Restoring NiMH batteries

[SilverSolder]

I have a batch of power tool batteries, NiMH 3800mAh types, that have been left discharged for a long time...

Any experience/suggestions on how to recover them back to a serviceable state - or is it completely hopeless?

[helius]

The IEC 61951-2 3.0 contains a procedure, section 7, which is the industry standard procedure for capacity testing single cells. It can also be used to recover capacity in deeply discharged cells like yours.
"A constant-current charge at 0.1C for exactly 16 hours including a 6 hour overcharge, afterwards stored for 4 hours, then discharged for at least 5 hours at 0.2C down to a 1.0V cut-off." You repeat the procedure up to 5 times until the rated capacity (measured during discharge) is reached.

[tunk]

What's the nominal and measured voltages of the battery?
And you may want to open it and measure the individual cells.

[RoGeorge]

Just charge them, and see if any of the packs are still usable.

If not, don't bother trying various cell recovery recipes found online.  They never work.  I've tried.

At most you can dismantle the packs and test each element individually.  Usually only 10-20% of the elements are bad inside a pack, the rest of the elements are still usable.  Replace only the most bad ones and thus you may end with a working battery pack.  This is usually very time consuming, ideally requires a spot welder, and there is no guarantee the resulting pack will last for long.  Expect the other cells to fail, too, sooner or later.

[SilverSolder]

Thanks for the suggestions.  I have opened the pack, separated the cells out, and am now using an external charger to try to refresh the cells one at a time.

Will report back on the results, this could take a while!

[Labrat101]

Thanks for the suggestions.  I have opened the pack, separated the cells out, and am now using an external charger to try to refresh the cells one at a time.

Will report back on the results, this could take a while!

I have also tried this process does not work that well 50/50 chance.
There is another way that does work and that is pulse charging . I used 10,000uf capacitor and pulsed
at 30 second intervals until the battery voltage starts to rise . then increase the pulse slowly.
If the voltage DOES not increase after about 5 minuets the battery  should be classed as RIP .
 And these batteries even if they do recover will Die again after a short time.
AS  RoGeorge said its better to replace all the batteries (cells) with new .
it will be well worth it in the end .