State of Health (SoH)
State of health (SoH) is a relative measure of a battery’s present condition against a defined new or reference state. A percentage is meaningful only when its health metric, baseline, and test conditions are stated.
What the number can represent
SoH is not one universal measurement. It may compare present ampere-hour capacity or usable energy with a reference value, describe remaining power capability, or use resistance growth as a health indicator. These results can differ for the same battery because capacity loss and resistance growth are related but distinct ageing effects.
A BMS or dashboard value is normally an estimate derived from vehicle data and an algorithm; it is not automatically the result of a controlled capacity test. UN Global Technical Regulation No. 22 makes a similar distinction for its energy-based durability metric, State of Certified Energy (SOCE): onboard SOCE is an estimate, while measured SOCE compares measured usable battery energy with certified usable battery energy.
SoH is not State of Charge (SoC), which describes the present charge level. For BMS estimation, see Battery Management System. For a practical capacity check and its limitations, see How to Verify Battery Health.