What is SoH?
The “State of Health” (SoH) of a battery is the key parameter for battery aging / degradation of a battery electric vehicle (BEV). Because the battery is the most important component of the vehicle it is very important to determine the vehicle condition and estimate its residual value. The SoH reveals which amount of the reference/promised capacity or energy of the new battery is available today.
SoH = "Current Capacity" / "Nominal Capacity" (like on the battery report)
It is therefore the indicator to identify the current phase of your battery-lifetime. In general, the expected end of life criteria for an electric car is a SoH of 70% - 80% depending on applied use case. There is not a unique method for calculation the SoH of a traction battery. The calculation is possible by focusing on range, energy or capacity.
Understanding BEVs: Capacity, Energy and Driving Range
If you ask electric vehicle owners about the parameters capacity, energy and driving range, the driving range will be the parameter most of them can relate with. When it comes to energy and especially to capacity, less people will have a common understanding of how they can translate these parameters to understandable performance values for their vehicle. Nevertheless, they will have a common understanding that their driving style has an influence on the driving range. Even if the range of their car is, let’s take 400 km or in terms of energy 60 kWh as an example, they will rather reach a destination 400 km away if they drive a constant 120 km/h than 160 km/h. The energy consumption increases with the driving speed.
This example shows that driving range and remaining energy are common parameters, but they are not reliable as they are highly dependent on the driving style and environmental conditions like temperature and recuperation as displayed in the image below. And let's be honest, even if a performance indicator is easily understandable, it is only of use for the driver if it is accurate.
Doing a one-time measurement on range or energy might be possible, but repeating this measurement under the same external conditions with the same driving style is only possible in a test bench and associated with huge costs. Therefore, the results will not be comparable as the same conditions for the measurements cannot be achieved. A low accuracy of these approaches is therefore to be expected.
Capacity might be a rather technical parameter, but it comes with the huge benefit of being a reliable and comparable indicator for battery aging. It is independent of external factors and the charging or discharging characteristics. Therefore, the capacity-based SoH calculation method is the one that deserves to be trusted when it comes to battery aging and SoH determination.
Battery Aging
Battery aging is the process by which a battery gradually loses its performance capabilities over time due to chemical and physical changes in the battery cells. Factors contributing to aging include cycle count, depth of discharge, temperature, and charge rate. Aging can result in reduced energy capacity, power capacity, and efficiency, and can increase the internal resistance of the battery.
In case of EVs this means that the possible driving range of your vehicle will decrease over time. The amount of decrease is dependent on the factors mentioned above.Batteries with such a SoH value can still be used as energy storages in second life applications before they need to be recycled.
