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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4 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.

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