Overly sensitive NiMH battery chargers

[13hm13]

I own two different AA/AAA NiMH battery chargers from  MAHA Energy (Powerex). I bought them brand new from Amazon in 2009.




Since day one, both chargers "fault out" (refuse to charge) a relative high % of batteries (including Maha's own Powerex brand batteries).

I have an older Lemnar charger which can charge the same faulted-out batteries.



Is there any way to adj the Maha chargers' circuitry so that they are a bit more "accepting"?

[ogden]

Many smart chargers refuse to accept overdischarged (below 0.5V or so) NiMh batteries. Solution is: use exclusively low self-discharge type, such as Sanyo Eneloop, IKEA Ladda and similar batteries, regularily check your battery stock (if any) and cycle all batteries every 2..3 months or so. Also avoid using NiMh in dumb (motor+switch+battery) devices/appliances that do not have low battery 0.8V/cell cutoff.

[13hm13]

Many smart chargers refuse to accept overdischarged (below 0.5V or so) NiMh batteries. Solution is: use exclusively low self-discharge type, such as Sanyo Eneloop,...

Almost all the NiMH batts I now use are Sanyo Eneloop.
I can charge them up to well above 0.5v on the Lenmar. But they still fault out if I, then, stick them in the Powerex chargers.
Resistance/impedance issue, perhaps?

[GLouie]

I also have a Maha C9000, use only Eneloops AA/AAA, and occasionally get "cannot charge" errors when older AAA batteries are reported as too high a voltage. I suspect it's something to do with the initial test algorithm and internal cell impedance. AFAIK there are no internal adjustments. I have never had a problem (high or low) with Eneloop AA, and by discharging the problematic AAAs in a flashlight for a few minutes, can proceed with charging. I never seem to have any overdischarged cells - my devices cut off inteliigently, or I just recharge before it's really needed.

My better solution was to get another brand charger (Xtar VP4+), which lets me charge 4 LiIons as well, and with the Maha, charge twice as many NiMH cells at a time. The Xtar has quirks of its own, but so far has not refused to charge a cell.

[ogden]

I can charge them up to well above 0.5v on the Lenmar.

So you overdischarge them. Long time below 0.5V leads to capacity loss. Charger may refuse to charge overdischarged, near-death battery with little remaining capacity - especially if it is high speed charger that does not support small < 1Ah batteries. Cycle those refused batteries, look what is reported capacity. If >=1000mAh and charger still do not accept such at >=0.5V - your charger may be faulty.

[IanB]

The C9000, and possibly other Maha chargers, test batteries before charging by passing a high current through them and checking the voltage rise. If the voltage rises too high it will reject the battery and refuse to charge it. This either means it is an alkaline battery, or it means it is an NiMH battery that has dried out and gone high resistance and has therefore reached the end of its life. NiMH batteries in this state will not be good for anything other than low drain applications and are best recycled.

If you look at the LCD display on the C9000 right after you insert the battery and start the charge process you will see the test voltage displayed for a second right at the start. If the voltage is above (I think) 2.5 V or so it will just display "High" and stop right there.

Yes, some chargers will accept old, failing batteries, but you still won't get much out of them.

[IanB]

Many smart chargers refuse to accept overdischarged (below 0.5V or so) NiMh batteries. Solution is: use exclusively low self-discharge type, such as Sanyo Eneloop, IKEA Ladda and similar batteries, regularily check your battery stock (if any) and cycle all batteries every 2..3 months or so. Also avoid using NiMh in dumb (motor+switch+battery) devices/appliances that do not have low battery 0.8V/cell cutoff.

No, the C9000 has no problem charging batteries at zero volts or less. It just trickles them up to a reasonable voltage and then starts charging at the normal rate.

What the C9000 rejects are batteries that have gone high resistance due to age or other circumstances.

[Rick Law]

Two of my NiMH chargers have firmware that rejects battery with low voltage and/or has "gone negative"  (have a negative volt).  They show "NULL" indicating the battery is a reject.   I used to re-plug my third charger to charge the rejected battery for about 5 to 10 minutes then switch the battery over to one of my two better chargers to finish charging.

Now I just keep a metal paper clip near by.  I put a good battery in the bay next to the bad battery and use the paper clip to short the two batteries' anodes (+ side) for a very brief moment.  This fools the charger into thinking that your bad battery has a good voltage.  In my case, within 0.1 to 0.5 second, the NULL went away and charging begin.  I put my paper clip back to the "paperclip holder", remove the good one and let it charge the formerly rejected battery.

