What is battery State of Health (SoH)?
State of Health (SoH) is the ratio between a battery's current usable capacity and its capacity when new, expressed as a percentage. A battery at 80% SoH stores 80% of its original energy; this threshold is often used to define the end of a battery's first life in a vehicle.
70 to 80% State of Health is commonly considered the end of an EV battery's first automotive life.
How SoH is measured
SoH can be estimated from capacity tests, internal resistance measurements, or continuously by the battery management system (BMS) using charge/discharge data and model-based algorithms.
Continuous on-board estimation is the most useful for circularity, because it produces a trustworthy history that follows the battery into its second life.
What degrades a battery
Battery ageing is driven by calendar time, cycling depth, fast charging, high and low temperatures, and time spent at high state of charge. Two identical packs can age very differently depending on use.
From SoH to Remaining Useful Life (RUL)
Remaining Useful Life predicts how long a battery can keep serving its purpose before reaching a threshold (for example 70 to 80% SoH). RUL turns a snapshot (SoH) into a forecast, enabling predictive maintenance and better reuse decisions.
Frequently asked questions
- What does battery SoH mean?
- State of Health: the current usable capacity of a battery compared with its capacity when new, expressed as a percentage.
- What is a good SoH for an EV battery?
- Most EV batteries retain 85 to 90%+ SoH after several years; 70 to 80% is commonly considered the end of automotive first life, though the battery can still serve in stationary storage.
- How can I check my EV battery health?
- Through the vehicle's on-board diagnostics or a professional battery health check; some manufacturers expose SoH directly in the vehicle app.
- What is Remaining Useful Life (RUL)?
- A prediction of how much longer a battery can operate before its SoH falls below a defined threshold, used for predictive maintenance.