Analysis of approximately 10,000 used electric vehicle battery tests across hundreds of UK dealerships, conducted by battery diagnostics firm Generational, reveals that while most packs remain healthy, a small minority harbor cell-level voltage imbalances that standard State of Health (SoH) metrics miss.
The study indicates that the median used EV battery is exceptionally well-balanced, showing a voltage gap of just 8 millivolt (mV) between its highest and lowest cells. However, 1 in 85 tested vehicles displayed a significant voltage imbalance exceeding 50 mV, while 1 in 200 exceeded 100 mV—a threshold at which, according to Generational, some manufacturers’ inspection procedures treat a pack as failed.
State of Health vs. Cell-Level Diagnostics
Standard State of Health checks measure overall battery capacity, but they do not guarantee that individual cells inside the pack are functioning evenly. Because hundreds of cells operate in series, the battery management system (BMS) restricts total pack output or charging based on the weakest cell to prevent damage.
- Median Variance: 8 mV difference between highest and lowest cells across typical used EVs.
- Moderate Risk (1 in 85): Imbalance above 50 mV requiring further investigation.
- Severe Risk (1 in 200): Imbalance exceeding 100 mV indicating a potential underlying fault.
“Reporting on State of Health is essential for used EVs, but it does not tell the full story of battery condition,” said Oliver Phillpott, CEO of Generational. “It measures capacity, not necessarily whether the cells inside the pack are functioning correctly. Cell imbalance problems are often imperceptible while the battery is developing a fault that could affect range, charging, or value later on.”
Financial Impact on Dealers, Lenders, and Buyers
EV batteries can account for over 40% of a vehicle’s total value. With UK used battery-electric vehicle (BEV) transactions reaching record highs in early 2026, hidden cell drift presents financial risks for dealerships, finance companies, and consumers.
Cell imbalance can trigger reduced driving range, incorrect state-of-charge readings, or altered charging behavior without illuminating a warning light or dashboard message until the issue becomes severe. This can lead to vehicle returns, contested finance agreements, and expensive module replacements.
When an issue is flagged, Generational provides dealers with instructions for a guided cell rebalance. Around half of these issues persist after the rebalance attempt, with larger voltage disparities more likely to indicate permanent component faults that require repair, often under warranty. Annual testing enables early detection before minor cell drift turns into complete module failure.
