# NiMH with reversed polarity... fixable or toast?



## milkyspit (Jan 25, 2005)

I've got some 3AA NiMH lights that drive LEDs around the house. Given the low current drain (most of 'em are nightlights for the kids) I leave them running 24/7 until the cells become obviously weak in terms of light output.

However, this evening I noticed that one of the nightlights had gone completely DARK. (Not good!) /ubbthreads/images/graemlins/ooo.gif Upon further analysis, two of the cells showed voltages around 1.2V, but the third had reversed polarity, showing something like -0.04V. I know NiMH aren't supposed to be drained that far! /ubbthreads/images/graemlins/icon15.gif

Now that it's happened, though, do I have any reasonable hope of rejuvenating the messed-up cell and perhaps fixing whatever damage might have been caused internally, or is this cell doomed to never return to its normal self? Or put another way: is this damage correctable, or irreparable?

/ubbthreads/images/graemlins/eek.gif


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## jtr1962 (Jan 26, 2005)

Try recharging the cell and see what happens. -0.04V isn't severely reverse-charged. The cell might return to near full capacity after a few cycles.


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## bwaites (Jan 26, 2005)

I have got them to come back to about 1/3-1/2 capacity but not more.

A few people have said that they recovered them by shocking them with like 80-100 amps but I can't imagine how they did that and haven't tried it.

I just asked Ginseng about this last weekend and his experience is similar.

Bill


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## koala (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Common 12V automobil lead acid can do at least 40 amps. That should be more than enough to zap a single cell.


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## greenLED (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I have a NiMH with 0.00v. I've tried with my 3 chargers (including the hack on the Vanson SpeedyBox) and it's a no-go. Shocking it with a car battery might resucitate it? Thanks for the idea, koala. How long of a shock? just a tap, hold the contacts for a few secs?


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## oldtimer (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

[ QUOTE ]
*greenLED said:*
I have a NiMH with 0.00v. I've tried with my 3 chargers (including the hack on the Vanson SpeedyBox) and it's a no-go. Shocking it with a car battery might resucitate it? Thanks for the idea, koala. How long of a shock? just a tap, hold the contacts for a few secs? 

[/ QUOTE ]

Yikes! Is your life worth 75 cents? /ubbthreads/images/graemlins/grin.gif

Have you tried using a trickle charger?


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## MrAl (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Hello,

I've had cells reverse on me several times.
I've measured -0.2 volts across the cell when i take it
out of a light. Usually the other cells are still like
1.00 volts or 0.900 volts or something like that.

After recharging, they still seem to work, although i didnt
make any effort to measure charge retention. They seemed
about the same. Cant say anything about the cycle 
life however, which may depreciate because of the reverse
voltage.

I've found it's almost impossible to make sure NONE of
the cells reverse, at least for a little while because
every light i have has at least 2, 3, or 4 cells in it
and sooner or later the weakest cell runs down first.
Once that happens, it's reversal time.
I'd have to have a battery protector on every cell in
order to make sure this doesnt happen ever, and that's
just not practical for several reasons..for one thing
when you buy a light already made and it's fairly small
there's not usually too much space to fit protection
circuitry except maybe in the big lights, which i hardly
every buy.

BTW, for safety's sake, DONT USE a car battery to 'fix'
any cells. The power output is too great. If you want
to experiment with 'zapping' cells (provided they really
are dead and cant be recharged because they look like a
short) use a small wall wart (1 amp or less rating) and
charge a large capacitor with it, then while standing 
at least 8 feet away and with safety glasses connect the
cell with jumpers. Leave it connected only a second, then
try to charge it with a normal charger or measure the
voltage across it. If the voltage rises, you've got some
success.


Take care,
Al


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## bwaites (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

MrAl is correct, the reason that RC racers always buy matched packs is that they then reduce the risk of reversing one cell accidently.

The secret with NiMH cells is that you MUST turn them off when the light FIRST STARTS to go dim! That protects reversal from happening.

Bill


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## milkyspit (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Thanks MrAl, for putting things in perspective! Sounds like it might not be the end of the line for this particular cell. /ubbthreads/images/graemlins/smile.gif


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## milkyspit (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Forgot to ask you guys: I vaguely remember something about charging a series string of cells all at once, and that this technique tended to pull the lagging cells up to the same level as the others. Is there any truth in this, and if so, is that the type of thing a charger like the Triton is good for?


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## bwaites (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Milky,

Hasn't worked that way for me, and I use a Triton.

Bill


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## eluminator (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I'm betting it won't recover it's full capacity.

You apparently didn't have matched cells in that light. Usually if most cells are at 1.2v, the weakest won't be as low as yours was, if they were matched to begin with.

If you use those same cells again, the situation will probably be worse because that weak cell is now even weaker.

I always check the voltages before I use them. It's important to do this on new cells, and run the weak ones through some charge/discharge cycles until they reach full capacity (and voltage).


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## rdshores (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I hook another fully charged AA cell in parallel for a few seconds and then check the voltage on the dead one. Then repeat and check the voltage again. If the voltage comes up to over 1.0 volts, then it should charge ok in your charger.


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## jtr1962 (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Yes, in general I've found that any cell which can hold 1.0V or better after being zapped will charge OK. As to whether or not it will retain its capacity probably not.

Another problem is that the cell became reverse charged precisely because it had lower capacity than the others in the group. It likely lost even more capacity now even if it can be restored so it will become reverse charged even sooner. This is one reason I always run new cells through a few cycles, check the capacity, and try to match them wherever they'll be used. While an exact match isn't always possible, if they're close the first cell which runs low won't have the chance to become reverse charged before the others run down. I think for N cells the recommended practice is to stop the discharge when the total voltage is (N-1) volts in order to prevent reverse charging.


