OK, I have to clear a few things up here.
IF the cells were simply STORED after being received and they did not discharge below 2.500V then it is most unlike that there is ANY loss whatsoever. Assuming the cells were tested to 280AH they should still be capable.
If the cells were static even heat / cold would not have much affect on them.
They will HAVE TO BE TOP BALANCED ! All LFP must to Top Charged & to 3.650 and then Balanced with the other cells. This reactivates the electrolyte and is recommended by every manufacturer.
- I would wager that the current Cell Voltage is likely around 3.000-3.100 volts per cell. (that would be quite normal even down to 2.900)
LFP will ONLY DEGRADE if ABUSED !
Abuse means operated outside of specs or if improperly stored. Neither is teh case in this instance.
IF it was me, I would buy them and call them 275AH and be happy with that.
I would suggest STRONGLY that you buy a Battery Tester like the YR1035+ and test the IR of the cells at the following Cell voltage points.
2.800, [ 3.000, 3.100, 3.200, 3.300, 3.400 ], 3.450, 3.500.
The values between the [ ] brackets is the Working Voltage Range which is the flat part of the voltage curve that delivers your rated Net AH.
The IR at the different voltage points indicate how much difference between the cells, try to match them up as close as possible.
Do be aware that LFP like ALL Battery chemistries has TWO Voltage Ranges.
The Allowable Range is from 2.500->3.650. This is the voltage range where there is no Harm/Damage done to the cells. The GROSS Capacity is measured from this and is typically 5-7% over the cell rating. IE a 280AH battery gross would be anywhere from 287-292AH. (quite typical)
The Working Range is from 3.000-3.400. This is the Flat Curve that delivers the NET AH Label Rating of the cells. Most people will use from 2.800 to 3.450 to get the most while remaining safely conservative.
Note that Non-Grade A Factory Matched & Batched cells (premium) can be pushed to the edges but B-Cells cannot.
In "GENERAL" LFP cells
will deviate within a pack after 3.425 Volts and below 2.750 which is most common.
When you Fully Charge your battery packs they will settle down as the charge stops, this is quite normal as well.
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