Just sharing / passing along some testing data.
One aspect that seemed interesting to me ... is the minimum sustained / trickle current rate for the A123 20Ah cells.
There are some people who insist that LiFePO4 cells can not sustain a slow trickle charge rate ... but I see a problem with those kind of absolute claims ... the cells do have a self discharge rate... see attached.
The issue with it is that when the OEM A123 tells you that the cells have a known self discharge rate ... about ~3% in 1 year @ 25 degrees C ... up to about ~8% @ 40 degrees C in 1 year... that comes down to a sustained trickle rate in order to counter that effect... the higher ~8% of 20Ah is ~1.6Ah per year ... or ~133mAh per Month ... or ~4mAh per day ... or a sustained trickle rate of ~180 uA .... the Lower ~3% Self Discharge Rate would be a sustained trickle charge rate of about ~67uA.
That is a extremely slow trickle / sustained rate ... so it does make sense why people would think it was nearly zero... and fairly useless ... and not useful in any practical sense ... even the higher ~180uA would take about ~230 days just to put 1 Ah of charge through the cell.
But when looking at the OEM Self Discharge rate guide there is a curve to it ... ie the self discharge rate is faster in month 1 then it is in month 6 ...month 6 is faster than month 12 ... etc ... where things like the ~3% to ~8% are the cumulative loss over the entire 12 months.... that curve to the rate may eventually flatten into a straight line ... but at least in the beginning there is a curve to the self discharge rate.
And this lines up with the Self Discharge rate testing I did for my 55 cells.... for example ... one specific cell over the 1st 2 months had an average self discharge of ~37.87 mWh per day ... but the same cell over a 12 month period had an 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 no rest period after charging.... ie a sustained trickle charge.... even if it ended up just being academic.
I also figured I would check / try and quantify the change ... if any ... at different SoC / SoE / Cell Voltages... I suspected the higher SoE would have a higher self discharge rate... maybe even get up to 1mA.
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So I started testing.
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... eventually the data ended up looking 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 1mA
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CV 3.2v 2A to to 25mA took ~45 Minutes ... went bellow 1mA
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CV 3.3v 2A to to 25mA took ~7Hours ... went bellow 1mA
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CV 3.31v 2A to to 25mA took ~14 Hours ... went bellow 1mA
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CV 3.32v 2A to to 25mA took ~47 Hours ... went bellow 1mA
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CV 3.33v 2A to to 25mA took ~24 Hours ... went bellow 1mA
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CV 3.34v 2A to to 25mA Took ~18 Hours ... went bellow 1mA
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CV 3.35v 2A to 25mA took ~27Hours ... went bellow 1mA
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CV 3.36v 2A to 25mA took ~50 Hours ... went bellow 1mA
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CV 3.37v 2A to 25mA took ~24 hours ... went bellow 1mA
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CV 3.38v 2A to 25mA took ~19 Hours ... went bellow 1mA
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CV 3.39v 2A to 25mA took ~23 Hours ... went bellow 1mA
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CV 3.40v 2A to 25mA took ~16 Hours ... went bellow 1mA
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CV 3.41v 2A to 25mA took ~15 Hours ... went bellow 1mA
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CV 3.42v 2A to 25mA took ~11Hours ... went bellow 1mA
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CV 3.43v 2A to 25mA took ~8 Hours ... went bellow 1mA
Graph
At this 3.43v point the discharge was showing I was near the top of the cell ... not wanting to over charge the cell quiet yet ... at least until I do other testing ... I can say ... that it does seem that any potential for a sustainable trickle charge ... is well bellow 1mA ... even on a fully charged cell.
I was not initially expecting the bumps ... or for the CV time to go up and down as it did .... interesting.
Eventually after other testing ... I might try another round of testing by putting all 55 of the cells together in parallel ... in order to try and use that way to amplify the average sustainable trickle charge rate ... at least for detection / verification ... but as it is bellow 1mA ( under C/20,000 ) ... I don't see it being of any use in application... other than just re-enforcing that we can't use a trickle charge to balance the pack.