Average state of charge versus depth of discharge: which variable buys lithium-ion battery life?

Battery life in a working system is governed by two variables that are easily confused: depth of discharge, which drives mechanically dominated cycle ageing, and average state of charge, which drives chemically dominated calendar ageing. This note separates the two mechanisms, explains why the relevant control variable depends on utilisation and rest profile, and shows how a supercapacitor buffer on the DC link couples them, narrowing the per-event excursion, reducing resistive heating and releasing the power margin that otherwise keeps packs near full charge. It closes with the six conditions a life-extension claim must state to be checkable. An earlier version of this note was published on the author's blog: https://hrohera27.blogspot.com/2026/09/average-state-of-charge-versus-depth-of-discharge.html