Nice Compilation of Charge vs Voltage for LiFePO4 batteries

Dave

I thought this was one of the nicer studies I have seen. It demonstrates what raj, mcgivor and others here have been saying, a final voltage of 3.4V per cell results in an essentially full cell, apparently 99% SOC.

https://www.powerstream.com/lithium-phosphate-charge-voltage.htm

BB.

Pay attention--Much from Dave's link is regarding LiFePO4 chemistry.

Different Li Ion chemistries can be vastly different charging voltages.

LiFePO4 is typical for "our usage" (solar, RVs, etc.). It is relatively safe (should not catch fire/explode if miss-charged or miss-handled).

However, exceeding high or low voltage limits can quickly damage even these cells.

-Bill

jonr

But only if you hold 3.4V until current drops to some low level (eg, ~1% as in the above test).  Charge at .2C to 3.4V and then stop charging and the result could be 70%.   See #17 below.     There is no single "this voltage = this SOC" number.
 
https://forum.solar-electric.com/discussion/351996/charging-lfp-lifepo4-batteries-to-90-soc-or-not/p1

mcgivor

Staying close to the knee of the charge curve rather than trying to achieve 3.600 VPS is recommended by many as it is not achieving much benifit and may actually be more stressful to the cells, same goes for the discharge curve there is so little potential energy below the discharge knee.

During an experiment I did set the charging voltage to 3.600 VPC to observe how the individual cell voltages responded, the battery accepted full current available until  3.500 VPC, the BMS then reduced the current dramatically from 30A to ~5A tapering down further as the cell voltages increased to a few mA, when the first cell reached 3.600 V the current became zero. The BMS then choose each lower cell and charged in pulses via the sense wire to a maximum of 75 mA until all were aligned at 3.600 V. Because the cells were top ballanced prior to installation the time taken was relatively short. This may be something I might consider if I see drift, but so far the cells are at maximum within 25 mV of each other when nearing the knee, but usually between 6 and 15 mV..

As far as dischargeing to lower limitations, I've never ventured down that avenue simply because my cells were top ballanced, they can be either top or bottom ballanced, but not both.

Dave

Now that I am trying to apply these results to my steep learning curve for my new batteries, it seems like I'd need to wait til my "absorb" current dropped to about 11A to reach full capacity with 3.4V target. That seems to happen after about 3 hours of absorbing. Will a 3 hour absorb damage the batteries in any way? there are loads coming off and online throughout the charging. Most experienced people are recommending much shorter absorb times and I am sure that's based on solid reasoning and experience but I don't see how the batteries will get fully charged unless the charge current drops to something like 0.013C, (about 11A for a 800Ah bank).