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## mrme (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

[ QUOTE ]
*milkyspit said:*
Forgot to ask you guys: I vaguely remember something about charging a series string of cells all at once, and that this technique tended to pull the lagging cells up to the same level as the others. Is there any truth in this, and if so, is that the type of thing a charger like the Triton is good for? 

[/ QUOTE ]

I don't know for sure, but I would suspect the weak cell would drag the others down to its level. 
If charged in a pack, the current will pass through all the cells. Once one is full, that's it for all of them.


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## greenLED (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

[ QUOTE ]
*b2p said:*
[ QUOTE ]
*greenLED said:*
I have a NiMH with 0.00v. I've tried with my 3 chargers (including the hack on the Vanson SpeedyBox) and it's a no-go. Shocking it with a car battery might resucitate it? Thanks for the idea, koala. How long of a shock? just a tap, hold the contacts for a few secs? 

[/ QUOTE ]

Yikes! Is your life worth 75 cents? /ubbthreads/images/graemlins/grin.gif

Have you tried using a trickle charger? 

[/ QUOTE ]

Woopeee!! :walks of to call the bomb squad: /ubbthreads/images/graemlins/yellowlaugh.gif Yikes! I didn't realize doing this would be that dangerous. I have no problem disposing of the dead cell now. Thanks for the heads up, guys!


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## PlayboyJoeShmoe (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

/ubbthreads/images/graemlins/icon23.gif

How does a poor man go about rating his cells?

I mostly mess with 2 cell stuff. CD Player, GPSr, Lights etc.

But I'll be getting into M*g11 and M*g85 soon....


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## eluminator (Jan 26, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

The way I charge the cells that have been reversed is to put them in my C. Crane charger. That charger will charge anything. 

I also use that charger to "rate" the cells. If I understand what you mean. I guess you could do the same thing with a voltmeter. I use cells from the same batch in a multicell device. I always keep them together. After charging, if the voltage of each one is within 1 or 2 hundredths of a volt, they should have nearly the same capacity. I use the analyze function of the charger to display volts. I suppose it measures voltage under load.


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## RussH (Jan 27, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I recently revived a reversed nimh AA cell that was at about -.06v. It wouldn't charge in my Vanson BC-1HU so I put it in my MAHA 2A4 dumb charger overnight. Since then it has charged and discharged normally. I only use my suspect batteries in my radio that has a low voltage cutoff. Unfortunately, I forgot to mark this one & I haven't found it because it hasn't gotten reversed again. I'm sure the capacity was at least somewhat reduced, so it will probably turn up again.

I used to 'ZAP' nicads with a 6volt battery charger - I don't know the amps but anything more than a touch would start to weld the clamp to the battery. Things get hot, where gloves and eye protection... I haven't tried it on NiMH and I doubt if it will work. Even on Nicads, it only brings the batteries back at 2/3 to 3/4 capacity, which is better than nothing. They only last a year or two afterwards.


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## eluminator (Jan 27, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I've read that NiCads will grow long thin needle like crystals called dendrites that eventually pierce the membrane and cause a short. A zap can melt the crystal and make the cell useful again. I'm also told that there will be more dendrites so the fix won't last long.

I don't know if dendrite growth is a problem with NiMH but I think the over-discharge problem is different and wouldn't be cured by zapping.

I'm told that slow charging can cause dendrite growth, but most chargers these days aren't that slow.

A bit of a low-powered zap might give the cell some charge and allow it to be charged in those stupid chargers that call themselves smart.


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## koala (Jan 27, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

There's a way to fix weak cells in a battery pack is to slowly trickle charge and fillup all the cells in a battery pack. The Triton can be set to do that with a small amount of current without the risk of overcharging. Trickle charging for long period will balance cells in a battery pack.

My apologies for suggesting a car/automobil battery for zapping cells, the idea just came up to me because the previous poster mention "80-100 amps" so a car battery seems to be a suitable candidate /ubbthreads/images/graemlins/grin.gif If anyone blow em self up that's your resposibility. /ubbthreads/images/graemlins/tongue.gif

The NiCd Pulsar.pdf is a good example of a pulse charger.

-vince.


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## matrixshaman (Jan 27, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

I didn't take time to read this whole thread but I would not stick it across a car battery - forget 40 amps - they can do more like 600 to 1000 amps for short periods. But if you equip yourself well you can stick them across a car CHARGER of 10 amps or even more and be safe - just put it across a few seconds to a minute, monitor with voltmeter - have on welding gloves, heavy coat and safety glasses - you get the idea - just in case and that will usually zap them - if it doesn't - toss it. I've never had one explode yet doing this but it certainly could happen so be safe if you try this.


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## Wingerr (Jan 27, 2005)

*Re: NiMH with reversed polarity... fixable or toas*

Even though a car battery can deliver that much current, the internal resistance of the battery will prevent it from actually drawing that much. So you won't really have youself an EMP device as seen on Alias - /ubbthreads/images/graemlins/smile.gif

The best way to try zapping open the internal short is using the capacitor Al mentioned; safest way because it will only deliver a short spike before discharging the cap back down, so it's self limiting. A car charger held on the cell for extended time may be just as dangerous as a direct battery connection, since it's working at a higher voltage and is still capable of pumping in some big current. If you must try that, just flipping the clamp onto the connection for a split second is all you need to do, extended time won't do anything more. You're not charging it, just trying to open up the short. 
Normal charging generally won't work because the internal short prevents it from doing any actual charging.


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