State of X Definitions and Calculations
Alexander Börger, Heinz Wenzl · 2025
State of charge ( SoC ), depth of discharge ( DoD ), and state of health ( SoH ) are widely used terms in battery science. However, the definitions of what exactly is meant by these terms vary, e.g. whether the SoC refers to the nominal capacity or the actual capacity of a battery, and whether a battery has reached the end of its life at a SoH of 0 or 0.8. The accuracy of calculating battery states, which is often claimed, is questionable. Rough estimates are straightforward, but detailed evaluations are extremely complex. Nevertheless, the practical usefulness of such values remains high despite these issues. For historical reasons, SoC is used instead of “state of energy,” which is a far more relevant term, and “state of function” would provide much better information concerning the end of life than SoH . The concept of state of safety ( SoS ) is problematic, particularly when a quantitative value is calculated. Who would fly in an airplane with a SoS of 0.8? However, EUCAR hazard levels or ASIL levels could be used to assess how safe a battery is.