Battery health reporting fails independent validation across manufacturers

Battery state-of-health (SOH) reported by on-board battery management systems (BMS) is the primary metric available to electric vehicle (EV) owners and regulators, yet no study has validated its reliability across manufacturers against independent measurements. We conducted an epidemiological study of 1,114 EVs across five manufacturers over 375 days to examine the accuracy of battery health reporting. Our findings indicate that battery health reporting is fundamentally unreliable, with real capacity differences ranging from 12–25% within every model, while BMS SOH correlations were low, ranging from ρ= 0.10 (non-significant) to ρ= 0.62 (only under restrictive filtering). Furthermore, 384 vehicles did not expose SOH at all. In contrast, a manufacturer-independent electrochemical marker achieved 74–89% degradation classification accuracy across all platforms without BMS data reliance. Laboratory validation confirmed that partial-voltage-window charge measurements reliably tracked reference capacity with ρ> 0.80 across all tested voltage windows (p< 0.001). These results unveil a significant structural information asymmetry that carries implications for various regulatory frameworks, including the EU Battery Regulation’s 2027 SOH transparency mandate and California’s ACC II durability requirements.