Charging NiMH at high and low rates -- what can go wrong

Using my newly assembled computer data logger (see here) I've been trying a few experiments.

In the first experiment I looked at what happens when charging an NiMH cell at two different rates. It is often advised to charge small NiMH cells like AAA and AA cells at a rate of 0.5C - 1.0C. Therefore an AA Eneloop was first charged at 2000 mA (approximately 1.0C) and the voltage and temperature were recorded during the charge. Then the same cell was charged at 400 mA (approximately 0.2C) and the results compared.

Here is the result of charging at 2000 mA on a GP PowerBank GPPB14:



After the peak voltage is reached a clear dip in the voltage can be seen. This is the −∆V signal that chargers usually look for to determine when to stop charging. In this case the charger detected the dip in voltage and switched off before too much over charge was supplied.

Next we have the result of charging at 400 mA on a Duracell Power Gauge CEF21:



In this case, after the voltage reached a maximum it flattened out and did not show any significant dip. Therefore the charger did not see a signal to end the charge and kept on charging. The cell was not accepting charge any more as can be seen by the rising temperature. Eventually I stopped the charge manually to prevent too much overcharge happening to the cell.

This comparison between two charge rates shows why it is that although a low charge rate might seem kinder, it does not necessarily lead to a good result. Depending on the design of the charger it might fail to detect the end of charge point and overcharge the cells. As for kindness, even when the Eneloop was being charged at 2000 mA the peak temperature reached of about 45°C was not that excessive.

Very nice! Thank you for posting this!

With such a dramatic difference between 1C and .2C I am curious as to what the results would be at .5C and .75C

Looks like a dT termination would be pretty good at low charge rates.

For NiMH smart chargers, where there is dV termination, isn't that often 0dV rather than -dV, especially for slower chargers?

0dV means constant voltage. -dV is drop in voltage.

Smart chargers look for the drop in voltage to terminate charge.

Perhaps the voltage drop wasn't big enough to see on the 400mA charge rate to see on the graph.

Popsiclestix said:

0dV means constant voltage. -dV is drop in voltage.

Smart chargers look for the drop in voltage to terminate charge.

Perhaps the voltage drop wasn't big enough to see on the 400mA charge rate to see on the graph.

Click to expand...

As long as the measurement period is long enough to prevent spurious early termination, 0dV is a pretty good backup termination method to -dV when smart-charging NiMH at low rates.
Charging ~4AH single cells and 3-cell packs at ~800mA on a homebuilt charger, short-period -dV generally triggers termination, but a 0dV based on a longer detection period sometimes cuts in first.
Some of the time, if a 0dV signal had been ignored, -dV would have triggered a little later, but pretty much all the time, if a short-period -dV had been ignored, then a longer-period 0dV cutoff would have happened reasonably soon (unless voltage were to suddenly start rising again).

There do seem to be commercial smart chargers around that do pretty well at low rates - a little 2-channel one I have that came bundled with an Olympus camera charges AAs at 550mA (~0.25C) and seems to consistently terminate with Eneloops being barely warm at the end of charge, and there are no obvious thermal sensors, so the cutoff must presumably be voltage-related.

I was wondering how on/off-charge voltage vary through the charging cycle - does cell impedance typically change much, and could that be used as another proxy for charge state?

What charger were you using? I top my cells off quite often at 200-400ma and never had trouble with the charger not terminating. I'm using the MAHA C9000.

Dont forget that cell age/prior abuse is a variable here too...

Mr Happy excellent findings. After reading cpf these last few months, this substantiated my personal belief charge hard and fast at 2a. I have a 801d and a c9000. I use the 8 bay most for charging and 90-95% charge is cool with me. Just seems if i am going to lose overall life of cells, say from 500 to 400 and i am not sure that is the case with high quality cells. If you do lose overall life of the cells, who cares thats why I work. Thanks again for your efforts

Nice job Mr H. :thumbsup: Your results concur with other's showing the difference between slow and fast charging, in relation to −∆V termination, quite well.

357mag1 said:

What charger were you using? I top my cells off quite often at 200-400ma and never had trouble with the charger not terminating. I'm using the MAHA C9000.

Click to expand...

357, keep in mind, that the newer (all but the very first ones) Maha C9000's primarily, use the 'magic" 1.47 Volt cutoff to terminate charging.

