Are these AA white Eneloops dead?

[lumen aeternum]

I put 6 into a worklight that is in my tool bag. Been there for months, probably somehow got turned on & drained the batteries. 1 is charging, the rest are flat at 0.00V. Sometimes leaving them alone for a while gets them back on scale, but its been a while, I think.

Do NiMh batteries have a low-voltage safety cutoff to avoid ruining them?

 

[jtr1962]

I put 6 into a worklight that is in my tool bag. Been there for months, probably somehow got turned on & drained the batteries. 1 is charging, the rest are flat at 0.00V. Sometimes leaving them alone for a while gets them back on scale, but its been a while, I think.

Do NiMh batteries have a low-voltage safety cutoff to avoid ruining them?

NiMH can be drained all the way to zero. Reverse voltage is what destroys them.

Sometimes the ones stuck at zero can be revived by briefly putting a few volts from a power supply across the terminals.

 

[vicv]

NiMH can be drained all the way to zero. Reverse voltage is what destroys them.

Sometimes the ones stuck at zero can be revived by briefly putting a few volts from a power supply across the terminals.

I think you're thinking of nicd. Nimh definitely get damaged from over discharge. Not as bad as lithium ion, but damage non the less

 

[alpg88]

simple way to test, charge them, see if they take charge, but monitor them for heat if they get too hot, throw them away. IDK where you are, but ikea sells rewrapped eneloops, ladda 1900

 

[lumen aeternum]

Sometimes the ones stuck at zero can be revived by briefly putting a few volts from a power supply across the terminals.

Do you mean briefly putting it in series with something like a fully charged 1.5v or 12v battery?

Or like with Lion batteries, momentarily putting them in series with the terminals reversed, in order to release the low-voltage shutoff ?

I've had them in the charger for 24 hours; sometimes when I push the start button and it still reads 0 volts, after a while they come up & take a charge. But as of now, 4 eneloops are still dead. They were new when I put them into the light!

 

[jtr1962]

Do you mean briefly putting it in series with something like a fully charged 1.5v or 12v battery?

I mean putting in parallel with either a battery or power supply (+ to +, - to -). A fully charged 1.5V cell might work. Sometimes you need a larger zap from something like a 5V or 12V power supply. Just briefly connect the cell for a second or so. Don't leave it connected or it could overheat. Another option is charging a large capacitor to perhaps 12V, and then connecting it in parallel with the battery. This will give a brief high current spike, but the cap won't have enough energy to overheat the cell or cause any other issues.

I've had them in the charger for 24 hours; sometimes when I push the start button and it still reads 0 volts, after a while they come up & take a charge. But as of now, 4 eneloops are still dead. They were new when I put them into the light!

If they don't take a charge after zapping them as I described a few times, just recycle them. Usually the zapping trick works within a few tries. If it doesn't by then, it's never going to.

 

[captianworkbench]

A lot of chargers will not charge a battery that is below a certain voltage. Sometimes they are too smart for their own good.

As mentioned, somehow getting a bit of energy into the cell in order to bring up the voltage at the terminals is necessary for the charger to recognize the cells.

If you have an adjustable bench power supply, that would be useful. Otherwise, an old 5V wall wart could be used.

Lastly, if you have an old dumb "overnight" trickle charger that is just timer based, it would probably work. They are low current and once activated they are often just a plain old constant current source.

If the cells have been held at 0V for too long then they will be toast. If you can revive them, you most likely will have lost at least a little bit of performance.

-CWB

 

[Dave_H]

If a cell is really dead, sitting at or close to 0v, or has too high internal resistance, attempted charging does not make sense.

I recover used NiCd/NiMH from recycle, and the 0v ones go straight back to recycle. Many others are still usable, to various degrees.

I recall using a "fruitcan" capacitor 78000uF charged up to 12v or so to "zap" NiCds. It had limited success. A cell needing this will always be weaker than other good ones in series so it's usually the end, or beginning thereof for it.

Dave

 

[WC8KCY]

I just don't get the whole "zapping cells" thing.

Putting zero-voltage NiMH cells in a 200 mA dumb charger for fifteen minutes raises their voltage sufficiently for my smart chargers to finish the job. Easy peasy--and it's safe.

 

[Dave_H]

I just don't get the whole "zapping cells" thing.

Putting zero-voltage NiMH cells in a 200 mA dumb charger for fifteen minutes raises their voltage sufficiently for my smart chargers to finish the job. Easy peasy--and it's safe.

NiCds could suffer internal shorts which sometimes could be "blown out" by a high-current zap.

If the cell is dead and shorted, no amount of charging is likely to help. Try for 15 mins as you said, if still zero or close to, it's gone.

Dave

 

[WC8KCY]

NiCds could suffer internal shorts which sometimes could be "blown out" by a high-current zap.

If the cell is dead and shorted, no amount of charging is likely to help. Try for 15 mins as you said, if still zero or close to, it's gone.

Dave

Agreed, Dave.

To me, zapping a cell should be a "when all else fails" last resort. The web is littered with recommendations of zapping cells as the first thing to do, which I cannot agree with.

 

[Monocrom]

Another vote for last resort only.

 

[IMA SOL MAN]

Yep, seems like I read somewhere that they develop little "whiskers" or crystallization that cause shorts, and the zapping destroys the little whiskers.

 

[lumen aeternum]

I set the 4 bad ones aside for a day. Now three have about 0.80 V on my external battery tester, they seem to be taking a charge now. The fourth reads 0.22 V and is not taking a charge, but I think the Gyrfalcon charger is still trickling to bring it up beyond 0.00 on its readaout. Maybe it takes a long time after removing from the trickle to recover the chemistry ?

 

[WC8KCY]

Giving your cells a day's rest was a good idea. In my experience, trying to recondition NiMH cells discharged as yours were is best done by:

Days 1, 3, 5, and 7: One charge cycle
Days 2, 4, and 6: One discharge cycle to 0.9V

Honestly, though, it's not looking good for that fourth cell holding at 0.22V. I'd wager that the cell ended up reversing, and after reconditioning, its capacity will be down significantly compared to the other three.

 

[lumen aeternum]

Giving your cells a day's rest was a good idea. In my experience, trying to recondition NiMH cells discharged as yours were is best done by:

Days 1, 3, 5, and 7: One charge cycle
Days 2, 4, and 6: One discharge cycle to 0.9V

Honestly, though, it's not looking good for that fourth cell holding at 0.22V. I'd wager that the cell ended up reversing, and after reconditioning, its capacity will be down significantly compared to the other three.

The 3 seem good, the last is going on the charger once more tomorrow.

 

[lumen aeternum]

Those last three stayed dead.