Reviving old NiMH cells using science?

I came across this scientific paper: https://www.mdpi.com/2313-0105/2/1/3/htm (full text and PDF are free, just click the link).

In the last section, it states

"After long-term storage, a few small-current charge/discharge cycles can bring back some of the lost capacity in Ni/MH batteries [322,323]. A more complicated method proposed by Li and Meng [324] involves 33% SOC small-current charge, high-temperature storage (45–60 °C for 20–24 h), and a small current charge/discharge cycle to restore at least part of the lost capacity. Its strategy is to redistribute the Co-conductive network that was destroyed during storage."

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Does anyone have any information about this?

I couldn't retreive the original paper that describes the 33% SOC method, however I have this anecdote from personal experience:

I had a pair of Sony AA 1800mAh NiMH batteries, about a decade old, that had degrated to the point where a charging voltage of 2.5V was needed just to force 0.5A through them. Internal resistance measurements were around 1500mOhm (Opus BT-C3100). Running the refresh cycle on the Opus at 200mA (it does three charge/discharge cycles per refresh) didn't improve the performance.

I put it on my hobby charger, gave it 2000mAh at 0.5A until the cell reached 60°C, discharged it at 1A down to 0.3V (tapering off down to 0.1A at 3V at the end of charge), then charged it at 100mA for 16 hours and did another 1A discharge. At the end of this, the internal resistance fell to 1100mOhm, and the charging voltage for 0.5A charge current fell to 2V (down from 2.5V). This improvement was after 10 charge/discharge cycles at 200mA on the Opus.

There seems to be some merit to the method described in the paper. Holding the battery at 60C isn't difficult using a high-current charge while the cell is in some sort of makeshift insulator (like a clean rag). Low-rate cycling is easy with a hobby charger. I'm not sure what they mean by 33% SOC - it could mean cycling 33% of the total capacity in and out (594mAh for my 1800mAh cell), or it could mean cycling the battery up and down keeping the average charge level at 33% (e.g. cycling repeatedly between 23 and 43% full).

Anyway, let's start a discussion - does anyone else have some experience or methods that worked for them for reviving old NiMH cells?

It's interesting if just playing around. Though it doesn't sound like you got much improvement out of the cell. I'm not sure there's any practical use for trying to revive a cell that is so bad.

I have a charger that does brief discharge pulses during its regular charging. They claim it improves battery performance. It does seem to do a decent job on very old cells, though that's because other smart chargers just quit when they see the cell has high IR. I don't think it does any better than a dumb charger.

Hello LMF5000,

In my limited amount of investigation on this it seems to me that the usual cause is a drying out of electrolyte in the cell. I like the suggestion of opening up the cell and pouring more electrolyte in. In my case that is unpractical, but it seems to be a good idea.

My usual procedure is to start with a Break In charge (0.1 C for 16 hours) followed by a 0.2 C discharge. This gives me an idea of how damaged the cell is. The next step is a series of 1.0 C charges and 1.0 C discharges. This tends to warm the cell up with the idea that the electrolyte may distribute evenly.

Sometimes this works...

Tom

SilverFox said:

Hello LMF5000,

In my limited amount of investigation on this it seems to me that the usual cause is a drying out of electrolyte in the cell. I like the suggestion of opening up the cell and pouring more electrolyte in. In my case that is unpractical, but it seems to be a good idea.

My usual procedure is to start with a Break In charge (0.1 C for 16 hours) followed by a 0.2 C discharge. This gives me an idea of how damaged the cell is. The next step is a series of 1.0 C charges and 1.0 C discharges. This tends to warm the cell up with the idea that the electrolyte may distribute evenly.

Sometimes this works...

Tom

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Agreed, a lot of my "bad" cells are ones that have been cooked by dumb chargers that continue pumping 200-400mA of current into them long after they're charged. There's usually some discolouration around the vents near the positive terminal where the electrolyte was vented out.

I've attempted to drill a small hole in the bottom of some cells and inject some distilled water in to replace the lost electrolyte (2-3 ml). Sometimes their performance improves drastically, but I haven't figured out how to seal the hole properly. Soldering is hard because when the cell gets up to soldering temperature the water inside starts boiling and outgassing from the hole so the solder is blown out. When I did manage to solder it, the electrolyte corroded the solder after a few weeks and I didn't want to risk having the cells leak into my devices so I threw away the cells.

Do you have any ideas for refilling AA or AAA cells?

Hello LMF5000,

Sorry, no. I just recycle them and replace them.

Tom

I have some Tenergy Centura LSD NiMH AAs that are acting strangely, even though they are new, and wondering if they are worth keeping. About 16-17 months ago I bought a couple dozen of them (6 packs of 4 cells), sold by Tenergy Corporation and fulfilled by Amazon. I broke open two of the packs right away and put them on the Maha Powerex C9000 charger to top them off. All seemed fine and they didn't take much charge (I forget how much) except for one cell that got really warm and took maybe 2600 mA before terminating. That was weird. A few months later I opened another pack and, again, one cell took a whale of a charge. When recharging these cells (mixed around, not paying attention to the specific ones) every so often I'd have a late terminator again.

