Thanks - that is basically what I was looking for.
I experienced something similar and can tell you what I did about it.
I did a first top-balance on 16 new 280Ah cells at 10A (0.0022C). This lasted weeks and was done indoors at 65-70C.
I then reconfigured into a 16S battery and performed a capacity test at 150W (3A / 0.011C). This allowed me to assess the capacity of my weakest cell and rank the others up to the strongest cell.
I charged the strongest 8 cells back to ~50% SOC using a 10A 8S LiFePO charger but it had become cold by then, so charging was performed at 45-50F (and at 0.036C).
I then charged the strongest individual cell to 3.65V using a 10A supply (0.36C), rested for one hour, and discharged at 10A (0.36C). All of this was performed in a cold room at 45-50C.
This strongest cell that delivered close to 280Ah in the 16S capacity test delivered only 160Ah of capacity when tested individually.
This was obviously very concerning so I repeated the exact same sequence with the second-strongest cell to the same result (<60% of the capacity from the 16S test charged at 65-70F).
After this second result of occurred to me that the change in weather and colder temps could have impacted these results so I reviewed EVE’s specs and decided to recreate them as closely as possible using a temperature control chamber.
My quick and dirty temp controlled chamber involved placing a brewer’s 40W heatpad on a layer of packing foam from the 280Ah cell packing. Cell was placed on the heatpad, heatpad connected to terrarium thermostat, thermostat probe taped to cell, and a cardboard box lowered over the entire assembly.
I set the thermostat to 77F (25C) and waited for the thermostat to settle (meaning cycling between in and off) before charging.
Charge cycle @ 10A / 0.36C was performed to 3.65V, followed by a 1 hour rest, followed by a 10A / 0.36C capacity test (all at 25C).
This time both the strongest cell as well as the second strongest cell delivered a full 280Ah.
I was relieved my cells have delivered rated capacity and have moved on to other priorities, but once my 8S battery is assembled, I plan to circle-back and compare capacity charged at 50F to capacity charged at 77F.
If capacity is as different as this initial experience indicated (~60%), I will build my battery into a closed and insulated container which I will heat control for at least 65F if not 75F.
If I don’t see much difference when testing the full 8S battery charged at 50F, I’ll chalk it up to some newbie mistake I made when charging individual cells for the first time.
Reviewing the EVE datasheet in detail, note that they specify discharge performance at -20C (>=70%) but this was following ‘standard’ charge at 25C. There is absolutely no specification indicating capacity when charged at temperatures lower than 25C.
So I don’t know if my bargain grey-market 280Ah cells underperform Grade A cells when charged at temps below 25C or not (since EVE’s spec says nothing about the performance to expect under those non-‘standard’ charge temperatures), but the fact that delivered rated capacity when charged under the conditions EVE specified s makes me satisfied with the performance of these cells.
If I confirm charge capacity degredation at lower temps and I’m the only DIYer unfortunate enough to recieve cells with that limitation, I’ll use temperature control in use and chalk the hassle up to the cost of getting the bargain I did.
But putting together a temperature control chamber for characterization is as easy as I outlined:
-Insulation from below (foam, wood, or cardboard)
-40W heating pad & terrarium/vivarium thermostat
-cardboard box to insulate the entire space from ambient air / temps
