# Crystal removal in old NiMH batteries



## desirider (Apr 10, 2012)

Hi folks,

I am a new member and this is my first post to the forum. I have scoured the forum and the net, but I still have some questions.

I am trying to restore a set of old Energizer NiMH batteries as a "project". The batteries were stored for about 5 years and I have been using them again for the past one year. I use the charger that came with it (http://www.energizer.com/charging/rechargeable-battery-charger/Pages/compact-charger.aspx) and they work OK in a set of portable speakers.

I recently got the MAHA C9000 and it rejects these batteries due to their high impedance. MAHA's impedance check voltage is about 2.45 when I plug in these batteries. I briefly shorted the batteries to warm them slightly and impedance check voltage dropped to 1.96 volts and I could charge and discharge them with the C9000. MAHA reports a discharge capacity of around 1200mAh. I can see they have quite some juice left in them as the discharge was cutoff early due to the high internal resistance. MAHA rejects them again after they cool down.

After discharging as much as the C9000 could handle (100mA down to 0.9V), the open circuit voltage was 1.25V and the short circuit current was about 100mA. As a test, I left one of the batteries shorted for several hours hoping to break the crystal formations but the short circuit current was still 10mA. When I remove the short, the open circuit voltage bounces back to 1.21V. I have been trying to run them down for more than a day now and the SC current is still 10mA and the OC voltage still bounces back to around 1.2V.

Does this mean the battery still has some juice left? Is the long 10mA discharge due to heavy crystalline buildup? Is it OK to let it continue discharging at 10mA? And lastly, when I charge it back should I use a high current (0.5-1C) or a low current (0.1C)? MAHA uses 0.1C for the break-in cycle, but I read here that slow charging encourages large crystal formation.

Thanks,
Desirider.


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## SilverFox (Apr 11, 2012)

Hello Desirider,

Welcome to CPF.

Crystal growth is associated with NiCd chemistry. NiMh chemistry may have similar symptoms, but it is usually caused by electrolyte starving or oxidation.

Cycling the cell through charge/discharge cycles or using the Break In function can redistribute the electrolyte and in some cases the performance of the cell improves.

High internal resistance is usually caused by oxidation damage to either the electrolyte or the electrodes, or both. This is not something that can be repaired and it is time to recycle the cell and replace it.

You mentioned that you are seeing a discharge capacity of around 1200 mAh. If the original capacity of the cell was 1500 mAh, then it may be worth spending some effort on the cells, but they are near their end of useful life. If the original capacity is in the 2500 mAh range, you will just have to accept that those cells are beyond their normal life.

I call those crap cells.

Think of a crap cell like a battery that will no longer start your car. It still will register some voltage, and may have enough power to run the lights or the radio for a period of time, but it won't perform in its intended function.

If your only purpose in having a car is to turn the lights on now and again or to listen to the radio, then the crap battery will work fine. However, your use will be limited.

The same goes for crap cells. You can measure a voltage and they do have some capacity, but they are no longer performing according to the manufacturers specifications. Chargers that check for internal resistance will refuse to charge them. If you use two or more of the cells in series, they will be mismatched and you will end up with a under performing use.

Some people don't want to dispose of a cell that measures any voltage on it, so there are those that search to fit the crap cell to an application that can tolerate it. This sometimes works, but once the cell has degraded to crap status, it usually continues to degrade so life expectancy is limited and not uniform.

Many people have formed bad attitudes about rechargeable cells because of their efforts to try to use crap cells. 

In general a crap cell is a cell that has its capacity reduced to below 80% of its initial capacity, is unable to hold voltage under load, or has developed a fast rate of self discharge. When those conditions are met, it is time to recycle the cell and replace it.

Tom


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## 45/70 (Apr 11, 2012)

Hi desirider. If you read about "large crystal formation" here on CPF, I may be partly to blame for the misunderstanding. As Tom said, this is actually a condition that occurs only with NiCd cells.

That said however, when NiMh cells sit for a long period of time, the grid of the electrodes can become, as it was described to me, "clogged". The symptoms and possible cure for this are practically identical to those associated with abnormally large crystal formation in NiCd cells. Perform several slow discharges, followed each time by recharging the cells at either a moderate rate (0.5C), or a forming charge rate (0.1C for 16hrs).

