Forming charge for old NiMH cells

[WC8KCY]

I am putting some long-unused NiMH AA cells back into service.

After six Discharge Test (Charge-Discharge-Charge) cycles in my Opus BT-C2400, I've gotten 8 cells back to just over 1000 mAh in capacity; 4 of them are Polaroid 2100 mAh from around 2007, and 4 are the dreaded Energizer 2500 mAh cells from around 2005.

I would like to perform forming charges on these cells (after discharging to 0.9V on the Opus) using my dumb chargers, the slowest of which outputs 100 mA.

Question: Should I...

• do a forming charge based on the current observed capacity of the cells (16 hours @ 100 mA)
• do a forming charge based on the labeled capacity (16 hours @ 210 mA for the Polaroid; 16 hours @ 250 mA for the Energizers)
• charge at a 100 mA for Panasonic's recommended maximum slow-charge time of 20 hours.
  

It occurs to me that putting 3360 and 4000 mA into aged cells that currently only hold 1000 mAh or so would result in damaging (and possibly dangerous) overcharge, but putting 1600 mA into these same cells may not provide enough "exercise" to regain lost capacity.

Thanks in advance....I couldn't find a definitive answer to this here at CPF using the Search function.

Regards,
Greg

 

[WalkIntoTheLight]

Honestly, you should just throw them out. Not only are they less than half their original capacity, but I bet their internal resistance is high and self-discharge is probably crap too.

If you're going to do it, use a slow charge of 100mA, and do it for at least 20 hours. Your results will not be good, though.

 

[Katherine Alicia]

Honestly, you should just throw them out. Not only are they less than half their original capacity, but I bet their internal resistance is high and self-discharge is probably crap too.

+1

Treat yourself to some Eneloops!

 

[Lynx_Arc]

I agree 100% with WITL as I've done hundreds of refresh cycles on my lacrosse and done the 100ma charing all day aned night and the returns on a 2100mah nimh that was damaged by reverse charging it in use in series and a dozen Energizer 2500mah I spent hundreds of hours and still ended up with cells that were only useful in low drain and low output devices. You are better suited to spend 1/3 of the time and effort out mowing a neighbor's lawn and using the money to buy new eneloops than the fruitless efforts of trying to "save" nimh cells especially non LSD types.
It is because of all this effort that I've gone to lithium ion 18650s for most of my stuff whenever possible.

 

[Burgess]

Totally agree with the helpful suggestions
you've already received above !


I'm still using (every single day !)
some Sanyo Eneloop AA's which are now
14 YEARS OLD !
(bought in 2007, mfg. in 2006)


As well as plenty of newer Panasonic eneloops.


Can't go wrong here --
Provided you avoid buying "counterfeits" ! ! !


< just sayin' . . . . >

:thumbsup:
_

 

[turbodog]

If this is all academic, then go for it. Otherwise, buy new cells.

I've revived some r/c nimh packs back in the day. I put them on the battery cycler, set for 10 cycles, and walked away. I would lean toward a more rapid charge/discharge cycle... maybe around 1 hour for an 'aa' cell. Would also discharge to around .5 volt if you can do single cells at a time.

 

[WC8KCY]

Thanks for the insights, everyone.

These cells are being reactivated to replace dying primary cells in remote controls. I wouldn't waste your time or mine inquiring about this if these cells didn't show promise of being up to that task. The four Energizer 2500 cells were last charged in April, and upon discharging yesterday to 0.9V at 100 mA, my Opus analyzer pulled 67, 241, 440, and 680 mA out of them. At least two will be satisfactory for remote control duty even if I can't improve their capacities.

I went the conservative route last night and charged the Energizer 2500 cells for 16 hours @ 100 mA. Discharging them today @ 200 mA to 0.9V yielded 1311, 1332, 1478, and 1618 mA.

Those are better numbers than I've ever seen from trying to rejuvenate them using the Charge/Test and Refresh modes on the Opus. Perhaps the dumb charger I used is putting out more than 100 mA, or the best two cells are 'vibrant' for their age.

A couple of weeks ago, I was able to rejuvenate the 2004-vintage 1900 mAh battery pack in my Fuji A340 camera to 1780 mAh using the Opus BT-C2400. I also toasted a couple of Polaroid 2100 mAh cells into useless oblivion. You just never know with old cells.

If I can get these old AA cells past 1500 mAh, they could be useful for applications beyond remote control duty. Since a forming charge already improved them, it seems worth a shot to keep trying.

 

[markr6]

Good timing. Just this morning I made a mental note to "refresh" all my Eneloops from about 2009-2016. I didn't plan on any specific procedure, just run them on the MH-C801D using the conditioning mode once or twice.

 

[WC8KCY]

...I would lean toward a more rapid charge/discharge cycle... maybe around 1 hour for an 'aa' cell. Would also discharge to around .5 volt if you can do single cells at a time.

With the Polaroid 2100 mAh cells, I've run them through a 1A charge/500 mA discharge on a different charger that doesn't throttle back the amperage as the Opus will do.

Discharge capacity remained about the same, but internal resistance did improve.

If the cells end up gaining enough capacity from reconditioning to merit use beyond remote controls, I'll further recondition them in the 1A charger to minimize internal resistance.