Fast forward to 2 weeks ago. I opened another pack and put them on the C9000. ALL of them got really hot and took anywhere from 3400 to 3800 mA of charge before termination! I thought, whoa, is my charger acting up? So I opened pack #5 and popped them into my Gyrfalcon All-44; it doesn't give a mA readout, but again all 4 cells were still charging after 2 hours and were really hot! I unplugged the charger to let them all cool. They weren't so hot that I would have burned myself, but they were uncomfortable to handle.

I wrote to Tenergy and they blew it off, saying they don't know anything about the quality of my charger and it's normal for cells to get hot during charging. I wondered, could my charger be malfunctioning? So I put 4 other AA NiMH cells in to charge: 1 Eneloop, 2 Eneloop Pro, and 1 (very old) Rayovac 4.0. Results: none got more than lukewarm. All terminated appropriately, and their accepted charges ranged from a low of 111mA (done in 7 minutes) to 561mA (37 minutes).

Next I broke open my last sealed 4-pack of Centuras. My fluke meter showed them all to be a hair under 1.26V. I did a discharge. All 4 had remaining charges that were very low for alleged 'low self discharge' cells: 483, 514, 492, and 503 mA respectively. I'd have expected about double that. Tenergy claims 85% retention after 1 year of storage and the package says, "READY TO USE – All batteries are pre-charged and ready to use out of the package." My next step was to run these 4 through a break-in cycle. This cycle showed capacity numbers in the 1710-1800mAH range. A follow-up discharge was carried out at 700mAH rate and the capacities came up another 30 to 50 mAH each, (1799, 1833, 1759, 1822) which seemed normal. Then I charged them at the normal 1000mA rate; 3 of the 4 Tenergy Centuras terminated in a normal fashion. Each one took 13% to 16% higher input than their last discharged capacity, which is completely normal because some energy is lost to heat. But the cell in bay #3 took 2678mA to terminate! (Note: this was the lowest capacity AA of the batch, 1759mA at previous discharge.)

After this I ran all my other Centura cells through a 500mA discharge and a 1000 mA recharge, and separated out the high terminators. I now have a pile of 7 Centuras that terminated late after accepting anywhere from 2600mA to 4100mA of charge. The tardy terminators get unusually hot on the charger.

I came across a post on BLF from someone else who'd had similar trouble with Centura LSD AAs: http://budgetlightforum.com/comment/991547#comment-991547 Are these 7 cells worth messing with, or am I better off tossing them?

Rexlion said:

Are these 7 cells worth messing with, or am I better off tossing them?

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Toss them. I haven't had great success with Tenergy Centura 9v NiMH batteries, either. At best, I can rate them "meh" after a couple of years of use. Great the first year, but they don't last.

For AA and AAA, stick with Eneloop.

I guess I'll run them down in devices one final time and then throw them away. I've used a fat black permanent marker to stripe over the "Tenergy" name so I don't forget which ones. And I just got 8 fresh Eneloops to replace the 7 duds.

Rexlion said:

I guess I'll run them down in devices one final time and then throw them away. I've used a fat black permanent marker to stripe over the "Tenergy" name so I don't forget which ones. And I just got 8 fresh Eneloops to replace the 7 duds.

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I just run a couple of charge/discharge cycles in my Maha C9000, If there is NO improvement i bin them, Eneloop Pro might get a break-in as they are quite expensive that takes like around 17hrs.

But my record of recovery of nimh cells is quite poor.

Stick them in a remote control even bad cells can run for months.

I would say replace them, You don't want them to fail in an emergency.

John.

So quick update, I've had some success with one of the cells I recovered from my old walkera 22e electric helicopter. It had a 9-cell 650mAh battery pack. Each sell is the size of an AA battery.

For my testing I chose the worst cell, this one had an internal short so terminal voltage would read 0.0V after a few days. The internal resistance measured some 8,000mohm - it needed 2-2.3V just to get 0.1A through it, and it was impossible to get any decent discharge current out of it.

The process:

I used a tiny 1mm drill bit to drill a hole in the base, stopping right as I punched through the metal. Then I added water to the cell. First I tried a syringe, but that quickly got tiresome, so I immersed the entire cell in a jar of tap water and left it there for half an hour. After this time, the weight of the cell had increased from 11g to 12g - a 10% increase. I then tried to seal the hole with superglue, which wouldn't bridge the gap, so I added a piece of tissue paper soaked in superglue. That seemed to hold.

A charge/discharge test at this point showed marked improvement. IR was down to 220mOhm and the cell was absorbing 0.7A of current at just 2V. However brown liquid and a lot of gas bubbles were coming out of the hole - the superglue and tissue seal had failed.

I mixed up a small batch of araldite (5-minute two-part epoxy) and put that over the hole, left the seal to cure for 24h, then tested again. There's still a small leak where one of the original spot welds were, but it's minor. I'm still cycling the cell. It's gone from basically dead to having a discharge capacity of 120mAh (0.21, 0.7V termination voltage) thanks to just the addition of tap water.

Some very encouraging results so far .