My vision/thoughts on/about the lifepo4 cells from China(btw i dont think bad of them! and i dont want to speak negative about them!)
If a cell is unused lets take a 100ah cell no matter what chemestry.
It has a left over of 100ah: rated new unused.
If a used cell test for example 97ah from the original 100ah it falls into the 95-100% soh rated: new A grade.
If a used cell test for example 93ah from the original 100ah it falls into the 90-95% soh rated: A grade.
If a used cell test for 86ah from the original 100ah it falls into the 80-90% soh rated: B grade.
For li ion cells is 80% soh the minimum, you can go lower but it is not advisable.
The soh "left" has to do with the cycle life of the cell.
If a cell can deliver 4000x 100ah cycles (new unused) and its soh 100% you pay a lot.
If the same cell can deliver 4000x at 93ah(used) its soh is obvius 93% and you will pay less.
But they can sell it as new a grade, how?
Simple: the lifepo chemistry can be total drained within its V limits until there is 1% left over aka 1ah from the 100ah.
With some other chemistry you
cant.
So if i am going to sell you a cell as new a grade 10ah but it was a original 100ah you can only tell by getting the fabrics IR value of the cell and compare that to the cell i sold you, with a IR meter.
But you can still use them.
All there cells are matched with IR and "left over capacity" aka State Of Health.
And yes there can be transport damage, transported upside down, monday morning measurements, faulty calibrated equipment(must be daily calibrated), or even a too large batch of a 1000 cells that checks oke in the first 500 and skip a few....ect
I also dont think they test one single cell with there highly priced yr1035 IR tester.(they are very accurate btw!!!!)
But i think more before they dismantle a battery, they test the whole device(via canbus/usb and/or via the bms) with a computer/laptop like some do with EV batteries before they buy them.
Personally i would not mind to buy cells that have a 3000 cycle life left or 80% soh, as long as it is in the price.
If i would do every day a full cycle that will give me at least 9 years!
My advice (after testing just 12.000 to 15.000 cells myself) for all that want to buy cells from a vendor, regardless the chemistry:
Make sure that the vendor has a good return/refund policy.
Have your test equipment in good running order.
Try to get the original fabric specs and not the ones from the vendor.
Perhaps Will Prowes can make a "corner" for this on his forum? So members can upload?
After receiving your cells connect them all parallel and charge them.
Even a 100a 5v psu will do...BUT!..Keep a very close eye on the V when they are almost full, please.
After charging leave them a day connected to let them balance, 2 days is better.
If there is time 14 days is optimum.
Do a valid Kelvin method IR test and volt on 0.01.
If your IR is more than 10% of the fabric specs and you payed to much, return them.
Discharge them separately/capacity test, with testers that have a cut off.
Divide your capacity with the fabric new capacity--->is soh%
BTW really new lipo4 cells needs a "special" first charging and discharging curve.
Regarding test equipment: it is advisable to have it anyway, to test your cells every one or two years or so.
Those are my thoughts on no matter what cell or vendor, i dont want to insult nobody.
Of course i want the best kwh for my euro's.
Looking forward to your replay,
With best regards Igor
PS
And I thought I had it bad with black flies in Maine
And i will put some Swedish mosquito's into the mix