SOC is not an absolute value. Its a relative value with no more than 80% of  available capacity as max limit. "Ageing" can happen in forms of -Capacity fade (Loss of active Li+ions) -Power fade (Impedance rise)   A good explanation using Nissan Leaf as an example. It seems there is a Lawsuit against Nissan because the battery can loose more than 27% of its capacity EVERY year in a Hot climate(like Sri Lanka!)