The fundamental issue is that this only applies if longevity is a high priority, and the use case allows for getting a high cycle count lifespan to start with. Also presumes that maximising every scrap of mAh capacity utilisation is not important.
The key is that the **charging setpoint** is an entirely different number from the **cell voltage** after a few hours of resting isolated.
Also when overcharging (in my longevity sense above) there will be, even after **days** of resting isolated, some degree of "surface charge" voltage over the true baseline, that does not proportionally represent any significant SoC / capacity utilisation.
Once that is dispelled by withdrawing say 0.1% of the pack' mAh capacity, call that "100% resting Full voltage" for that chemistry / model cell.
As a side note, IMO even a low current but constant Float charge at a voltage **below** that definition of Full is not optimal for longevity, beyond the damage inflicted by sitting at a high SoC.
OK back on topic.
Say you define your endAmps spec for "Precise 100% Full" for benchmark testing calibration etc, as
4.04V at 0.5C, held for Absorb / CV until trailing current tapers down to 0.05C then stop
Note this is a higher SoC than you would use for your definition of "daily usage cycling Full", which might as well be defined as "charge
to a voltage and stop", IOW Bulk / CC stage only, no Absorb / CV stage at all, both gentler more conducive to longevity and very simple to implement in a failsafe manner.
Now back on topic again.
What happens if your charging current, rather than the normal rate say 0.2C or higher, is down way low, even below the endAmps spec as discussed above?
Then if you are trying to do a CV stage, you can't even tell when to stop!
That is why when charging current is very high, it is safe to use a bit higher V setpoint, the "after charging sag" is greater, represents a lower SoC point than at much lower current rates.
At **very** low current rates like below 0.1C even if using a CC-only profile, to be safe the termination setpoint should be tweaked downward a bit.
Examples from a real life bank, using LFP chemistry in this case 160Ah Thundersky cells, where this issue is more both critical, and the delta between charge setpoint vs resting cell voltage is a bit higher:
Vendor spec from Mr Winston Chung Hing Ka might be
way too stressful except for when longevity is not desired but every scrap of mAh capacity is.
My own definition of "100% Full, for benchmark testing calibration etc" is
3.46Vpc at 0.4C, endAmps 0.05C, then stop
My usual "daily usage cycling Full" is
Charge to 3.52Vpc and stop
for normal current rates, but maybe
Charge to 3.35Vpc and stop
in a situation where the C-rate may fall below 0.1
Note "resting Full" cell voltage is 3.34-3.36Vpc, anything over 3.37V is just surface charge.
Usually this whole idea is only relevant with solar-only contexts, and the danger of overcharging IRL is mitigated by concurrent loads and the fact that in most places the sun does set every night.
I hope this is clear and helps explain that overly terse edge-case generalization.
