## 2 Theoretical Background  
Overcharging is one of the most critical situations in battery applications. According to [10], the consequences of overcharging the cells are much more critical compared to deep discharging. Overcharging can cause undesirable side reactions such as the decomposition of the electrolyte, cathode material, and reactions of the electrode with other cell components, which can consequently lead to catastrophic cell failure, like a fire or explosion. There are various reasons for cell overcharging, i.e., a defective charge controller or a faulty BMS. False voltage measurement (measured voltage against actual voltage) can also contribute to cell overcharging. For example, if the BMS has an integrated balancing circuit to monitor the cell voltage, the BMS will try to balance the cell based on faulty voltage values and will eventually overcharge the cell, leading to a potential thermal runaway of the cell. The extent of the thermal runaway depends on the composition of the cells and the resulting chemical reactions inside [11]. Depending on the materials and chemicals used in the cell, oxygen is produced during the decomposition of the cathode (depending on the State of Charge (SOC) and cathode material) [12]. The oxygen reacts with Carbon and the electrolyte solvent which leads to the release of flammable gases like Carbon Monoxide (CO), Carbon dioxide (CO\({}_{2}\)), and Hydrogen (H\({}_{2}\)). In such scenarios, Lithium Nickel Manganese Cobalt electrodes (NMC 622 and NMC 811) and Lithium Nickel Cobalt Aluminum electrodes (NCA) behave critically, while the Lithium Iron Phosphate electrode is known as the safest material due to its low release of toxic CO [12] gas. An electrolyte is the primarily responsible element for the gas development in the cell [13]. The gas formation in each cell builds high pressure, and [13] due to the hermetic sealing of Li-Ion cells to the environment, the produced gases escape. Together with the stable metallic outer shell, the gas pressure can go up to 20 bar, and in an uncontrolled event of failure, these gases can explode [6]. To reduce such potential hazards of energy storage devices, various safety devices and control mechanisms are employed. These act both at the cell level and at the system level and serve primarily to prevent abnormal operating conditions such as overcharging of the cells. At the cell level, internal safety measures such as the Positive Temperature Coefficient (PTC) device and the Current Interrupt Device (CID) [6] are adopted. On the system side, BMS is implemented as an external safety measure to monitor the cells continuously. If something goes wrong, it will disconnect the cells from the load or power supply. The PTC is a small metal part integrated at the positive pole, with a Positive Temperature Coefficient, which means that at the time of heating, the PTC increases its resistance and reduces the current flow [7]. The CID consists of two components, (i) the top disk (orange components in Figure 1) and (ii) the bottom disk (dark blue components in Figure 1). The top disk is a flexible, ring-shaped conductor connected to the bottom disk by a weld in the center of the top disk. Both disks are separated from each other by insulation. Thus, the welded connection between the top and bottom disk is the only electrical connection to the active material, i.e., this represents a weak point in the current path. The top disk is connected to the jelly roll (gray component in Figure 1) by a bottom disk. The light blue areas are plastic insulation, and at the top is the plus pole for external connection to a load or power supply [6]. In the event of overcharging and a resulting increase in pressure inside the cell, this structure causes the top disk to bend upwards and causes the weld joint to break mechanically from the metal foil, severing the contact and thus the current path to the active material (see Figure 2). Triggering the CID is comparable to opening a switch under load [7].