Try it, it probably will work on your charger as well since most chargers likely share common ground and other common characteristics.  Please do so with the understanding that there is some risk shorting the + for 1/2 second.  For me, both my better chargers worked with the paper clip trick.  But given the risk, I must add: if you try, you are trying at your own risk.

Do let us know how this trick work out for you...

[13hm13]

Now I just keep a metal paper clip near by.  I put a good battery in the bay next to the bad battery and use the paper clip to short the two batteries' anodes (+ side) for a very brief moment.  This fools the charger into thinking that your bad battery has a good voltage.  In my case, within 0.1 to 0.5 second, the NULL went away and charging begin.  I put my paper clip back to the "paperclip holder", remove the good one and let it charge the formerly rejected battery.
...
Do let us know how this trick work out for you...

I tired it on the Maha MH-C204 (see photo in my OP) and it seems to work! Too early to tell how if it will work with other batts. and/or with the C9000. Hopefully, same good results.

I assume that modern chargers have protection circuits that will protect them and/or the batts in case something goes wrong in the middle of a charge? In any case, the rapid charger (C204) ALWAYS ends its cycle with batts in a VERY warm cond.

Just a few thoughts on Maha/Powerex. Back in 2009, I bought three of their chargers and several sizes of their Powerex batts (AAA, AA, 9v). All products had issues: The batts ALL failed within a little over a year. And the charger have the aforementioned sensitivity issue. The company is still around and offers a decent catalog of chargers and batts. Maybe their quality has improved ??

EDIT:
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.

[ogden]

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. NiMh is "unique" chemistry - at the end of the charging it will rapidly heat-up and voltage will slightly drop (not typo). When shopping for fast charger - you make sure it does support all terminations - timer, dT and dV. Thou I am not sure that mentioned 25$ Maha MH-C204 support temperature monitoring, anyway don't worry about hot batteries in the end.

https://lygte-info.dk/info/batteryChargingNiMH%20UK.html

[Rick Law]

...
...
EDIT:
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.

May be I should have mention this caution in my last reply also:

When your battery has low voltage and/or gone-negative, they are damaged battery.  You cannot expect it to be normal.  The hope is that it is not too damaged and became useless.

I had recovered 800mAH AAA (gone negative) back to nearly 800mAH, and I had recovered 2000mAH AA back to just 17mAH.  At 17mAH, it is good as dead.  Damaged battery likely have higher internal resistance and will heat up more.  It should be charge as lower current.   As to it is useless or not, it is up to you to decide.

I don't know the options on the Maha MH-C204.  Both my better chargers (LaCross BC700 and Opus BCT3400) have a cycle mode called "discharge-refresh" to improve your battery.  It is what I used for damaged battery recovery.  It charge full and discharge (while measuring the mAH) a number of times.  This "exercising" the battery do help unless your battery is at or near-death.  But if your battery gets hot, run your charger at a lower current likely will help it from getting as hot.   1/10 C is what I use for recovery, but you can try even lower.  (If you don't know C: C is the capacity.  2000mAH battery is C=2000, so 1/10 C is 200mA)

My LaCross BC700 NiMH charger (when selected) will do such cycle indefinitely until the mAH stop getting better (higher).  Sometimes it runs for days cycle after cycle.  In the end, the battery does come out working much better.  My Opus BCT3400 NiMH+LiIon will do such cycle but limited to 3 cycles.  That 800mAH battery that I recovered was at 400mAH range at the first cycle.  By the time it was done, it was 790mAH range (798 if I recalled correctly).

If your Maha MH-C204 have such options (charge full then discharge with mAH measurement), try it a few cycles and see if it helps your battery recover some.  If your battery is not too damaged, it probably would recover a bit - but never back to as good as before damage.

Don't forget - If you want it to run cooler, try it with lower current setting.

[13hm13]

As i noted in the OP, in the old Lenmar charger, I can successfully re-charge the batts. that faulted in the Maha chargers. Using those charged batts in devices yields fairly normal results (maybe a bit on the low side).
I question the quality and design of Maha products.

[ogden]

As i noted in the OP, in the old Lenmar charger, I can successfully re-charge the batts. that faulted in the Maha chargers. Using those charged batts in devices yields fairly normal results (maybe a bit on the low side).
I question the quality and design of Maha products.

Fact that dumb charger is able to charge battery do not mean that both Maha chargers are faulty

As you do not tell measured capacity just  "fairly normal results" which could be as low as 100mAh. For 2A pulse Maha Powerex charger my estimate is that you need at least 0.5AH battery for charger to not refuse it with "too high voltage" (meaning high impedance) claim. Before continuing to blame Maha (Powerex) charger, you really shall cycle/test batteries and measure their real capacity. When Maha charger refuses to accept battery - take your time and measure it's voltage using multimeter.

