Nice Compilation of Charge vs Voltage for LiFePO4 batteries

DaveDave Registered Users Posts: 51 ✭✭
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.

12 * 300 W (10 fixed rooftop, 2 movable pole mount), Morningstar Tristar MPPT 60, Magnum 4448 PAE, 64 200Ah CALB in 4p16s arrangement with 16 LED Balancers and a Choice BMS300 (It is lousy and I don't recommend but it provides high and low voltage cutoff)

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,760 admin
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,379 ✭✭✭✭
    edited December 2019 #3
    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

    I am available for custom hardware/firmware development

  • mcgivormcgivor Solar Expert Posts: 3,439 ✭✭✭✭✭
    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.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • DaveDave Registered Users Posts: 51 ✭✭
    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).
    12 * 300 W (10 fixed rooftop, 2 movable pole mount), Morningstar Tristar MPPT 60, Magnum 4448 PAE, 64 200Ah CALB in 4p16s arrangement with 16 LED Balancers and a Choice BMS300 (It is lousy and I don't recommend but it provides high and low voltage cutoff)
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