longlivity with lifepo4

[1000kw-hunter]

My system is more winter sized, and that gives other problems.

What my settings are per cell:
Low end cut off 2.7v inverter
low end cut off 2.6v bms
Balance at 3.30v and up
high end cut off 3.5v, mppt stops charging.
high end cut of 3.6v bms.

To get the most out of my cells, some advice to let them sit for not to long at 100%, in my case: 54v max on the chargers.
From the first signs of spring (beginning of march) the batts stay full up to oktober.
That is 8 months in a row.
I think charge at max 70%? at 3.35v? during the summer and one time a month let a mppt charge them to full?

 

[Korishan]

I say set it to 3.6 and let it ride. Don't worry about keep changing the charge voltage.

 

[OffGridInTheCity]

Will Prowse recently did this one - 'full charge' discussion/recommendations.

View: https://youtu.be/K9Tfivf5bAI?si=ipZbtACS7JQvKQl0

 

[Korishan]

There's also this one:

View: https://youtu.be/w1zKfIQUQ-s


Even tho he's talking about EV's, the chemistry is still LFP

 

[1000kw-hunter]

Will Prowse recently did this one - 'full charge' discussion/recommendations.

this vid and some other vids of him made me question the fact if it is wise to let them sit at 70% during the summer months.
In one of his vids he showed a graphic (i can not find it anymore....aaaaargh) what degradation those cells have if left for more than 3 months fully charged. and that toke off almost half the calendar age.

I say set it to 3.6 and let it ride. Don't worry about keep changing the charge voltage.

Above 3.5v there is not much left, really almost nothing.
And the inverters (one chinesum) are doing better with lower v.
Also the expansion of the cells, but with this i am not sure yet, there is a experiment going on right now.
A row of 32 cells is put on the shelf with no compression, just snugging up.
One row is moderate tight.
One row is put on the shelf with some force.

All were 45% soc when they went on the self.
After testing and full charge, i will see how much force i must use to get one cell out.
And of course, test a couple of cells for their remain capacity.
I think packing them with to little space will cost you capacity.
With too much space will cost you kalander life and too much stress on the terminals.
It is all an educated guess, end of our summer i hope to have some answers.....yt is to full of misinformation these days. while some like offgrid garage and Will provide decent and good info on lifepo4, hb on li ion.

My settings are more to protect the inverters from overheating, cells will outlive me and the wife.

 

[Korishan]

Above 3.5v there is not much left, really almost nothing

This is true. However, if you set your limits to 3.5, you could start to have cell imbalance easily and the BMS won't even know it. The voltage curve where it starts to spike is is barely starting at 3.5V. It doesn't get moving until about 3.55-3.6V.

Just because there isn't *capacity* beyond 3.5V, doesn't mean that it's not a good idea to still set that value just beyond. These are LFP's, not NMC's, they act completely different on the charging curve.

So you need that little bit of extra voltage to make sure the LFP's are actually fully charged. This is one of the issues I've been having with my cells. I have 2 cells that are slightly ahead of the rest as far as SoC is concerned, and when the charger is pumping in those cells will spike before the rest and the BMS kicks off the charger. Because of my current setup, I'm limited on how fast I can initially charge (cabling is getting redone this weekend, actually; new cells delivered today!!) and so I run out of charging light before they can fully get saturated. The BMS balancing just can't keep up. Hindsight, I should of gone with the 2A balance not the 1A, JK BMS as this one just can't keep up with size of battery and variations. But that's a whole other topic atm.

 

[1000kw-hunter]

I think i found the video?

View: https://www.youtube.com/watch?v=1G8WxY_jcUM

and a screenshot/

 

[Korishan]

Yes, Will goes over the merits very well in that video, and the first 20 seconds he basically summarizes the whole video up.
Short version:
If Storing the cells, then have them around 50% SoC
If Cycling the cells, aka using them, then you charge to 100% and discharge to around 0%.

I personally think 0% is a bit too low, especially considering that most inverters won't like that low of voltage anyways. And as voltage drops, current increases, so below 2.7V/cell you'd be putting more and more stress on the cells for the inverter to keep up with the Wattage output.

The graph shown Journal of The Electrochemical.... is showing storage conditions. Not cycling conditions.

To get the most out of the cells, charge to 100%, or close to 3.6V, and then discharge to about 2.7V. Going lower than that just means you are putting undue stress on the cells. Unless you have absolutely have to do this, I would not recommend it.