Overly sensitive NiMH battery chargers

[RoGeorge]

For NiMH is either delta V = 0 or a jump in temperature, since the energy pumped into battery is no longer stored as battery charge, but wasted in heat instead.

[Rick Law]

First, As said already, battery with low voltage is damaged battery so you cannot expect it to be "normal."

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Just because it's been said doesn't mean it's true.
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I was a little surprise at this statement initially...  Thinking it over, it may NOT be a binary disagreement but disagreement to a matter of degree.

I think you would agree that keeping an NiMH at low or negative voltage is not a good thing for the battery.   If so, that implies it harms - to some degree: from very little damage to fatally damaged.  That matches my personal experience.

So, I should rephrase that state as "That battery may be damaged to some degree...  The hope is that it is not damaged to the point of uselessness."   If it was possibly damaged, it just make good sense to be extra careful in the first re-charge and until one is comfortable that it can sustain normal use and normal recharge.

EDIT: forgot a "NOT" in the line, which kinda mess things up.

[IanB]

You have two options:

A battery which is old, degraded or damaged may self-discharge down to zero in storage, so that when you measure it it has no voltage. From this you would draw a conclusion that the battery is probably bad and discard it, especially if it had a good voltage when stored.

On the other hand, a good battery may be discharged down to zero by a device it is installed in. This says nothing in itself about the battery, it just says the battery has been mistreated. You may charge the battery up again and it may be fine.

A third option is you may intentionally discharge a battery to zero volts and leave it that way. In the case of NiCd chemistry this is believed to do no harm. In the case of NiMH the situation is less clear. Some people have discharged Eneloops to zero volts and stored them that way for a considerable time, and on later testing found no loss of performance. Other people have claimed otherwise. So it is hard to draw a conclusion.

In my own experience, occasionally draining an Eneloop down to zero volts does not seem to be a problem. So I do not worry too much about it.

[james_s]

I have never seen any noticeable damage from draining a NiMH battery down to near 0V, I have lots of them that have done that repeatedly and they still test at greater than min rated capacity and show no signs of degradation.

I think you are confusing these with Li-ion cells which most certainly do get damaged from excessive discharge. The only way this situation can really harm NiMH is if you have multiple cells in series and one gets discharged so low that the other cells partially charge it in reverse. I had that happen once with a defective flashlight and it damaged one of my Eneloop AAA cells. 

[Rick Law]

I think you are confusing these with Li-ion cells which most certainly do get damaged from excessive discharge. The only way this situation can really harm NiMH is if you have multiple cells in series and one gets discharged so low that the other cells partially charge it in reverse. I had that happen once with a defective flashlight and it damaged one of my Eneloop AAA cells.

Nope, I was not thinking LiIon.  I was thinking AA/AAA NiMH.  But I did consider going-negative and going low-voltage as the same thing in what I wrote.  There is a difference between the two, so my treating it as the same is probably wrong.  Good to be more precise here.

You have two options:

A battery which is old, degraded or damaged may self-discharge down to zero in storage, so that when you measure it it has no voltage. From this you would draw a conclusion that the battery is probably bad and discard it, especially if it had a good voltage when stored.

On the other hand, a good battery may be discharged down to zero by a device it is installed in. This says nothing in itself about the battery, it just says the battery has been mistreated. You may charge the battery up again and it may be fine.

A third option is you may intentionally discharge a battery to zero volts and leave it that way. In the case of NiCd chemistry this is believed to do no harm. In the case of NiMH the situation is less clear. Some people have discharged Eneloops to zero volts and stored them that way for a considerable time, and on later testing found no loss of performance. Other people have claimed otherwise. So it is hard to draw a conclusion.

In my own experience, occasionally draining an Eneloop down to zero volts does not seem to be a problem. So I do not worry too much about it.

Your option 1 and 2 are really saying that the battery is bad and low/negative are just indicators of damage, but the damage was caused by other reasons such as age.  If I understood you right, I can agree with what you said. 

Your statement "... Eneloop down to zero volts does not seem to be a problem..." caught my eyes and it is very interesting.  This got me thinking: LSD vs non-LSD robustness in dealing with over discharge.

May be just coincidental - I do have better luck on recovery with Eneloop and with the Chinese work alike Tenergy Centurion brand, but I do not have nearly as good recoveries for my other batteries.  The Centurion AA has nearly identical specs to the first generation Eneloop AA.  So at least in my case, LSD vs non-LSD appears to have a very different characteristics when ran down to zero charge (or negative) and left in the device for extended period of time. 

It would be interesting to see any studies out there on LSD v non-LSD when overdischarged.  That will likely be my web-search/reading for the next few days...  May be it was simply my LSD's are just newer.  That would be disappointing.

[ogden]

I have never seen any noticeable damage from draining a NiMH battery down to near 0V, I have lots of them that have done that repeatedly and they still test at greater than min rated capacity and show no signs of degradation.

Again - it depends. Near 0V is fine up-to few days until you find mains plug for your charger, yet you would not want to store overdischarged NiMh's for weeks because it degrades usable capacity much faster compared to charged cell.

But I did consider going-negative and going low-voltage as the same thing in what I wrote.  There is a difference between the two, so my treating it as the same is probably wrong.

Right. Quote from Energiser appnote explaining polarity reversal (both electrodes reversed): At the point both electrodes are reversed, substantial hydrogen gas evolution occurs, which may
result in battery venting as well as irreversible damage.

[james_s]

Either way very deep discharge is generally best avoided due to the risk of cell reversal, however sometimes stuff happens and it would be convenient if all chargers had an override function to jump start the charging process on cells that are too low to detect. Draining down below 0.5V is not ideal but it is something that I have had happen often enough to be annoying. Unlike Li-ion, there is no risk of "exciting" failure modes so even if a cell is damaged there is rarely any harm in attempting to charge it. Even if one is no longer in tip-top condition I see no reason to dispose of it if it is still reasonably usable.

[13hm13]

After a test with the paper-clip trick on the C204, the charge did conclude (green light) ... but both batts. were too hot to touch. This is not normal.

Hot NiMh's in the end of fast charge is completely normal.

Batteries too hot to touch at the end or charging is not normal. It is a sign of bad batteries or bad charger. If you have a "fast" charger that regularly does this it should not be used.

At the end of charging batteries should be mildly warm at best, certainly not hot.

Yes.  The Maha 204 (or the 9000 in fast mode) does leave the batts. quite warm. I've never been comfortable with this in any case. But the paperclip trick was prob. a good 20 degrees hotter.

[Rick Law]

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Yes.  The Maha 204 (or the 9000 in fast mode) does leave the batts. quite warm. I've never been comfortable with this in any case. But the paperclip trick was prob. a good 20 degrees hotter.

If I understand you right, you are saying when you start it with the paperclip (verses other way of starting charge with the very same battery), that made a difference in the battery temperature at termination?

Did you remove the paperclip after charging starts?

Once it starts charging and the paperclip is gone, it should not make a difference how you start it -- unless with your charger under the mode you selected, it does something such as testing how much current it can take and use that as the charge current.  If it does auto-detect charging parameter, it might have started with parameter the battery doesn't agree with.  In this case, the paper-clip trick is not for you.  Your charger doesn't agree with it.  Please don't do it.

Regarding fast charge, many articles I've seen suggest that lower charge current is better for long-term health of the battery.  If you are already uncomfortable with fast charge, that is one ready reason to switch over to lower current charging.

[David Hess]

As far as I know, the only danger from low or zero cell voltage for NiCd and NiMH batteries is cell reversal.  Lithium and lead-acid should never be deeply discharged.