Dave

uk_caver said:

Looks like a dT termination would be pretty good at low charge rates.

For NiMH smart chargers, where there is dV termination, isn't that often 0dV rather than -dV, especially for slower chargers?

Click to expand...

uk_caver said:

There do seem to be commercial smart chargers around that do pretty well at low rates - a little 2-channel one I have that came bundled with an Olympus camera charges AAs at 550mA (~0.25C) and seems to consistently terminate with Eneloops being barely warm at the end of charge, and there are no obvious thermal sensors, so the cutoff must presumably be voltage-related.

Click to expand...

It is interesting you ask these questions, because I had a similar motivation for doing this test. The 400 mA charge was using the Duracell Power Gauge CEF21. This charger usually seems to do a good job of terminating with AA cells and I was curious to see how it did it. The CEF21 does have individual bay temperature sensors inside the negative battery contacts.

However, when I did this test, the CEF21 did not terminate! I am not sure why that was so far. It may be because I tested a single cell with the battery cover open causing the cell to run much cooler than usual. Normally I charge two or four cells with the cover closed. I am sure temperature has something to do with the termination condition and maybe I will repeat the charging test using a more realistic scenario of two cells with the cover closed and see what happens.

I was wondering how on/off-charge voltage vary through the charging cycle - does cell impedance typically change much, and could that be used as another proxy for charge state?

Click to expand...

That would be interesting to know, but I am not able to measure it. My meter only samples four times per second and that is not fast enough to catch the short period between charging pulses.

357mag1 said:

What charger were you using? I top my cells off quite often at 200-400ma and never had trouble with the charger not terminating. I'm using the MAHA C9000.

Click to expand...

In fact the C9000 is special as it has a max voltage test of 1.47 V open circuit. In most cases this is what causes charge termination and is one of the reasons the C9000 provides a cool and reliable charging solution. (The C9000 termination would have occurred at about 55 minutes on the 2000 mA charging graph. You can see an example of the C9000 charging an Eneloop here.)

Mr Happy,

Please add CHARGER and CELL SIZE to the title of your graphs (i.e. Eneloop AAA Charged @ ~400mA on Duracell CEF20). Since the Eneloop AA Charged @ 2000mA hit -DeltaV @ ~1.625 (-0.8 = 1.545VDC), it obviously wasn't on the Maha C9000 and was probably back on the GP PowerBank Smart 2, although the dropoff in this thread appears to have taken a bit longer and has a bigger 'hump'. :thinking:

Interesting visual that will be useful for LINKing to for those "Slow Charge Aficionados".

Note: I put "~400mA" because without a display, we don't really know the EXACT Charge Rate on this category of chargers - only what the specs says it SHOULD be.

I did not mention the chargers used since the voltage and temperature profile would be essentially the same with any source of charging current. In fact if I had a suitable bench power supply I would have used that. But you are right in this case that the two chargers are the GP PowerBank Smart 2 @ 2000 mA and the Duracell Power Gauge CEF21 @ 400 mA.

Also I would really prefer for people to link to the whole post rather than just the chart. The post contains extra details and context.

I do have plans to further investigate the lack of charge termination at 400 mA, and I will post further details when I get to it.

Great post, the graphs really make it clear what is happening in a smart charger.

I'm glad my CHUN uses both feedback and temperature sensing to sense complete capacity. And I always charge at least with 900 ma. (mostly since this is simply the low setting on the charger I have and is in recommended range for my eneloops).
When would I use the high setting, about 2000 ma.?

I'm building a higher amp hotwire atm that will pull about 10 amps so I am going to use high current elites instead of enelopes, would I be correct in assuming that these are more suited to the higher setting when charging?

357, keep in mind, that the newer (all but the very first ones) Maha C9000's primarily, use the 'magic" 1.47 Volt cutoff to terminate charging.

Dave[/QUOTE]

Mine is definitely one of the newer ones. Bought it from Zbattery about 2 months ago. My initial unit had bay 1 go bad and the service from Zbattery was awesome.
I have two different chargers that take the batteries over 1.5 volts when charging and I thought they might be shortening the battery life. I made a post about it and was reassured that it was normal. So I started thinking these chargers were giving me a bit more charge but haven't been able to prove that in the numerous discharge tests I've done.

