SoH calculation methods - range based, energy based, capacity based
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. The different methods, a comparison as well as their advantages and disadvantages will be covered as follows.
Range-based method
The most straightforward parameter you might think of to estimate the “State of Health” (SoH) of your vehicle’s battery is the driving range.It seems pretty simple to compare the current range you can drive your EV compared to the range you were able to drive when the car was new.
SoH = current WLTP range/ initial WLTP range
You might ask yourself now what WLTP means. WLTP stands for “Worldwide Harmonised Light-Duty Vehicles Test Procedure”. This is a standardized process to obtain driving ranges by testing with clear instructions on driving style and test setup. To achieve comparable results for driving ranges one needs to acquire the driving ranges in an identical way. This means the driving style and external factors like temperature need to be identical.
Additionally, the same measurement method has to be used to get comparable results. A drawback of this method is that you are not analyzing the battery, but the whole vehicle. That means that the overall state of your car with bearings, wheel size and all its components influence your results.
The vehicle attributes list the WLTP range which is determined in a standardized procedure. To determine the current WLTP range reliably the car needs to be driven according the WLTP standard which requires a test bench and lab conditions. Therefore, this approach does not pay off. The calculation of the SoH with the range-based method is therefore an easy to understand method, but neither an easy to perform, nor a reliable one.
Advantages
Initial WLTP range given by the manufacturer (measurement / calculated value)
Disadvantages
The current WLTP range is very difficult for vehicle users to get as the test scenario has very strict rules
Range depends on driving style, temperature (defined by the WLTP) and vehicle parameters (especially powertrain losses which also increase during lifetime)
Expensive testing
Energy based method
Energy is the total amount of work a battery can perform and is measured in units such as watt-hours (Wh) or kilowatt-hours (kWh). Imagine it as the total distance a vehicle can travel on a full tank of gas - the work the battery can accomplish.
The energy-based calculation takes the energy the battery can store into account and compares the current value with the initial one.
SoH = available extractable energy/initial extractable energy
The initial extractable energy is provided by the vehicle manufacturer (net energy content), but normally this is not the maximum amount of energy that can be stored in the battery (gross energy content). The maximum amount of extractable energy of the battery is usually bigger than the defined size. The vehicle manufacturer limits the usage of the battery to increase safety and to minimize battery degradation (Safety buffer). The battery is “oversized”. These limits are calculated by the battery management system and depend on temperature, charging / discharging power, battery age and internal estimations by the vehicle manufacturer. These limits are not transparent and change on user behavior and external conditions, so that the available energy within these operation limits varies both negative and positive.
Advantages
Initial extractable energy value given by the manufacturer
Enhances interpretability for end-customers by focusing on range or energy metrics
Disadvantages
Available energy depending on variable operation limits defined by vehicle manufacturer
Accuracy depends on measurement method like charging/discharging power and external factors like temperature, driving style and limitation defined by vehicle manuafcturer
The calculation of the SoH with the energy-based method is therefore an understandable method, but neither an easy to perform, nor a reliable one, although it is not dependent on the whole vehicle degradation as the range-based method. Recreating testing environments to measure energy for both new and used batteries is nearly impossible.
Capacity based method
Capacity is the amount of charge a battery can store, and it's measured in units such as ampere-hours (Ah) or milliampere-hours (mAh). Analogous to the size of a gas tank, the capacity represents how much energy a battery can potentially hold. We expect that energy extraction will decrease steadily as the battery capacity reduces over time. Comparing the capacity of a new battery with that of a used one provides the most accurate indication of performance decline and State of Health (SoH).
SoH = current extractable capacity/nominal capacity
The capacity is a battery parameter that is independent from external factors. It is therefore easier to get comparable results. The nominal battery capacity is given by the cell manufacturer. This is the capacity the battery is in any case able to deliver in the new state, but in reality, the initially extractable capacity is higher. This is due to the limits the vehicle manufacturer sets. It is also the reason for this higher extractable capacity.
This method actually outperforms the range and energy-based methods in terms of reliability and independence from external factors.
Advantages
Independent from vehicle manufacturer as nominal battery capacity (=initial extractable capacity) is used
Comparable result
Independent from external factors
Disadvantages
Vehicle manufacturer provides only energy content to users since it is more understandable, while cell manufacturers provide the nominal capacity → battery know how necessary.
