# Application of Infrared Thermography in E-Bike Battery Pack Detail Analysis--Case Study
## 3 Analysis and Diagnostics of the Battery Pack
### Thermography in the Battery State Assessment and Analysis
Thus, there was a need for a non-invasive analysis method that would not interfere with the functionality of the battery under operating conditions. Figure 6 shows a thermographic recording of the battery pack during charging, near the end of its life. The image was taken when the range of the bike was significantly decreasing. Figure 7 shows the same battery and its thermographic pattern during charging, taken when the battery was at the beginning of its life, long before the images in Figure 6 were taken. The battery in Figure 7 shows a nearly uniform thermal pattern of a fully functional battery that is in excellent condition. However, this uniform thermal pattern has a slight disturbance in the heat distribution pattern caused by the spatial distribution of heat sources in the battery pack or cells. Next, irregularities in the thermal patterns, shown in Figure 6, are a consequence of changes in the internal ohmic resistance of individual battery cells as well as their capacity. That fact leads to improper and spatially irregular thermal dissipation within the battery pack confinement space. When the battery started to show signs of reaching the end of its service life, the bicycles started to shut off/down unexpectedly. Soon after noticing the aforementioned first signs, the thermographic analysis of the bike battery pack was performed, and thermal hot spots were noticed. Additionally, asymmetry and spatial irregularity of heat dissipation within the pack now was augmented significantly, Figure 8. Hot spots with a temperature difference of \(\geq\)2 \({}^{\circ}\)C represent a possible battery defect that warrants further investigation, according to NETA (International Electrical Testing Association). The pattern disturbance is located in the spatial center of the battery and is normal since it represents the core of the entire battery. Furthermore, the comparison of the thermograms in Figures 7 and 8 inevitably leads to the conclusion that a homogeneous heat distribution pattern may indicate a healthy condition or proper battery behavior. Figure 8: Battery pack assembly thermal pattern, at the end of the lifetime, (**a**) top layer, (**b**) bottom layer. Figure 7: Thermal pattern of the healthy battery pack: inside the bike holder (**a**) and outside (**b**). Since the battery was no longer suitable for cycling, it was disassembled, and the internal circuits and visual condition of the battery cells were analyzed. Figure 9 depicts the interior of the subjected battery pack, fully disassembled with focus on areas where the hotspots are noticed (suspected faulty cells and a BMS circuitry). The battery pack structural analysis resulted in a circuit principal diagram, shown in Figure 10. The results can be summed up as follows: the battery pack consists of 40 cells Panasonic CGR18650 CG type, cells nominal voltage is 3.6 V and nominal charge capacity of 2250 mAh, individual cell weight is 45 g [33]. The electrical connections arrangement was made in the form of ten cell blocks connected in series (10s), and each cell block consists of four parallel connected cells (4p)--a so-called the 4p10s battery cluster. The way cells are joined together is crucial for packet behavior [34; 35]. The balancing leads connection points, for the sake of clarity, are marked with colored circles at the BMS and in between cellblocks. Each balancing line has temporally thermal overcurrent protection accomplished by Polymer PTC Resettable Fuse of JK-D Series; furthermore, the BMS has two additional thermal monitoring and protection probes. The further analysis was followed by connecting the open battery to a charger device, and at the same time, the pack was monitored with a thermal imager during the charging process--charging cycle. The thermographic analysis of the thermal pattern of the pack's cells shows that the first two pack cells were significantly warmer than the adjacent cells and that remedial action must be taken according to [30].