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I have seen information supplied with branded NiMH cells (e.g. Duracell) saying "do not leave cells for more than 6 months without recharging."

I have some AA size cells which were fully charged around 18 months to 2 years ago, but have since been left unused. During that time they have not been installed in any battery powered devices, and kept at normal household temperature and humidity levels. They have no visible damage.

Attempting to recharge them with my usual charger doesn't work - I assume their voltage is so low that the charger protection circuit is cutting in to protect the charger. If it's relevant, the charger is designed to fully charge either 2 or 4 cells, either AA or AAA size, in about 2 hours.

Is there any relatively simple (and safe!) way to recharge them to the point where a commercial charger will at least try to recharge them fully?

It wouldn't cost a huge amount of money just to replace them with new ones, but my natural inclination is "don't scrap stuff unless you really have to!"

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I do not expect these cells to be damaged. Contrary to Lithium based cells, which are indeed damaged when fully discharged, NiMh cells can handle this as long as they are not reverse charged.

Reverse charging can happen when the cells are used in for example a camera and one of the cells has less capacity. When it is empty it will be reverse charged while the other cells still provide current.

But you appear to have left the cells on their own so reverse charging cannot happen in this case. The cells just emptied themselves due to self-discharge.

Personally I would use my lab supply to "kick the cells into life" with 0.5 A of current for 10 seconds or so while measuring their voltage.

If you do not have a lab supply but you do have a "dumb" charger (one where it takes 10 hours or so to a full charge) you could try to charge the cells in that. Since such a charger does not check the cell's voltage but simply charges with a small current, the cells might become alive again after an hour.

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  • \$\begingroup\$ +1 for good answer, but I'd explain the "Personally I would use my lab supply to "kick the cells into life" with 0.5 A of current for 10 seconds or so while measuring their voltage." part in more detail lest he wires that up backwards and reverse charges it. \$\endgroup\$
    – Trevor_G
    Apr 20, 2017 at 19:45
  • \$\begingroup\$ @Trevor Everybody makes mistakes sometimes, but I like to think I'm not quite dumb enough to do that completely by accident ;) \$\endgroup\$
    – alephzero
    Apr 20, 2017 at 20:07
  • \$\begingroup\$ @Trevor Appreciate your concern but I assumed that someone who has (access to) a lab supply knows how to use it ;-) \$\endgroup\$ Apr 20, 2017 at 20:15
  • \$\begingroup\$ Based on this idea, I actually kicked them into life with the charger, by charging one of the old cells in parallel with a fully charged cell for about a minute. After repeating that with another discharged cell, the charger was happy to start charging the two old cells together. Time will tell if they have survived unharmed, of course. \$\endgroup\$
    – alephzero
    Apr 20, 2017 at 23:55
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Those cells are probably irreversible damaged (the capacity will be much lower than nominal).
You can use a bench power supply. Just set constant current mode and limit voltage to say 1V.

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  • \$\begingroup\$ In constant current mode, shouldn't I be setting the current limit, not the voltage? (But I can see the point of setting a voltage limit as well, in case the cells are effectively open circuit.) \$\endgroup\$
    – alephzero
    Apr 20, 2017 at 19:05
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    \$\begingroup\$ @alephzero you have to set BOTH. Set current limit (depends on capacity of your cells) and set voltage limit (to 1V, so power supply will not go over that). \$\endgroup\$ Apr 20, 2017 at 19:06

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