Automotive E-ssentials

Health status of a battery: SoH is not always SoH

Only those who know the State of Health (SoH) of the traction battery can meaningfully indicate the value of a used battery electric vehicle. Unfortunately, the current methods for determining a neutral and independent SoH value are not comparable.

Deutsche Automobil Treuhand (DAT) is moving with the times. In future, the experts for calculating the value of vehicles will also take into account the State of Health of the Battery. When determining the value of used e-vehicles in the Silver-DAT Software, there is now also a field for the SoH - in addition to several other parameters such as Mileage and vehicle age.

This alone shows that the reliable and independent determination of the state of health of the traction battery in an electric car is a up-to-date topic and important for pricing in the used car market. The battery is the most expensive component in the vehicle - its performance and ageing condition at the time of sale are therefore important factors in determining its value.

But what are we talking about when we talk about the SoH value? What is behind it and how can the State of Health be reliably determined? Markus Gregor, Technical Expert Battery Lifetime and Safety Certification TÜVSÜD, clarifies: ‘There is still no standard for determining the SoH. Because the calculation itself is not specified and defined, everyone can do it differently.’

Vehicle manufacturers also issue a SoH value for the battery at the push of a button. However, these values are not comparable due to a lack of transparency in their calculation. There are now several reputable methods on the market for determining a neutral and independent SoH value. Due to a lack of methodological specifications, providers pursue different strategies and sometimes rely on different parameters and references, as Gregor explains: ‘At the moment a neutral, independent SoH must be determined in a dynamic process, which means that the battery must be stressed. In concrete terms, this means that the battery must be charged or discharged.’

We asked Markus Gregor, Technical Expert Battery Lifetime and Safety Certification TÜV SÜD and PhD student at Ingolstadt University of Applied Sciences (THI), Carissma, Institute of Electric, Connected and Secure Mobility, some questions.

What is meant by the term State of Health (SoH)?

Markus Gregor: There are several definitions for the state of health, depending on the physical quantity to which it refers. However, the SoH value most of the times indicates a ratio of the current state to the new state of the battery - in relation to a specific parameter. The SoH is usually given as a percentage value. The SoH is often related to the amount of energy but can also refer to the remaining capacity or the resistance in the battery.

How robust are today's batteries in electric cars?

Markus Gregor: In reality, batteries are proving to be quite robust: the cells are already extensively tested by the car manufacturer. Today, batteries manage four-digit charging cycles without ageing dramatically. With a real range of 300 kilometres and an assumed 1,000 charging cycles, a lifetime of 300,000 kilometres is already achieved. At this point, the battery has often only lost 20 per cent or less of its energy. But you also have to bear in mind that there can always be outliers in production. Not every battery that leaves the factory is equally good. Temperature, driving behaviour and, above all, charging behaviour also have a major influence.

How comparable are the available SoH-Tests?

Markus Gregor: The results are not comparable. There is still no standard for determining the SoH. Because the test itself is not specified and defined, everyone can do it differently. When comparing the SoH values determined using different methods, we found deviations of up to seven per cent.

What data does the car manufacturer provide?

Markus Gregor: There are very different data on residual capacities, and we don't know how the values are calculated. It's not transparent. Of course, this has nothing to do with the actual performance of the battery.

Definition of SoH

A possible SoH value can, for example, be the ratio of the amount of energy currently available in the fully charged battery to the amount of energy initially available when the battery was new. Here too, however, there are different approaches to calculating some of the parameters. Accordingly, there are currently no equivalent methods or results according to definition and standardisation.

In the Euro 7 standard which is based on GTR-22, the battery status is referred to as SoCE (State of Certified Energy) and SoCR (State of Certified Range). The SoCE - this term originates from the area of type approval - is most comparable to the SoH value.

But why are the different SoH values not comparable? This is not only due to different calculation methods, but also to different reference values, as Markus Gregor explains: “It is not so clear which value is used as the new condition value. A distinction is made between the gross energy quantity and the net energy quantity.” The net value indicates how much energy can be used for driving, as a certain amount of energy is used as a buffer and to keep the vehicle ready to drive (Infotainment, air conditioning).

In the case of the VW ID.3, the difference is considerable: the gross energy output is 62 kWh, while the net energy output is only stated as 58 kWh. For example, if a SoH value refers to the gross value, this is inherently somewhat worse than a test method based on the net value.

Not all cells are the same

Further uncertainties arise from battery production itself. Gregor: ‘The individual cells can have a range of a few per cent in terms of energy content due to the production process alone.’

One uncertainty factor lies in the manufacturer's battery management system: nobody knows how the battery management system will regulate the battery if less energy is available over time. It could well be that the system then releases energy over the lifetime so that losses are compensated for and are less noticeable to the user.

Manufacturer information

Caution should also be exercised with the values shown by the Manufacturer on the display, explains Gregor: ‘There is a physical State of Charge (SoC) of the battery and a state of charge that is shown on the display. And there are deviations! We have found deviations of up to four per cent!’ For example, it can happen that the vehicle suddenly displays a few per cent more battery power after an over-the-air software update.

Coypright: Autoservicepraxis