Back to the point of this thread my C9000 probably doesn't prove anything with respect to slow charge if it cuts off at 1.47volts.

but but but, exactally the opposite could also be said:
look what you did to it when you charged it at such a high rate the temperture goes crasy, the poor thing faulters.

lower the low rate to 250ma and do it for the next 3 weeks, then take it off, cycle it one time and tell me how bad off it becomes? (enloops handle this ok)

Applets and cotlets, the asumption with the low rate is that it will overcharge, but it is within its capability to cope with such overcharge.

SURE if were using termination chargers we want them to terminate, but the LAST THING i would want on a charger that is timed or lame or doesnt terminate or terminate properly is a high rate.
and they SELL chargers that charge at MED rates (350ma), without termination, and the wrong application of this information would be berry berry bad. those chargers suck.

And try that same trick in series, and offset multiterminations are not so easily seen by brainless computers.

so that has to be part of the knowlege. it has gotta say that this information is usefull WHEN using Voltage dropping terminations, and is really bad in parelell, and can be not so wonderfull in series.

and lets not forget the 3rd choice. you got torture 1 and torture 2, but given a good alogrythm you could get a fast charge going, that tapers down automagically neer the end of charge, based on a set high voltage, and have cake and eat pie too.

Mr Happy said:

I do have plans to further investigate the lack of charge termination at 400 mA, and I will post further details when I get to it.

Click to expand...

Any chance you could rig together a third meter for a current reading? I'm wondering if the charger was ramping down the charge current. And perhaps sacrifice a cell or two to see if the unit will actually self-terminate on other conditions (pre-set temp maybe?)

Seems to me that the Duracell charger isn't exactly cheap stuff and I would have hoped that they would have covered their bases adequately.

an enloopie can handle 350ma non stop unpulsed actual 350ma for more than 300 hours without showing a visable V-drop.
that being a usable low resistance one.
and basically i cant tell that it hurts it that much either, but 400, well from my tests you would be getting into slow bake territory,

depending on the charger would depend (again) on if it was "Averaged current" or "actual constant current" too. big difference when say using a 900 at 200ma, which is really 1000ma/5 <--- yup hurts my head too.

Popsiclestix said:

Any chance you could rig together a third meter for a current reading? I'm wondering if the charger was ramping down the charge current. And perhaps sacrifice a cell or two to see if the unit will actually self-terminate on other conditions (pre-set temp maybe?)

Click to expand...

How many $70 DMMs do you want me to buy?

Actually the CEF21 normally does a perfectly good job of terminating on time when charging AA cells and I was expecting it to do so this time. I was rather surprised when it didn't.

I think temperature has a lot to do with the −∆V signal (in fact, I think it has everything to do with it), and I think my test circumstances kept the cell too cool.

Next time I run a test I will try to recreate a normal charging scenario more closely and see what happens.

Seems to me that the Duracell charger isn't exactly cheap stuff and I would have hoped that they would have covered their bases adequately.

Click to expand...

The CEF21 contains an ATMEGA88 microcontroller, and who knows what programming is inside it? It is possible it would have terminated on 0 dV or temperature if I had left it a bit longer. Eneloops can probably take a 400 mA overcharge for a little while before there is any risk of serious damage.

Mr Happy said:

...In this case, after the voltage reached a maximum it flattened out and did not show any significant dip. Therefore the charger did not see a signal to end the charge and kept on charging. The cell was not accepting charge any more as can be seen by the rising temperature. Eventually I stopped the charge manually to prevent too much overcharge happening to the cell.

This comparison between two charge rates shows why it is that although a low charge rate might seem kinder, it does not necessarily lead to a good result. Depending on the design of the charger it might fail to detect the end of charge point and overcharge the cells. As for kindness, even when the Eneloop was being charged at 2000 mA the peak temperature reached of about 45°C was not that excessive.

Click to expand...

357mag1 said:

What charger were you using? I top my cells off quite often at 200-400ma and never had trouble with the charger not terminating. I'm using the MAHA C9000.

Click to expand...

IMO, these results relate DIRECTLY to the 'Experiment' that PeAK proposed on 11/19/09 (Is Your Charger Terminating Properly?) and that, it appears to be, only I implemented and reported back on (Test for how well batteries suit your charger).

IMHO, *EVERYONE* should determine how well their current charger(s) terminate (*BEFORE* they damage their cells!).