State of Health

What it measures

State of Health is the long-term equivalent of State of Charge - where SoC tells you how full the battery is right now, SoH tells you how much capacity the battery retains compared to the day it left the factory.

A new vehicle starts at 100% SoH. After years of charging cycles, thermal stress, and calendar ageing, that number falls. A battery at 85% SoH has 85% of its original usable capacity. If it was rated at 77 kWh new, it now effectively holds around 65 kWh.

How batteries age

Two processes drive degradation: cycle ageing and calendar ageing.

Cycle ageing accumulates with each charge and discharge. It’s why the 80/20 charging habit matters - shallower cycles cause less stress per cycle than full 0–100% swings. High-power DC fast charging also accelerates cycle ageing compared to slow overnight AC charging, because the rapid current flow generates more heat in the cells.

Calendar ageing happens regardless of use. A battery stored at high SoC and high temperature ages faster than one stored at 50% SoC in a cool garage. This is less relevant for daily drivers but matters for vehicles left parked for extended periods.

Real-world data

A large-scale study by Recurrent Auto across tens of thousands of EVs in active use found that most modern batteries degrade far more slowly than early adopters feared. The average vehicle with 100,000+ miles showed SoH above 90%, with only a small percentage of outliers below 80%.

Earlier-generation vehicles - particularly first-generation Nissan LEAFs without active thermal management - did degrade more severely. Those experiences shaped the narrative around EV battery fears, but modern packs with liquid cooling and sophisticated battery management systems behave quite differently.

Warranty thresholds

Most manufacturers in Australia now offer a separate battery warranty distinct from the general vehicle warranty. The threshold is typically 70% SoH over 8 years or 160,000 km - whichever comes first. Some manufacturers are more generous: Kia and Hyundai cover battery degradation below 70% SoH for 7 years or 150,000 km on most models, and their unlimited-kilometre powertrain warranty applies on top.

If SoH drops below the warranty threshold within the coverage period, the manufacturer is required to repair or replace the battery module. Claims are rare in practice, but it matters to know the threshold before buying used.

Checking SoH

Most EVs don’t display SoH directly in the standard vehicle interface. Options include:

• OBD-II adapters with EV-specific apps (Leaf Spy, Car Scanner, Torque Pro with the right PID) - works on many vehicles
• Manufacturer dealer diagnostics - all service centres can pull SoH from the BMS
• Third-party services - companies like Recurrent offer SoH checks that are increasingly useful for used EV valuations

When buying used, asking for a dealer-printed SoH report is reasonable and increasingly common in Australia.