Thursday, November 8, 2012

Unexpected Results

I have been setting up a battery testing bench to do some of my own Fast Charge tests on cells at a high amperage rate. I have a power source, JLD Volt Meter, and AH meter from Jack Rickards shop to keep watch on the cells and to turn them off when I wanted. Well I have not quite figured out how to set up the JLD switches yet. But with that I do have it all set up with my power source, meters and my PowerLab to monitor and log the voltage graph. I started out with an A123 cell that had been discharged to 2.9 volts at 20 amps then shut off. I then turned on the power source and began to monitor and log over time and on video. The power source is a 5 volt 40 amp MegaPac module. It holds 5 volts well and pumps out a solid 45 amps. During the entire test the voltage rose and the amperage stayed solid on 45 amps with no hiccups. The voltage rose to more than 4.5 volts at 1 ah pumped into the cell. The voltage rise was far higher than expected but at 4.7 volts and 10 ah's the test was terminated. It was terminated not because of heat but because of swelling of the cell. I saw an ever so slightly swelling of the pouch and knew I needed to shut down. The two unexpected things was the fast voltage rise and the fact that the cell and terminals were not hot at all. Heck they were not really warm either. So I am baffled at the voltage and as it was only 10AH were pumped into the cell so it was not overcharged. The cell was plenty empty. It has a similar curve to what Jack had with his test but mine had much higher voltages and it was confirmed that the high voltage was not a fluke on my system. I even did the same test on a Hi-Power Cell and even went to 4 volts and 45 amps and put in 77 ah's into a 100 ah cell. But at those high voltages damage does occur and gassing does occur. Yes even with LiFePO4 cells. I used the A123 because I was able to charge at a much higher C rate. The Hi-Power only got charged at under 1/2C. Not terrible high.

My power source is a MegaPac Power Factor Corrected output. It holds a solid 45 amp output at 5 volts real well. I am going to assume that the issue is with my power supply. 5 volts should not be that much of an issue for a cell that can or should handle with no trouble 3.8 volts and it should handle a solid 3C of charging and I was not even at that current rate. Yet I got slight puffing and pulled the plug early. You will see on the video what I am taking about. One thing I did notice was that there was more puffing after the cell sat overnight. I looked and felt it and you can see and feel the puffiness of the cell. It is much more so today than right after the test. So gassing continued even after the power was pulled. Voltage is sitting at a cool 3.34 volts.


Here is a rough graph I exported from the PowerLab 8 after the test. I have to go back and reexport the text file for Excel.






3 comments:

  1. Keep an eye on that cell. I would keep it in a metal can filled with sand. Better safe than sorry.

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  2. Thank you for that test Peter.
    Its all very odd. We supply a higher Voltage to a cell so it takes an appreciable charge current. But the cell appears to develop symptoms of over volting. It appears to me an electrolytic reaction happens to create bubbling above a certain delivered voltage.

    I'm definitely going the laptop route on charging. A fixed voltage supply that should not bring any cell over 3.35V.

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  3. Andyj,

    I have done some subsequent testing on another cell and kept it below 3.7 volts but I can't say for sure that was the problem. The only way to know for sure is to do it again and take a cell to 3.7 volts and see what happens. At this point I know the other cell can handle 3.5 + C charging current with no problems up to 3.65 volts at a CC rate. That process will put in about 16ah into the cell. That is near dead on the 80% full mark where we want to be and well below the peak where we don't want to be.

    I feel that the cell here that gave the unexpected results was a faulty cell from the beginning. I say that because we lost fully 1/2 its capacity. It could be that it has shorted a foil or two within and caused the loss. When I am ready to sacrifice another cell to science I will charge up a good cell to 3.7 volts and see if the results are the same as this.

    Pete :)

    Until then just keep your fast charge end voltage limited to 3.65 or less. 3.65 being about exactly what you should do under high current or low. Nothing more or damage may result.

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