10 years is my magic number for many things to represent good value.
my electric car is a 2012 so cells are 5 years old . 16kwh bank that I have seen 63kw discharge and 45kw recharge (regen) often using 80% of the capacity and all 88 cells always recharge at the top to within 5mv.
I guess my point is that good cells can give incredible performance for many years. I will see how they are in another 5 years a the ten year mark.
I do actually have some (about 50 cells) 10ah capacity Headway lifep04 that are now 9 years old. performance is still very good for the age, partially if they were being used for a stationary storage application at low discharge rates they are able to put out close to there nominal capacity still.
In the real world there is only one way to test things and that just takes time............
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.