SoH / Battery health status

What does that actually mean?

Myths and facts about the condition of an EV battery: What you need to know.

Nikolaus Mayerhofer

In this text, we answer questions about the battery health (SoH) in electric vehicles and plug-in hybrids. It is important to understand the different definitions of SoH and which one is actually relevant. Discover a new perspective and start seeing electric vehicles through different eyes.

SoH stands for "State of Health," which is the health of an electric or plug-in hybrid car's battery. To know what state the battery of an electric or plug-in hybrid car is in, the SoH must be determined. However, electromobility is still a young industry in which there are no uniform standards and norms. The lack of standards also means that there is no uniform method for calculating battery health (SoH). Our starting point is what every EV driver cares about - range.

The formula is simple! Theoretically...

Determining the SoH using range could be based on the following calculation:
"Range current" (corresponding to the current state of the battery) divided by "Range new state" - the result is then given as a percentage value. However, the decisive factor in this calculation method is that the same driving style is used as the basis for both values. Easier said than done! After all, every driver has his or her own individual driving profile. External influences also affect the extractable energy of the battery and thus the range of the vehicle being tested.

But that's what WLTP is for! Unfortunately also only theoretically...

The most transparent calculation method, independent of external influences, would be the SoH determination based on the so-called WLTP driving cycle. WLTP stands for "Worldwide Harmonized Light-Duty Vehicles Test Procedure", a test procedure used to determine the energy consumption of vehicles. The WLTP data is intended to reflect a driving profile that is as close to reality as possible, taking into account various influencing factors. Of course, one could now argue again about the term "closeness to reality," and it is no secret that for many cars the real-world range is far below their WLTP range. The advantage, however, is that this method is the same worldwide and is therefore comparable and independent of the driver's driving style.

In principle, the formula "WLTP current" divided by "WLTP new" would again be used - the value for "WLTP new" is that specified by the manufacturer for the range. To determine "WLTP now", the car to be tested would actually have to be driven according to WLTP standards. However, this procedure costs vast amounts of money, is very time-consuming and is therefore not an option for the average consumer. The reference value, the WLTP range new state, on the other hand, could however be conveniently taken from the manufacturer's data sheet to calculate the percentage value.

A simple solution is to measure during the charging process! Unfortunately, that is not so simple after all...
The simplest solution would be to measure the quantity to be charged. However, this is influenced by external factors, the charging method, etc., and will thus turn out to be larger than the amount of energy actually stored in the battery. Therefore, it is useless as a determinant of battery condition.

The performance-determining parameters of a battery are its nominal voltage in volts [V], its capacity in ampere-hours [Ah] and the resulting amount of storable energy in kilowatt-hours [kWh]. However, these parameters change not only due to the amount of use, but also due to environmental influences (such as temperature), discharge characteristics (driving profile), etc.

AVILOO solution - manufacturer-independent and objective

In order to still be able to present our customers with an honest, meaningful result, our calculation of the battery's state of health is based on the extractable energy in kWh. The extractable energy in kWh is comparable to the tank capacity in liters of gasoline or diesel that can be extracted from an internal combustion vehicle.

As mentioned before, however, there are other serious differences or peculiarities in this comparison that are not known from the internal combustion drive. As we already know, the extractable energy in kWh is strongly dependent on the temperature of the battery, as well as the discharge characteristics and other aspects. Compared to the tank capacity of an internal combustion engine, for example, the amount of energy stored in the battery cannot always be used to 100%.

Therefore, it is important to AVILOO to consider numerous correction factors when calculating the result. For example, AVILOO calculates in accordance to a battery temperature of 25° and a WLTP-like driving profile for each evaluation.

This is how the SoH (state of health) is calculated using our PREMIUM test:

You connect the AVILOO box to the OBD interface of your vehicle and drive in your individual driving style until the battery is discharged from 100% to 10%. You don't need to pay special attention to anything or adjust your driving style.

The analysis is based on all data collected during the discharge. Millions of battery-relevant data points from the vehicle are transferred to the AVILOO Battery Data Platform in real time. Once the discharge is complete, the transmitted data is validated and from this the state of health (SoH) the state of the battery is analyzed.

All data is evaluated and converted to a comparable result on our AVILOO servers using the necessary compensation factors. The calculation is conducted according to the SoC display in the vehicle on the total 100% and is transparently displayed in kWh on the battery certificate. The result is the value used for the assessment of a battery condition.
With the calculation "energy content now" (measured usable energy of the test vehicle /100% to 0% according to the display/) divided by "energy new condition" (measured usable energy of the tested vehicle model in new condition /100% to 0% according to the display/), we obtain a percentage value that provides information about the state of health (SoH).

The basics of the analysis:

The State of Health (SoH) of the battery is conducted using complex calculations, algorithms and models. Two important factors considered in the calculation are temperature compensation and discharge rate compensation (type of driving).
To ensure temperature independence during the battery test, each measurement result is compensated to a battery temperature of 25°C.
To ensure discharge rate independence during the battery test, each measurement result is compensated to a discharge rate typical according to the WLTP cycle.

Example:

The measured battery capacity (100% to 0% according to the display) for a car model is 60 kWh, which is the value for "energy content new state".
In the AVILOO PREMIUM test, only 54 kWh can still be removed, this is the value for "energy content now".
54 kWh / 60 kWh = 0.9 * 100 = 90%
This means that the SoH of this battery is still 90% compared to new condition.