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Old Apr 18, 2006, 09:28 AM
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Charles (and others) were right… NiMh AA Cycle Testing

NiMh Cycle Testing Observations…

I am a big fan of the Energizer 15 minute charger. I am amazed that the batteries don’t just melt down into a puddle and burn up. It actually does a very good job of charging in a very short time.

Charles, along with several others, pointed out that in order to enjoy maximum cycle life you need to charge at a slow rate. I contacted Energizer, and they pointed out that the 15 minute charger was a new product and they offer it for convenience. If you want long cycle life, you charge at a slower rate.

I have a set of test cells that I use for reference. Some of these cells have several cycles on the 15 minute charger, and they are still showing capacities similar to when they were new. This got me wondering just how hard fast charging is.

Perhaps the Energizer engineers have found a way to skirt the laws of physics so we could have our cake and eat it too…

The objective of this test was to compare the effects of fast charging with slow charging, and to take a look at life cycles for a high capacity cell.

I took two Sanyo AA 2500 mAh cells, performed a forming charge of 200 mA for 16 hours, cycled them 5 times, and proceeded with the testing.

Two chargers were compared. The Energizer 15 minute charger was the fast charger, and the Sanyo NC-MQH01U was used as the slow charger by utilizing one of the slow charge slots. The Sanyo charger charges at roughly a 1.0 amp rate in that slot.

The first test was done after charging both cells on the slow slots of the Sanyo charger. This became my base line capacity. The cells came out reasonably close in capacity and the testing continued.

I saved the data collected every 25 cycles, and it is presented below.

The procedure involved charging the cell on its respective charger, letting it rest for 20 minutes, then discharging it at 2.5 amps down to 0.8 volts.

High capacity cells are not the best choice for high charge or high discharge rates. A cell is usually considered fully discharged at 0.9 volts, and going to 0.8 volts is a bit of an over discharge. These values were chosen to accelerate the aging process and reduce the number of cycles needed to see a trend. I also only let the cells rest when I was asleep. I believe you can get improved life by less rigorous usage.

A general consensus from a variety of battery manufacturers suggest that you can expect around 500 cycles from high capacity batteries, and around 1000 cycles from lower capacity batteries. In the AA consumer battery size, lower capacity batteries are those with less than 2200 mAh of capacity, however Energizer was the only one that suggest a 2200 mAh cell as lower capacity. Most consider 2000 mAh cells as the high end of the lower capacity cells.

Keep in mind that when the battery manufacturer comes up with these estimates for the number of cycles, they are doing a 5 hour discharge and a 14-16 hour slow timed 0.1C charge.

Also, keep in mind that all of the information I have been able to find indicates the number of cycles to reach around 60-80% of the cells original capacity. It is understood that the mid point voltage will be reduced, but the performance is measured in reduction of capacity.

It has been interesting to watch the mid point voltage drop during these cycling tests.

Here is the data from the slow charge through 150 cycles.



Here is the data from the fast charge through 150 cycles.



Here is a comparison of the slow charged cell at 125 cycles with the fast charged cell at 100 cycles. Based on this limited testing, it looks like you lose around 25 cycles by fast charging.



In my notes I observed that the fast charge cell was about done after 125 cycles. I then switched to “topping off” the fast charge at a slower rate on the Schulze charger. This did not seem to have much effect.

At 148 cycles, the fast charge cell was no longer able to be charged on the Energizer 15 minute charger. I would put the cell in, and would get a blinking red error light. The final charge cycles were done on the Schulze.

Out of curiosity, I did another cycle (number 151) on both cells. This time I charged both cells on the Sanyo charger and did the discharge at 1.0 Amps.



Conclusions:

Fast charging high capacity cells, followed by fast discharging to an over discharged state is harder on the cells than slow charging followed by the same conditions.

The mid point voltage drops with use and it seems to drop faster with fast charging.

Cycle performance based on a percentage of remaining capacity does not tell the whole story.

Cells stressed and damaged by heavy usage may still be suitable for lower drain applications.

The performance lost by cycling is permanent.

There does not appear to be an increase in the self-discharge rate of high cycles cells.

Tom
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Old Apr 18, 2006, 11:18 AM
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Haralson County GA. USA
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Hi Tom,
Thanks for a great series of test. One observstion I would make is that 1A is not exactly a slow charge for a 2.5A cell. Much slower than 10A but still what I consider a quick charge. To me a slow charge is C/10 for 16 hours or so but then you start the great deabtes about not detecting peak etc. A slow charge does not use a peak detection circuite. It is my understanding that C/3 (about 1A here) is the minimum for reliable peak detection with NiMH cells. My Tritons work great with 2200-2500 NiMH AAs at 0.5-0.7 A which is as fast I every charge my Transmitter pack and then only when cycling approximately every 3 months..