[IanB]

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.

For safe, moderate charging of Eneloops I recommend you to check out Panasonic chargers, for example the BQ-CC17. Or if you have a C9000 use that, but keep the charge rate at 1000 mA for AA batteries (use 400 mA for AAA).

[ogden]

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.

Please define "too hot to touch" in degrees (Celsius).

[edit] Panasonic pro NiMh recommended charger specs attached. Note that rapid charge ambient specified is 0..40oC. As 2Ah AA self-heating at 1C charge is +10 degrees and delta T termination needs couple of minutes of ~2oC/minute raise, some who charge his batteries in hot place could get quite a hot batteries in the end. Also note that AA have max 55oC allowed which as far as I know shall be considered as "too hot to touch". As you see - it depends.

[Rick Law]

*************** ***************
By the way, this is important and I kept forgetting to put it in
*************** ***************
The trick is for NiMH only!  Do NOT attempt the paper clip trick with LiIon battery.  Doing that could give you a very very unpleasant afternoon - not just that day but there after.

Shorting a good LiIon with a bad/damage LiIon can easily cause major problems!
******

Now back to the conversation:

As i noted in the OP, in the old Lenmar charger, I can successfully re-charge the batts. that faulted in the Maha chargers. Using those charged batts in devices yields fairly normal results (maybe a bit on the low side).
I question the quality and design of Maha products.

Fact that dumb charger is able to charge battery do not mean that both Maha chargers are faulty

As you do not tell measured capacity just  "fairly normal results" which could be as low as 100mAh. For 2A pulse Maha Powerex charger my estimate is that you need at least 0.5AH battery for charger to not refuse it with "too high voltage" (meaning high impedance) claim. Before continuing to blame Maha (Powerex) charger, you really shall cycle/test batteries and measure their real capacity. When Maha charger refuses to accept battery - take your time and measure it's voltage using multimeter.

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

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.

For safe, moderate charging of Eneloops I recommend you to check out Panasonic chargers, for example the BQ-CC17. Or if you have a C9000 use that, but keep the charge rate at 1000 mA for AA batteries (use 400 mA for AAA).

Second, I am not sure it s fair to blame the Maha charger on it getting hot.  Recall one has to trick the Maha to charge a rejected battery that it doesn't think it should charge - so, one cannot trick it to start then blame it for not doing it right.

No doubt Maha could have designed a much better charger.  In practice, they really can't do that without making it exceedingly complex and/or expensive.  Imagine having to look up the battery spec to set "termination voltage", "termination temperature", etc. etc.  So they have to make trade off decisions.  If the Maha is meeting it's own specs, that all we can ask for.

(13mh13, below is mostly for you)  Since you did the tricking of the Maha to start so you can try to "rescue" a damaged battery, you should expect things wont be normal and extra precaution has to be taken.  I don't do as much safety measure with NiMH as I do with LiIon, but if/when it reached "too hot to HOLD" (which would be cooler than "too hot to TOUCH"), I would be very cautious.  Likelihood is, if it is that hot, the battery is probably too close to death to be worth using.

Take note on the end-charge temperature, take note on other clues.  Remember you ARE the Emergency Room trying to "bring it back from the dead".  You "got the heart beating" by using a paper clip.  Now don't send the patient out the door right the way to drive home by himself as if all is well and normal.

I don't think you necessarily need a lot of tools.  You do need the a way to measure charge temperature, charge voltage, the charge time, and the actual "fully recharged" capacity.  Those will be like the blood pressure, heart rate, and temperature of your patient in the ER.  The ER wont let you leave without looking carefully at your "pre-exit" vital signs.  You shouldn't put your battery into circulation (treat as normal) until you scrutinize the battery's vital's.

[ogden]

You shouldn't put your battery into circulation (treat as normal) until you scrutinize the battery's vital's.

When you have many (more than 4) NiMh's, you want to know actual capacity of each battery so you can "balance" pairs/4-packs of them by capacity to minimize polarity reversal of weakest in the pack. This conversation is quite long already, yet we still don't even know actual problem - charger is refusing batteries due to low voltage or lost capacity. I don't want to hurt anybody's feelings, but to me this thread looks like smearing of Maha Powerex charger&batteries to promote Lenmar charger&batteries.