As Tom mentioned, the forming charge rate will help to redistribute the electrolyte throughout the cell a bit better than a faster charge rate. It all depends on whether this is a part of the problem, or not, but it doesn't hurt anything.

Also I'll through in that if the Energizer cells that you are attempting to restore are the old "2500mAh" version, the cells are probably dead. These cells were never very good to start with. They exhibited high self discharge rates, short life, and generally poor performance, out of the box.

Dave


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## desirider (Apr 11, 2012)

Thank you for your detailed responses gentlemen. These are 1850mAh Energizers.

I was a little unsure about the crystal build-up. One of the symptoms of crystal build-up I read here was the fluctuating brightness of a light bulb connected across the battery during a deep discharge. I never saw this happen with my batteries.

Anyway, this was just a project and was worth a try. I learned a thing or two. These are definitely crap cells now. I have several dozen Eneloops powering my toys (flashlights and flashes mostly) and my kid's toys, so I won't miss the Energizers.

Regards,
Desirider.


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## 45/70 (Apr 11, 2012)

You might still give them a try. The "traditional" (non-LSD) variety NiMh cells of 2000mAh, or less capacity typically, are of a more robust construction, than the higher capacity cells. It still may be worth looking into.

My experience with rejuvinating old NiMh cells has mostly been unsuccesful though. They often come back to life somewhat, only to return to there original unsatisfactory condition after a few use cycles. On the other hand, I've never tried it with cells that have been sitting idle for years. Most of my rechargeable NiCd/NiMh cells are cycled in use, at least several times a year. I would think that cells that have just been sitting for a few years, might have a better chance of being restored.

Then again, nickel based rechargeable cells in general like to be used, not stored away. They are more or less a "use them, or loose them" proposition. Fortunately, with the newer LSD cells, this doesn't hold true, as much anyway.

If you cycle the cells and see no improvement with the first few cycles, then the cells are toast. I've often said that charger analyzers such as the Maha C-9000, are better at preventing your cells from falling into a degraded state, than actually reviving them. Still, if you want view this as a "project", as you said, just to see what happens, give it a shot.

Dave


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## LMF5000 (Apr 13, 2012)

I don't have anything to add regarding the revival of dead batteries (I'm more of a lead-acid desulfating kind of guy), but many are mentioning 80% of stated capacity as a cutoff point. It's funny how none of my batteries come anywhere near the label capacity during discharge tests. Maybe it's due to bad luck, or maybe poor technique. 

The closest I ever got were with real eneloops, the worst are Energiser or (*shudder*) GP batteries. Some examples, all using an Imax B6:

Brand new 2450mAh energisers. Bought in store, taken out of pack, charged straight away at 2A, then discharged at 1A to "auto" cutoff voltage - 2150mAh on discharge

Four GP batteries, in series. Wrapper said 2700mAh. Less than 20 cycles on them. Charged at 1A in automatic mode (took 2 hours 30 minutes to charge), discharged at 1A to 3.7V total pack voltage - gave 2020mAh on discharge. Charging at 2A and doing the same discharge gave 2080mAh. Charging at 0.3A and doing the same discharge gave 2040mAh. 

Charging at 1.3A and this time discharging at 0.5A down to 3.7V gave 2160mAh.

A set of 8 eneloops in series. Sticker capacity of 2000mAh, less than 5 cycles on them. Gave 1822mAh at 0.4A down to 7.4V for the pack.

Some hong-kong batteries, bought on ebay for £0.99 for a pair - sticker said 3000mAh (which was obviously fake because they felt VERY light in the hand) - gave 600mAh at 1A down to "auto".

I've had more success with Sony branded AA batteries, but I seem to have misplaced the numbers for those right now.

And the self discharge on normal NiMH cells is horrible. I don't use them much anymore, because I store them charged but when it comes to using them, at least one cell is always dead enough to not power the device. I've since switched to low self-discharge ones (mainly the "instant" brand on ebay, because they're much cheaper than eneloops), and now 18650 lithiums for devices that can take them.

I much prefer 18650s. They're so gratifying to charge - they don't get warm at all during charge, and the charger never misses the correct cutoff point. The self-discharge is negligible and they can take a decent charge rate. And most of the capacity that goes in during charge comes back out during discharge (as opposed to losing over 10% of the energy with Ni-MH). Though having said that, I've had some mixed experiences with the capacity of ebay cells too.

