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Thread: What is the Chemical cause of Amp steps in LiFePO4 CV Charging Phase?

  1. #1 What is the Chemical cause of Amp steps in LiFePO4 CV Charging Phase? 
    Forum Freshman IamIan's Avatar
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    Oct 2013
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    I'm trying to figure out what the chemical cause of the amp steps I'm seeing in the CV phase of charging LiFePO4 Battery. I'd appreciate any insights others might have.

    It's a A123 20 Ah rated cell.

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    Back Ground:
    I was initially curious if what I've read about LiFePO4 cells not having a trickle charge rate was true.

    I was curious if it was not actually true, and was more functionally accurate. Like was it so low a amp rate that people just called it zero. Maybe very small. So small as to not be useful. But maybe still actually non-zero.

    One piece of potential data that looked to me like it was actually non-zero. Was the published self discharge rates. The OEM A123 Link tells you that the cells have a known self discharge rate. About ~3% in 1 year @ 25 degrees C. That seemed to suggest to me a potential very low amp rate trickle charge. ~3% of the 20Ah cell is ~600mAh in that 1 year, about ~68uA on average. The graphs also showed what looked like a higher rate initially that slowed down over time. Thus it seemed the trickle rate at t=0 would be higher than that yearly average of ~68uA.

    That is a extremely slow trickle / sustained rate. So it does make sense why people would think it was nearly zero.

    And this , slowing in the self discharge rate over time , lines up with the Self Discharge rate testing I did for my 55 cells. One specific cell over the 1st 2 months had an average self discharge of ~37.87 mWh per day. The same cell over a 12 month period had a yearly average self discharge rate of only ~9.8 mWh per day.

    So I was curious about how high the self discharge rate would go as someone gets closer and closer to t=0 rest period after charging. ie a sustained trickle charge.

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    Testing:
    So I started testing. Using a Regulated power supply good down to +/- 1mV and +/- 1 mA.

    At first it seemed to be going as expected. CV to 1mA was getting longer and longer. The curve of the amps seemed to continue to suggest that there was a sustained trickle charge point out there. It seemed like I just had to go to a higher SoE / terminal voltage to get it up to at least 1mA. Then the data got a bit odd , and not as expected.

    CV 3.0v 2A to to 25mA took ~16 Minutes ... went bellow 1mA Graph
    CV 3.1v 2A to to 25mA took ~28 Minutes ... went bellow 1mAGraph
    CV 3.2v 2A to to 25mA took ~45 Minutes ... went bellow 1mAGraph
    CV 3.3v 2A to to 25mA took ~7Hours ... went bellow 1mAGraph
    CV 3.31v 2A to to 25mA took ~14 Hours ... went bellow 1mA Graph
    CV 3.32v 2A to to 25mA took ~47 Hours ... went bellow 1mA Graph
    CV 3.33v 2A to to 25mA took ~24 Hours ... went bellow 1mA Graph
    CV 3.34v 2A to to 25mA Took ~18 Hours ... went bellow 1mA Graph
    CV 3.35v 2A to 25mA took ~27Hours ... went bellow 1mA Graph
    CV 3.36v 2A to 25mA took ~50 Hours ... went bellow 1mA Graph
    CV 3.37v 2A to 25mA took ~24 hours ... went bellow 1mA Graph
    CV 3.38v 2A to 25mA took ~19 Hours ... went bellow 1mA Graph
    CV 3.39v 2A to 25mA took ~23 Hours ... went bellow 1mA Graph
    CV 3.40v 2A to 25mA took ~16 Hours ... went bellow 1mA Graph
    CV 3.41v 2A to 25mA took ~15 Hours ... went bellow 1mA Graph
    CV 3.42v 2A to 25mA took ~11Hours ... went bellow 1mA Graph
    CV 3.43v 2A to 25mA took ~8 Hours ... went bellow 1mA Graph

    Stopped at this 3.43v point the discharge was showing I was near the top of the cell. ( ~99% SoE / SoC ).
    It does seem that any potential for a sustainable trickle charge, is well bellow 1mA.

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    Thus my current problem / confusion.
    I was not initially expecting the amp bumps. Nor did I initially expect for the CV time to go up and down as it did.

    Any insights?


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