[edit] Answer to original question is: No, you can't change anything about Maha's charger because it is microcontroller-based and for sure it's firmware is read-protected.
[edit1] If Maha do not work for you, then get Lacrosse BC1000

[james_s]

My BC-900 charger fails to recognize batteries that have become overly discharged. My solution has been to keep a regular dumb charger around for times I encounter this issue. I pop them in that for a few minutes and then move them to the LaCrosse charger and it then works fine. It's annoying and I wish the BC-900 had an override button to deal with this but it hasn't been too big of an issue overall.

[ogden]

My BC-900 charger fails to recognize batteries that have become overly discharged.

Yes, treshold for LaCrosse BC-chargers is 0.5V. If you have such NiMh batteries - either you are using battery set(s) with huge capacity spread, your tool/device/whatever is not designed for rechargeable batteries or your batteries just lost capacity and shall be recycled.

[dnwheeler]

I have a set of 4 Energizer NiMH batteries that my Eneloop charger won't charge. This has been true since the batteries were new and doesn't matter if the batteries are charged or discharged - the Eneloop charger always reports an error. The original Energizer charger (came with the batteries) and an intelligent universal (NiMH + Li-Ion) charger both work fine.

[IanB]

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

Just because it's been said doesn't mean it's true.

I have a Harmony remote that eats batteries. I will put 4 AAA Eneloops in it, and often by the time I notice the remote has switched off one of the batteries has been drained to zero or even reversed. I never have any trouble recharging a battery in that state and there are no particular signs of damage. Such a drained battery still tests just fine.

Summary: a low voltage battery may not be the fault of the battery, it may be a result of battery abuse. Good batteries can take abuse better than bad batteries, and abused batteries are not necessarily damaged.

Also, none of the chargers I regularly use complain about charging batteries that have been drained to zero. They charge them just fine. (These are Panasonic chargers and the Maha C9000.)

[IanB]

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.

Please define "too hot to touch" in degrees (Celsius).

[edit] Panasonic pro NiMh recommended charger specs attached. Note that rapid charge ambient specified is 0..40oC. As 2Ah AA self-heating at 1C charge is +10 degrees and delta T termination needs couple of minutes of ~2oC/minute raise, some who charge his batteries in hot place could get quite a hot batteries in the end. Also note that AA have max 55oC allowed which as far as I know shall be considered as "too hot to touch". As you see - it depends.

If a battery is "hot" during charging rather than "warm" then I consider it battery abuse and will not use that battery/charger combo again. I also will not use a 1C charge rate regardless of datasheets that suggest it is OK. Whatever a manufacturer might claim for whatever reason, common sense also has to be applied.

For similar reasons I don't drive my car fast over holed and rutted roads even though the manufacturer may test it that way. I don't regularly drive my car on racetracks at the limit of its performance. I don't routinely drive my car with the revs close to the red line, even though the rev limiter hasn't cut in.

There is a difference between upper operating limits and good practice.

[magic]

NiMH certainly can be discharged to absolute zero and charged back with no problems. They will even self-discharge in a few years, or the crappier ones in a few months.

Cell reversal supposedly harms them, perhaps it's less of a problem at low currents like in a remote
And I think it was NiCd which really, really hated cell reversal and easily got damaged.

And yes, I hate "smart" chargers which aren't smart enough to charge a completely drained battery.

[james_s]

My BC-900 charger fails to recognize batteries that have become overly discharged.

Yes, treshold for LaCrosse BC-chargers is 0.5V. If you have such NiMh batteries - either you are using battery set(s) with huge capacity spread, your tool/device/whatever is not designed for rechargeable batteries or your batteries just lost capacity and shall be recycled.

It's things not being designed specifically for rechargeable batteries mostly, generally not a problem but sometimes I forget to be proactive and batteries get drained too far, doesn't seem to hurt anything but it's an inconvenience to have to put them in a dumb charger for a bit to resolve that. I also have a few older non-LSD batteries which work fine in some applications, no sense in disposing of something that still works, but if they are not used for a while they self discharge and I have to use the same trick. It's a minor annoyance but it could have easily been solved by a simple firmware change to allow one to override the detection and force it to start a charge cycle.

[David Hess]

NiMH end of termination is tricky to detect.  One thing to watch out for is charging too slowly which makes it even more difficult.  I use C/4 now as the slowest charge rate.

I have had a Maha C9000 long enough to wear out a complete set of AA and AAA Eneloop cells.  At one point, my Maha C9000 started having problems like you describe.  I found nothing wrong with the power adapter so I took it apart and suspected that the aluminum electrolytic capacitors had worn out.  I replaced them with some 20+ year old solid tantalums and now it works like it should.

[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.
...
...

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.