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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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Last edited by SilverFoxCPF; Apr 19, 2006 at 05:48 PM.
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