Glossary
State of Health (SoH)
The State of Health (SoH) describes the health of an EV battery: the ratio of the currently usable capacity to the original nominal capacity, expressed as a percentage.
What does State of Health mean?
The State of Health (SoH) is the key indicator of the ageing of a traction battery in an electric vehicle. It relates the currently usable capacity to the original nominal capacity when new and is expressed as a percentage: an SoH of 100 % corresponds to the as-new condition; a lower value shows how much storage capacity the battery has lost through ageing and use.
The SoH should not be confused with the State of Charge (SoC), which merely indicates the current charge level – comparable to the fuel gauge, whereas the SoH describes the size of the tank itself.
Why does a traction battery age?
Lithium-ion batteries lose capacity over time. Conceptually, a distinction is made between:
- Calendar ageing: chemical ageing processes that occur independently of use and are influenced, among other things, by temperature and the state of charge during storage.
- Cyclic ageing: wear caused by charging and discharging; frequent fast charging, high currents and extreme charge levels can accelerate it.
How quickly a battery ages depends heavily on the vehicle type, cell chemistry, thermal management and usage profile. Many manufacturers offer warranties that typically guarantee a certain minimum SoH over a defined period or mileage.
How is the SoH determined?
There are various approaches to determining the SoH, each with a different level of reliability:
- Reading out the battery management system (BMS): quick and widely used, but dependent on the manufacturer's estimation algorithms.
- Diagnostic and measurement procedures: specialised test methods that analyse the electrical behaviour of the battery and enable a manufacturer-independent assessment.
- Capacity testing via charge/discharge cycles: the most accurate, but time-consuming.
In assessment practice, experts increasingly use certified battery tests to document the SoH reliably – for example as part of valuations, condition reports or used car purchases. DIAVAG also supports experts with digitally documented battery tests in this growing field.
Practical example
A prospective buyer is interested in an electric vehicle that is several years old. The seller's range figures and on-board computer readings vary considerably depending on driving profile and weather. An independent battery test as part of the pre-purchase inspection provides a specific SoH value and thus documents the actual condition of the vehicle's most expensive component. On this basis, price negotiations can be conducted objectively – and the buyer knows whether the manufacturer's battery warranty is likely to become relevant.
Relevance for vehicle value and assessments
The SoH is becoming the key value factor for used electric vehicles: the traction battery accounts for a substantial share of the vehicle's value, and a well-documented SoH noticeably supports the residual value and selling price. Conversely, a low SoH can significantly reduce the market value.
For experts, this creates a new field of expertise: SoH measurements are increasingly incorporated into valuation reports, condition reports for lease returns and pre-purchase inspections. Standardised, traceable test documentation creates the trust that buyers, sellers and leasing companies need.
Frequently asked questions
What SoH value is considered good?
That depends on the age and mileage of the vehicle. As a rough guide, an SoH close to the age-typical average of comparable vehicles is considered unremarkable; many manufacturer warranties typically guarantee a minimum SoH of around 70 % over the warranty period.
Can I read out the SoH myself?
On some vehicles, the menu or an app shows an estimated value; diagnostic adapters can also read out BMS data. However, these figures are based on manufacturer estimates. For reliable statements, for example when buying or selling, an independent battery test is recommended.
What is the difference between SoH and SoC?
The State of Charge (SoC) describes the battery's current charge level, while the State of Health (SoH) describes its remaining total capacity compared to the as-new condition. The SoC changes with every charge, whereas the SoH changes slowly over the battery's lifetime.
Does the SoH affect the value of my electric car?
Yes, considerably. The battery is the most expensive component of an electric vehicle. A well-documented SoH supports the resale value, while a significantly reduced SoH can lower the market value. A certified battery test creates transparency here.
Does the SoH decline evenly over time?
No. Typically, capacity loss is somewhat faster at first, then flattens out and may accelerate again towards the end of the battery's life. Usage profile, charging behaviour and temperature conditions have a considerable influence on this progression.