -). Your goal is to increase the voltage, not to increase the amperage, so parallel should be the answer vs. series.

  3. Aditya says:

    I do not have a nutcracker or a wine cork opener, would you please provide me with an alternative and explain it more clearly how to do it??

    • Don says:

      See my reply to Jonathan_UK. The goal is to increase the voltage in the “dead” cell. You can connect the batteries with standard wire. I’m not recommending this method myself, I stand by the original post about using a battery charger to do it. BTW, in later readings from the web I’ve seen people saying that leaving a “dead” battery in a “smart” charger for an extended period (2-3 days) will bring the battery back up enough for the charger to recognize it and then do a standard charge cycle. I’ve not tried that yet myself (and given the speed of my solution I’m not that interested).

      • Rudy says:

        Leaving dead batteries in a smart charger for an extended period, like the Energizer charger, actually worked for me. Not on all dead batteries though. YMMV

  4. Ted Maudsley says:

    Not so much a reply, as another question! I have some ni-mh cells which seem to be ‘whiskered’ like the dreaded ni-cds (no volts, but s/c both ways). I’ve been trying to revive them by splashing them from a higher voltage battery. I’ve yet to see how they hold up, but they seem to have taken a charge. Any other ideas, just in case?

    • Don says:

      Sorry, you’re venturing where I’ve not gone before. The folks I worked with used a charged capacitor to burn away the whiskers on NiCd batteries (so using a rapid discharge current instead of higher voltage) but I’ve not heard anything about similar issues with NiMH batteries and what to do if so. Be sure to report back if you’re successful!

  5. Ted says:

    So far, so fairly good, with two from five successes. One is helping to power a drill, and the other is holding up OK off load. The ‘splashes’ were from a 12v motorbike battery, so they had to be very brief!

  6. mc says:

    If you don’t have a nut cracker but you have something like a stereo cable – or any kind of connector cable – that will work – it’s just a wire. Just tried this on a “dead” cell:

    1) put the two positing tips touching – leaving the flat negative ends exposed
    2) touch an end of the cable to each end of a battery
    3) make sure loop is closed
    4) sing two choruses of your fave song or count to thirty
    5) pop into charger.


  7. Trevor Edmond says:

    Great posting which has probably saved me $$$
    Tried using my car battery charger to “flash” the dead NiMH 6 V that was showing 0.0 V. No Joy!
    Got an old car battery that had been hanging around for about 6 mths. It had 9V on it. Hooked up the charger which started to output 13V
    Used the battery terminals (while still hooked to the charger) to “flash” (just struck the terminal about 3 times) the NiMH battery. It tested 5V. Stuck it into the NiMH charger and it started to charge.
    When I first tried to charge it (before flashing) the charger green and red lights kept alternating rapidly. Now its steady on red and I am presuming it is charging as it should. After 10 minutes it was showing 7V (its a 6V battery for Paslode Nailer).

  8. Graham Foley says:

    Thank you so much. I’d previously thrown a few away before reading this. Thought it was odd for relatively new rechargeable batteries to show null after a few uses.

    I used a Duracell AA battery +ve terminal to +ve terminal of the rechargeable and used a large paperclip (unfolded) and then held in place. I could get away with counting to just 10 seconds before slotting back in the small Techno charger I have.

    Thanks again for this great tip.

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