# Explainable Artificial Intelligence for State of Charge Estimation of Lithium-Ion Batteries
## 3 Methodology
### Data  
The battery-related data used within this work were collected by AVILOO GmbH [38]. AVILOO is a company that specializes in the diagnostics of electric and hybrid vehicle batteries, specifically the testing of lithium-ion batteries with independent, accurate, and reliable diagnostics. AVILOO's battery diagnostics provide transparency and confidence in order for potential buyers to be informed when purchasing EVs. The data were directly obtained from the Battery Management System (BMS) of EVs. Currently, AVILOO has data from more than 70 different car models, including long-term monitoring data as well as several full discharge cycles, albeit sparse with exceptions. Within this work, we focus on the vehicle model of _Volsswap e-Golf_ (e-Golf), due to the vast amount of data available and the variety of age, mileage, and SoC amongst the Volkswagen e-Golf cars. The details are available in Table 2.  
**Signal description:** The e-Golf dataset contains various battery-related signals, such as voltage, current, and temperature data of the battery. Table 3 offers an overview of the available signals. The battery pack of an e-Golf consists of 27 battery modules. A module consists of cell blocks connected in series, and each cell block is a parallel connection of single cells. Two different modules were used in the e-Golf battery pack: 4S3P, and 2S3P. A 4S3P module consists of four cell blocks in series, and each cell block is a parallel connection of three single battery cells. The 2S3P is a series connection of two cell blocks. These modules (17\(\times\) 4S3P and 10\(\times\) 2S3P) were connected in series, resulting in 88 units of cells put in 3P (3 cells in parallel). The battery cells used in the e-Golf had a rated capacity of 37 Ah and a nominal voltage of 3.7 V. Therefore, the overall capacity of the battery pack was 111 Ah, and the overall nominal voltage was 325 V.  
Two different SoC signals were obtained from the BMS. The SOC REAL signal is related to the 111 Ah overall capacity of the battery pack. However, to protect the battery pack and increase its service life, the voltage limits were restricted. This means that only 100 Ah were available for capacity and discharging when the car was driven. The SOC DISPLAY signal is related to the restricted 100 Ah capacity and is also the signal that is provided to the driver. Figure 1 shows the two SoC signals and how they relate to the battery pack's capacity. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Vehicle Model** & **Cars** & **Sessions** & **Full Cycles** \\ \hline Volkswagen e-Golf 35 kWh & 200 & 7000 & 208 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Vehicle model: Volkswagen e-Golf overview. We have introduced two new features derived from the existing measurements: differential capacity and delta resistance. Both features were calculated based on the measurements of current, voltage, and charge. They were used to aid SoC estimation. Differential capacity represents the change in charge concerning the change in voltage. It was calculated as follows:  
\[C_{diff}=\frac{\Delta Q}{\Delta V} \tag{1}\]  
where \(Q\) represents the charge and was computed using the cumulative sum of the product of current and time intervals. Figure 1: SOC DISPLAY and SOC REAL signals.