## 2 Materials and Methods
### Test Process  
The tests were executed on the dynamic platform of the ZalaZONE Automotive Test Track in Hungary [46; 47]. Fault diagnosis requires many measurements from each cell, a time and resource-consuming process. In addition, the test results can be affected by several factors, such as external temperature, driving style, terrain conditions, heat exchange systems, and the operation of other onboard consumers. Therefore, test cycles were performed under the authors' defined test conditions. The structure of the test measures is defined according to different criteria and factors. First, to test the stability of the high voltage battery system's specific data extraction and storage process. Three different cases were considered in the measurement methodology:  
* constant speed: 30 km/h, 40 km/h, and 50 km/h;
* stepped constant: 30-40-50 km/h;
* dynamically variable speed. It is important to note that each test was carried out separately, first with three steady constant progress tests, followed by two stepped constants, and finally, three dynamic tests. Furthermore, the vehicle was placed on the test track in a fully charged condition. The tests were uniformly 900 s in duration, and the diagnostic data to be observed and logged from the CAN network were as follows:  
* system voltage [V];
* current [A];
* 88 cell voltages [V];
* four separated battery temperature values [\({}^{\circ}\)C];
* vehicle speed [km/h];
* GPS (Global Positioning System) speed [km/h]. The system-level voltage represents the voltage of the entire system, with all modules connected in series and parallel. The 2020 Volkswagen e-Golf lithium-ion battery used for the measurements has a nominal capacity of 35.8 kWh, of which 32 kWh can be used. The battery pack has a 3p88s structure, so it contains 264 cells (type: NCM333), of which three are connected in parallel and 88 in series. The 400 V architecture has a nominal voltage of 325 V. During data collection, information can be extracted from 88 cells, i.e., elements connected in a row. In order to monitor and record the data coming over the CAN (controller area network) during the measurements, it is necessary to decrypt the car's communication system and use a CAN BUS (controller area network bus) decoder file. When mapping the vehicle communication system, the CAN communication of the vehicle's Central Gateway control unit was first observed as contactless, but little battery cell-specific data were accessed. In the second procedure, without a galvanic connection, the communication between the VAG-COM VCDS interface and the Central Gateway was investigated, through which it was possible to retrieve cell information; the schematic of the method is shown in Figure 2. As soon as the cell voltages were requested via the VAG-COM software, messages were displayed on the CAN bus. Network access, serial communication conversion from CAN communication, message reading, and information exchange with external sensors are all provided by this telemetry system based on the CAN protocol. The following elements and devices were applied in the measurement system: Kvaser (Molndal, Sweden) Memorator R-SemiPro CAN USB Interface; AEM (Hawtrone, CA, USA) 30-2206 GPS Sensor; ELM (London, ON, Canada) 327 OBD-II Interface; VW e-Golf MK7 Vehicle Central Gateway; Murcal Terminating Resistor 120 \(\Omega\) (Palmdale, CA, USA); 12 V Power Supply; CAN Hub (Velke Porici, Czech Republic). Experience has shown that it is not enough to monitor the CAN BUS network but to send a CAN message to the central gateway to retrieve the data. Then the parameters of the data retrieval were analyzed, where it was found that the voltage of each cell has to be retrieved from the BMS individually, i.e., a requesting CAN message is needed, which is answered by a CAN message with the voltage value of the desired battery cell. The simultaneous query cannot obtain the desired data; therefore, the query messages must be sent one by one in rotation. It is probably necessary to avoid overloading the bus, and the method cannot be changed.