What is the optimal discharge and charge cycle for LifePO4 batteries?

Kulagin
Kulagin Registered Users, Users Awaiting Email Confirmation Posts: 1 ✭
edited December 1 in Energy Use & Conservation #1
I have 4 LifePO4 battery cells. What is the optimal discharge and charge cycle for them for better health?

I have 4 LifePO4 cells 304A. We get power outages for 2-7 hours total per day. I use 200Wh. I measure the voltage after each usage. In one place I've read that it's best to store it at 3.2-3.3V, and then in other I places I've read that I should fully charge them once a month to 100%. Then there are articles that say that it's good to store them at 50% and then there are other that say that it's good when the battery between 20-80%. And then there are images like this one showing bad and good voltages for the batteries:

But none(not the ones that I've found and read) give recommendations on the optimal discharge and charge cycles.

So if I have a choice, what voltage should I discharge the batteries to before charging and then what voltage should I charge them back to before stopping for optimal battery health? Is 3.22 - 3.32V good?

Also, should I charge the cells after every usage back to the target voltage(like 3.32V) or should I charge it to like 3.32V, then discharge it to 3.22V and only then charge it back to 3.32V?

And another situation: let's say the batteries went down to the lowest acceptable voltage(3.22V in this case) and then I charge them to 3.29V, so I don't fully charge them to my target voltage. Now I use them for 2 hours and the voltage drops to 3.28V, should I continue charging them to my target full charge voltage(3.32V in this case), or should I discharge them back to the lowest acceptable voltage(3.22V in this case) before charging them again?

For the charging current, I figured from this video: https://www.youtube.com/watch?v=qZg4Jxftw-c

It's better to charge at 0.05C. I can't exactly get to 0.05C, I'll charge at 0.035C(10A) or 0.07C(20A).
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  • mikekimble73
    mikekimble73 Registered Users Posts: 3 ✭
    #2
    Don't be too critical of the mess, wires and mismatch of BMS and other components. Those were testing days. Most has been wired permanently now and I have settled on JK BMS over the Daly components. I was trying to sort out a wiring scheme when using Daly but none of that is now required with the JK BMS as it has active balance built in. I was trying to keep the lights on and the wife happy so safety and looks were on the back burner.

    Do a search on Off Grid Garage Australia, as you have posted link to here. Andy gives great advice on compression as to why it is good and why it doesn't matter. He also gives great advice on storage. Balancing your cells is very important and because of that, charging them to 3.45v ~90+ percent SOC, is the only real way to know their SOC just above the * knee * of the charge curve. I have ~4000AH @ 51.2Vnom ( 304ah EVE cells ). I prefer to bring my cells back up 3.45v each to better keep them balanced during use. I currently have 2700+AH @ 51.2Vnom installed, compressed. I have ~1300AH @ 51.2Vnom yet to be installed and some are reaching 6 months in storage state. I store them at 30 to 40% SOC. I have checked a few of them by bringing the capacity down to a voltage of 3.00Vdc and recharging them with 100AH. They seem to only lose a few AH after 3 month of storage. EVE recommends compressing the cells when charging and even Andy says that something like 80% SOC is when the cells start to potentially off gas those gases left at production time as that is believed to be the cause of micro cracks. I honestly don't recall the exact explanation. Your cells may be more self supporting than mine, I have the aluminum case cells. If you have the older ABS plastic cases, I wouldn't worry one bit about compression for charging or discharging. Be very careful using any voltage as reference to SOC, as you can tell in the screenshots, AH usage and voltage are near flat. It is only the charge curve that really gives an indicator of SOC. Voltage near the 10% SOC are is tough... 

    Look at the screenshots for the last 6 months. At one time, I had only ~20 percent capacity. The voltage was not an indicator. Voltage is tough because I use 300 to 400 or more Amperage loads at times. And, to top it all off, load and charge are not predictable 90 percent of the time. The only things I have found consistent is top balancing at 3.45vpc and counting AH and AH with a Whizbangjr connected to one of my charge controllers, even that is a fairly rough indicator.

    As far as cycles... that is a deep subject... I prefer to use them and have as much capacity as I can get every chance I get. We have days of no sun too often to prepare for.

    I have also included a few pictures of one of the test racks. These are compressed with temporary wiring connecting the system. The system is in progress. 

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  • mikekimble73
    mikekimble73 Registered Users Posts: 3 ✭
    #3
    Here is one for those who gasp...  There are several strings connected yet that are to the right of the picture screen. I have built a second rack but have not placed it yet.
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  • mikekimble73
    mikekimble73 Registered Users Posts: 3 ✭
    #4
    Here it was when I used AGM cells.
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