  The battery state of health (SOH) based on capacity fade and resistance
increase is not sufficient for predicting Remaining Useful life (RUL). The
electrochemical community blames the path-dependency of the battery degradation
mechanisms for our inability to forecast the degradation. The control community
knows that the path-dependency is addressed by full state estimation. We show
that even the electrode-specific SOH (eSOH) estimation is not enough to fully
define the degradation states by simulating infinite possible degradation
trajectories and remaining useful lives (RUL) from a unique eSOH. We finally
define the deepSOH states that capture the individual contributions of all the
common degradation mechanisms, namely, SEI, plating, and mechanical fracture to
the loss of lithium inventory. We show that the addition of cell expansion
measurement may allow us to estimate the deepSOH and predict the remaining
useful life.
