LiFePO4 batteries' state of charge versus charging voltage

Barkuti · 2018-12-02T18:13:53.000Z

As the title says. After reading some evidence claiming state of charge of LiFePO₄ cells has an impact in their calendar aging (briefly check this John Catton thesis if you will), something which is in line with what happens for li-ions too, I do wonder what sort of impact reducing the maximum charging voltage has over the cell/battery capacity. For li-ion batteries I can more or less infer this from published discharge curves because there's little to no voltage drop after top off, but this is not the case for LiFePO₄ cells which seem to collapse from 3.6V down to 3.4V or a bit below.

For those of you who do possess the required equipment (CC/CV supply and LiFePO₄ battery discharge setup) I'd be glad if someone could at least make some sort of testing in this regard. For me it could be enough with the following capacity measurements:

After charging to 3.6V (for reference).
After charging to 3.52V (44/45th).
After charging to 3.44V (43/45th).
After charging to 3.36V (42/45th or 14/15th).

Of course feel free to contribute with whatever you would be willing to anyway.

Cheers

Rdubya18 · 2018-12-09T17:48:22.000Z

Hey thanks , that thesis was an interesting read . I just started delving into this subject . I didn’t realize how many variables , or to the extent , impacted life / performance . It seems almost undaunting to account for them all in determining effects .

Barkuti · 2018-12-09T20:05:03.000Z

The no-load resting voltage for LiFePO₄ cells is significantly lower than their maximum charging voltage, ≈3.4V versus 3.6V. I wonder what sort of impact does reducing this value grants with regards to cycle life and stored capacity/energy. This is something of importance for cranking batteries and/or maybe SAI replacements. After all, what is this expensive LiFePO₄ thing worth if it cannot seriously outlast state of charge sensitive lead - acid batteries?

Cheers ^:)

ImA4Wheelr · 2018-12-09T20:36:12.000Z

I don't have any technical knowledge to contribute. I can say that my gas mileage increased 3 to 5 miles per gallon when I switched over to a self-built LiFePO4/Boost Cap rig. The capacity is quite limited. I can only start my car a few times without charging. My think the mileage increase is due to the higher operating voltage (about 14.4 volts) and fast charge time. In Short, the alternator is under much less load now that it is not continuously charging the lead acid battery up to 14.4 volts. So there is that benefit.

I do notice resting voltage of 13.1 volts is reached in a short period of time (before my 9 hour work day is over). My pack is 4SP LiFePO4 parallel to 6S3P BoostCaps (350 Farad D Cells).