SOH (State of Health)
Battery health: current capacity vs nominal. 100% = new battery; 70-80% = end of practical service life.
SOH (State of Health) compares the CURRENT capacity of the battery to its NOMINAL factory capacity. A new battery has SOH 100%. With use, sulphation, internal corrosion and loss of active material reduce effective capacity: at SOH 80% an 80 Ah battery actually delivers 64 Ah. Practical end of service life for automotive use is considered when SOH drops below 70-80% — at that point CCA has fallen enough to cause cold-start failures. Professional diagnostic equipment (Bosch BAT, Midtronics) measures SOH by injecting a known load and observing the dynamic response; basic voltmeters do NOT measure SOH, only SOC.
In Start-Stop vehicles the ECU continuously estimates SOH; when degradation is detected, it automatically disables the Start-Stop function to protect engine starting — usually the first sign that the battery is reaching end of life.