Planning to buy a used electric car? The battery check is the key step. Use these practical steps to review State of Health (SoH), charging habits, service history, a road test, and independent diagnostics.

What “State of Health” (SoH) really means

  • SoH estimates how much energy the battery can store compared to when the car was new. It’s normal for SoH to decline over time.
  • Decline is influenced by temperature, how often and how the car is charged, driving style, and storage conditions.
  • SoH is reported by the Battery Management System (BMS). Treat it as a solid starting point, but confirm it with additional checks.

Questions and documents to request from the seller

Ask for and review:

  • The vehicle’s service history, especially any work on the high-voltage system and any battery module replacements.
  • Charging habits: how often the car was charged on home AC vs. DC fast charging, and to what percentage it was typically charged.
  • Any official battery reports or in-car screenshots showing SoH and system logs (if the vehicle provides them).
  • Information on software updates and whether a recent BMS calibration was performed (for example, a supervised full charge/discharge cycle).
  • Warranty terms if still active, and the criteria under which a module replacement would be approved.

Quick on-site checks

  • Visual: inspect the charge port, cable condition, any warning lights on the dash, and everyday operation of HVAC/thermal management (important for the battery).
  • Charging test: if possible, briefly connect the car to AC charging and check for stable power without unexpected interruptions. Note fan/pump noise—excessive or unusual activity can point to thermal stress.
  • Test drive: reset the trip and energy consumption, drive in typical conditions, then compare energy used to the battery percentage drop. This gives a practical snapshot of usable capacity and range in your own driving.

In-depth diagnostics: BMS and OBD data

If you have access to tools compatible with the vehicle (e.g., an OBD interface with a suitable app or a professional service tool), request or read:

  • SoH and the total usable capacity reported by the BMS.
  • Cell balance (differences in module voltages): larger deviations can indicate imbalance or aging of certain modules.
  • Battery temperatures under load and while charging: large differences between modules can suggest an issue with thermal management.
  • The number and type of charging cycles (if available), and logs of high-temperature events or power limitation. Note: BMS data is best interpreted in context—combine it with the test drive and service history.

Independent expert inspection before purchase

If you’re unsure, invest in an independent diagnostic at a specialist EV service:

  • A capacity test under controlled load and a repeatable check of real-world range.
  • Analysis of cell balance, thermal control, and any faults in the system log.
  • Inspection of high-voltage components (connectors, insulation, contactors), the battery cooling system, and the 12 V system.
  • A written report you can use to make a decision and negotiate the price.

How to make the call

  • Compare SoH, test-drive results, BMS readings, and the independent shop’s findings. Look for consistency across sources.
  • Maintenance plan: ask about recommended charging habits and periodic checks to slow future battery aging.
  • If signs of cell imbalance, frequent power limits, or cooling issues are present, factor in the added risk—or consider another vehicle.

For more practical tips on buying and maintaining vehicles, Browse all guides.