All of the cells which I charger in the 15 min. zapper have their positive ends painted Red. They are only used for charge and go non mission critical applications.

Thanks again.

Charles
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Old Apr 19, 2006, 06:04 PM
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Hello Charles,

It seems like every time I do a test to answer a question, several more questions pop up...

I designed this series of testing to accelerate the aging process, but was still surprised that the cell charged at the slower rate did not last longer.

I have been told that if I performed the same test with 2000 mAh cells the results would be drastically different. I will have to see if I can find another block of time so I can consider doing this.

It would also be interesting to run this test with sub C cells.

The latest theory is that during fast charging, the pressure inside the cell builds up and the cell actually slightly vents during each charging cycle. The reduction in performance is actually caused by a loss of electrolyte.

Have you heard anything like that?

Tom
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Old Apr 19, 2006, 06:12 PM
Southern Pride
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Quote:
The latest theory is that during fast charging, the pressure inside the cell builds up and the cell actually slightly vents during each charging cycle. The reduction in performance is actually caused by a loss of electrolyte.

Have you heard anything like that?

Tom
No ,I thought they only vented at something over 250 PSI and the China made cells have reportably a one shot vent. This may be BS but that is what I heard anyway.
I do believe that fast charging / heat drys the paste next to the plate and can cause a decrease in life but then I never claimed to be an expert.I have however sawed a few old cells in half length to see what the insides looked like.


Charles
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Old Nov 13, 2006, 08:15 PM
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Another round of cycle testing.

This was pretty much the same charging and discharging conditions as the previous test. The same chargers were used.

This time I used some Titanium 2000 mAh cells. Charging was done at about 1 amp on the Sanyo charger and at around 7.5 amps on the Energizer 15 minute charger. Discharge was done at 1C.

2000 mAh cells hold up a lot better.

The one difference in this test was that after the first 50 cycles, I did a 0.1C charge for 16 hours after every 25 cycles. I am not sure what influence this had on the cells, but I didn't seem much difference in the curves during the first 50 cycles vs the remaining 100 cycles.

Here is the data:

Slow charging at 1.0 amps.



Fast charging at 7.5 amps.



Comparison of the slow charged cell at 125 cycles with the fast charged cell at 100 cycles.



And finally, cycle 151 with a discharge at 1.0 amps.



As you can see, the lower capacity cells held up a lot better than the high capacity cells did. I retired my high capacity cells after the test, but these lower capacity cells are still going strong.

I guess if you are going to use the 15 minute chargers, you should do so with lower capacity cells.

Tom
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Old Nov 13, 2006, 08:55 PM
Southern Pride
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Thanks once again for such a thorough test Tom. I am of the opinion that lower capacity cells are just better all around in most applications. They tend to last much longer,have a lower self discharge rate and do not have nearly as high of a failure rate.
My 11 year old NexCell AA NiMH cells just refuse to die. They are so old that they do not even have the capacity listed on them and the first time I tested them they were approx. 8 years old and tested approx. 1100 mAh.

Any real testing on the Ray O Vac Hybrid AAs yet? I purchased some a little over a week ago and initial testing is not bad but not great either..

http://www.rcgroups.com/forums/showthread.php?t=595283



Charles
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Old Nov 14, 2006, 12:24 PM
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Hello Charles,

I have not looked at any of the low self discharge cells except for the Eneloop cells. Toward the end of January, I will have data on 6 months of self discharge. Once I understand how the Eneloop cells perform, I will branch out to the others.

I must say that I find it interesting that the Eneloop cells are 2000 mAh. I also find it interesting that the Ray O Vac IC3 15 minute charger came with 2000 mAh cells, and the initial Energizer 15 minute chargers came with 2200 mAh cells. There seems to be something magical about 2000 mAh. Perhaps they are the best overall performers.

On a side note, we are beginning to see the same problems you observed with the Energizer 2500 cells with the Sanyo 2500 cells. Strong performance off the charger, but they are self discharging to nothing in a week or two.

Tom
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Old Nov 14, 2006, 12:43 PM
Southern Pride
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Well my Sanyo 250 Green sleeves are still doing great however the Sanyo 2700s are a big disapontment.

http://www.rcgroups.com/forums/showt...12#post5871612

Post 18 and 19 have the latest dismal performances listed.

Charles
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