Battery health reporting fails independent validation across manufacturers

Battery state-of-health (SOH) reported by on-board battery management systems (BMS) is crucial for electric vehicle (EV) owners and regulators, yet its reliability across manufacturers has not been previously validated against independent measurements. This study aims to address this gap through an epidemiological analysis of 1,114 EVs from five manufacturers over 375 days. The findings reveal that BMS SOH is fundamentally unreliable, with real capacity differences of 12–25% observed across models, where correlations between SOH and actual capacity varied from non-significant to moderate only under restrictive filtering conditions, and over one-third of vehicles failed to report SOH altogether. In contrast, a manufacturer-independent electrochemical marker demonstrated a degradation classification accuracy of 74–89% across all platforms without relying on BMS data, while a controlled laboratory evaluation confirmed that partial-voltage-window charge measurements correlate highly with reference capacity across numerous voltage windows. These results highlight a critical information asymmetry related to battery health reporting, which has significant implications for regulatory practices, including the EU Battery Regulation’s 2027 SOH transparency mandate, California’s Advanced Clean Cars II durability requirements, and considerations for warranty enforcement, resale value, and second-life battery markets.