The famous trustfire cells with flames on the sticker - claimed 2400mAh, actual 2200mAh at 1A discharge rate (to 3V). Best cells ever tested.

The infamous "3000mAh" ultrafiires with red/silver wrapper and black lines - 600mAh at 1A (sent back for refund the next day)

A pair of blue batteries labelled "S.S. battery" - claimed 3000mAh, one gave 660mAh and the other 800mAh at 1A (got a refund, seller never asked me to return them because the description was so obviously flawed).

6 cells scavenged from my own 5-year-old laptop battery that wouldn't take a charge as a pack - two of them were leaking and dead shorted. The other four had over 600mAh each, at 1A discharge.

Moral of the story - always take sticker values with a giant pinch of salt. Unless you discharge them at like 0.2A. But where's the fun in that? :naughty:


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## LMF5000 (Apr 13, 2012)

desirider said:


> Hi folks,
> 
> I am a new member and this is my first post to the forum. I have scoured the forum and the net, but I still have some questions.
> 
> ...



P.S. quick thing I forgot to add. If you're doing a lot of reviving of dead or seemingly-dead batteries, I suggest getting a hobby charger. Something like the imax B6 (possibly the cheapest computerised all-battery-chemistries charger on eBay). It's smart enough to charge things well, and it checks open terminal voltage before charging (so you don't blow up lithium or lead based batteries)... but, and here's the important part, it's dumb enough not to care if the batteries have high internal resistance or funny characteristics. So instead of refusing to charge them like a smarter charger like the maha would, the B6 still forces them to charge.

True, you might be flogging a dead horse in 99% of cases that won't charge on the Maha, but at least with a hobby charger it at least lets you try. The other option would be to use another battery, or a CC/CV power supply...


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## desirider (Apr 13, 2012)

Thanks for the hobby charger tip, LMF. I have already been reading up on a lot of things. I'll add Imax B6 to the weekend reading list 

Yes, sometimes I also feel that smart chargers like MAHA are a little too smart. I would certainly like them to handle cells with a little higher resistance and discharge them to maybe 0.8V. But I guess "crap" cells are not exactly their target market.

Desirider.


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## LMF5000 (Apr 13, 2012)

desirider said:


> Thanks for the hobby charger tip, LMF. I have already been reading up on a lot of things. I'll add Imax B6 to the weekend reading list
> 
> Yes, sometimes I also feel that smart chargers like MAHA are a little too smart. I would certainly like them to handle cells with a little higher resistance and discharge them to maybe 0.8V. But I guess "crap" cells are not exactly their target market.
> 
> Desirider.



Welcome. Just a little info - there's also the imax B6AC with (you guessed it) comes with a built in AC adapter!

And the next step up (in terms of price and performance) is the imax B8 which can take a higher wattage. The B6 is limited to 5 amps charge, or 50 watts total output, whichever is lower. The only time I hit those limits is when I charge my RC car's batteries (two 11.1V 3Ah lipos) or a full size car battery (12V 42Ah). So probably a B6 is fine for your needs.

There's also a charger which contains four B6's in the same unit and has four separate displays, four sets of buttons and four outputs. Useful for doing four things at once. No idea what it was called, but it'll be easy to find.

Oh, and if you're wondering if it has any drawbacks, the biggest one is that the resistors the factory puts in for the charger to be able measure cell voltage when balancing lipo packs have too high a tolerance in their value, i.e. there's quite a big variation. The charger can handle up to 6 cells, so it should have 6 identical resistor, but this is far from the case, so when you try and balance lipo packs with it you'll find that it actually ends up more unbalanced than when you started! The unbalance is up to 0.05V in some cases. Mine has this problem too, so I never balance charge, and to actually equalise the cells I just give the cells a top-up charge individually one by one by connecting the charger to one cell's pair of pins on the balance connector, charge it up, move on to the next cell and so on. I only repeat it twice for two-cell packs and 3 times for 3-cell packs. If this last paragraph didn't make any sense whatsover, then you don't need to worry about anything contained in it because it won't affect you unless you balance charge li-po packs (which is what I'd bought the B6 for in the first place) :thumbsup:

Edit - just checked, the B8 can take 150 watts or 7 amps (whichever is lower at your charging voltage), but whilst the cheapest B6 is $21.50 with adapter, the cheapest B8 is $83 and it doesn't even include the AC adapter...


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