Anyway to resurrect some NiMH Cells

[kvk]

I have some NiMH AA cells that are a few years old. Most of them charge but about 1/2 dozen don't. I put them in the smart charger and the light goes red (meaning charging) for about 10-20 seconds. Then the light goes out (which means the charger doesn't like the cell; not sure why, maybe no current runs through it?). For the good cells, then charge for 12-16 hours and then go green.

Anyway to poke these back to health? Maybe give them a little jolt or something like that?

[purplepeople]

After about a year and half of not using them, a jolt is exactly how I brought some dead NiMH cells back to life, after reading about it a number of times on the web.

Warning... the following procedure is contrary to all manufacturers recommendations and may result in explosion or other danger. Anyone trying this accepts all responsibility for any property damage, injury or death that may result. Should you attempt this, use safety equipment, including goggles, gloves, grounded wrist strap, etc.

That said, using a 12V 1A adapter (transformer), I zapped each 1.5V cell positive to positive (and neg to neg) for as short a time as I could muster with my pliers. There will be a small spark. Wait some time before applying the normal charging routine. Et voila! Working cells. I have no idea how much charge is held vs. new, but they all take and hold a charge.

Good luck.

:)ensen.

[Ron E]

measure the voltage - I seem to recall that if REALLY discharged, they reverse polarity. They may be unrecoverable, or might only last half as long or less than normal and put stress on the better cells you might mix them with and accelerating damage to the poorer cell. Sort of like a spreading cancer.

I would just replace them. If it is significant money, try putting them on a non-smart slow charger, they might come partially back, or they might start a fire or something...

your mileage may vary, I am not a battery expert.

[Coulomb]

Actually what works really well and a lot safer than using an Arc welder to jump start them is, put them in a freezer baggie and throw them in the freezer overnight.

NiMH develop crystaline shorts overtime which reduce the battery capacity and "fool" the charger into thinking it is fully charged.

After removing from the freezer let the battery "thaw" for 12 hours, wipe off any condensation and recharge. I have rarley had this fail to bring back a NiMH. I do not usually bother with this process unless it is a custom pack size.

I have a Romba vaccum and after many recharges the "special" read really expensive, pack stopped recharging. One treatement and it has benn working good as new for several months now.

Eventually, like everything else, it will have to be replaced.

Anthony

[soongsc]

Originally posted by Coulomb
Actually what works really well and a lot safer than using an Arc welder to jump start them is, put them in a freezer baggie and throw them in the freezer overnight.

NiMH develop crystaline shorts overtime which reduce the battery capacity and "fool" the charger into thinking it is fully charged.

After removing from the freezer let the battery "thaw" for 12 hours, wipe off any condensation and recharge. I have rarley had this fail to bring back a NiMH. I do not usually bother with this process unless it is a custom pack size.

I have a Romba vaccum and after many recharges the "special" read really expensive, pack stopped recharging. One treatement and it has benn working good as new for several months now.

Eventually, like everything else, it will have to be replaced.

Anthony

Is using a freezer baggie necessary?
I tried some ordinary NiMH batteries, threw them in the freezer without the baggie, and let them sit out for 12 hours after that. Some will charge, but somehow they either never get full or cannot deliver power when the charger says it's fully charged.

[Coulomb]

Baggie is just to keep moisture of the surface.

It is possible that the batteries are just to far gone.

Anthony

[TroelsM]

Hi

Will the freezer-method work with NiCd also?

Regards TroelsM

[unclejed613]

Ni-Cd cells short out. best way to rejuv these is to take a large cap charged to 12V and discharge (observing proper polarity) across the shorted cell. this removes any Ni "needles" from between the electrodes. then you do several charge-discharge cycles to full voltage and discharging through a low resistance (to generate heat inside the battery and remove battery "memory"). the battery, while not as good as new, will at least be useable. the charge-discharge trick works also for sulfated gel-cell batteries as long as they're not too far gone.

[7n7is]

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[soongsc]

Originally posted by unclejed613
Ni-Cd cells short out. best way to rejuv these is to take a large cap charged to 12V and discharge (observing proper polarity) across the shorted cell. this removes any Ni "needles" from between the electrodes. then you do several charge-discharge cycles to full voltage and discharging through a low resistance (to generate heat inside the battery and remove battery "memory"). the battery, while not as good as new, will at least be useable. the charge-discharge trick works also for sulfated gel-cell batteries as long as they're not too far gone.

I used to open up my power amp, charge a cap with 70+ voltes, and the discharge it though the NiCd battery, works 90% of the time.