Every electric vehicle has a battery management system that reports state-of-health — a single number, usually a percentage, that tells the owner how much capacity remains relative to new. This number governs warranty claims, resale valuations, second-life battery markets, and regulatory compliance. It is, for most practical purposes, the battery's medical record.
Wassiliadis et al. tested 1,114 EVs across five manufacturers and found that this number is essentially fiction. Correlations between manufacturer-reported SOH and independently measured capacity ranged from rho = 0.10 to rho = 0.62 under the most generous filtering. Real capacity differences of 12-25% existed within every model, but the BMS readings failed to track them. Worse, 384 vehicles — more than a third of the fleet — did not expose SOH data at all.
The mechanism is a combination of algorithmic opacity and misaligned incentives. Each manufacturer uses its own proprietary algorithm to estimate SOH, calibrated to its own reference conditions, updated on its own schedule. There is no standard for what SOH means, how it is calculated, or how often it is recalibrated. A battery that one manufacturer calls 90% healthy, another might call 82% or 97%. The number is not a measurement. It is an opinion, issued by the party with the strongest interest in the answer being favorable.
Meanwhile, a manufacturer-independent method — partial-voltage-window charge measurement — tracks actual capacity with rho greater than 0.80 across all test conditions. The physics-based measurement works. The proprietary metric does not.
When the entity responsible for reporting a system's condition is also the entity whose liability depends on that condition, the report will converge toward the reporter's interest rather than toward ground truth. Reliable assessment requires either an independent measurer or a measurement method whose physics cannot be reinterpreted. The gauge must answer to the world, not to the manufacturer.