## 2 Theoretical Background
When opening current-carrying connections, an electric arc may ignite [14]. This can be transferred to the CID of a cell, which carries the current flow from the cell windings to the positive terminal. Higher voltages are responsible for the formation of electric arcs. For cylindrical cells with CID, a voltage of 18 V is sufficient to generate an electric arc (according to [7]). A single cylindrical cell will not reach such a high voltage value, but it can occur in series connections (see Figure 3). Figure 1: Current Interrupt Device before opening, with a representation of the components of the part [6]. Figure 2: Current Interrupt Device during the disconnection, with a schematic representation of degassing process [6]. If the CID opens on one cell in the circuit, it becomes highly resistive, while the resistances of the other cells remain low, which would drop a large part of the voltage at this cell [15]. This is particularly dangerous when the battery is in a charging state and an external voltage source is connected. However, in such a case, the main question is if there is a potential risk of using cylindrical cells with CIDs in serial connections, which leads to subsequent questions of whether the test standards like the ECR R 100 [16], the ISO12405 [17], UN38.3 [18], and the FreedomCAR [8] cover this problem. One of the regulations that is significantly important for cell testing is the UN recommendations on the Transport of Dangerous Goods. The recommendations in this document are based on several national laws and international agreements [18]. It is divided into two parts, (i) the Model Regulations and (ii) the Manual of Tests and Criteria. The latter part is important for this research as it encapsulates the information about cell testing and provides information about tests to ascertain the hazards of dangerous goods including battery cells. Newly developed cells must pass special tests before they are allowed to be transported. Multiple test requirements are outlined in section UN 38.3 T3 [18], which must be carried out by the manufacturer or a testing laboratory [5]. These correspond to the same principle and are carried out as follows. Test T.7 is an overcharge test from the UN standard [18]. Due to the electrolyte's decomposition at voltages above 4.2 V, increased gas production happens in overvoltage situations [10]. This leads to overpressure inside the cell and to the opening of the CID (see Figure 4). To determine whether the cells will act dangerously in this abuse case, the overvoltage test T.7 is used. The intention of the test is as follows: 'This test evaluates the ability of a rechargeable battery or a single cell rechargeable battery to withstand an overcharge condition' [19]. Therefore, the cell should be overcharged to a voltage level of two times the maximum charge voltage. For instance, a cell that can withstand a charge of 4.2 V will be overcharged to 8.4 V. The test will be performed under ambient temperature and with twice the cell data sheet's maximum continuous charge current [19]. If the cell does not disassemble or catch fire during the 24 h test, it fulfills the criteria. Regulation No. 100 of the UNEC is the legal binding for the approval of EVs in the European Union (EU). One of the tests that this regulation describes is the overcharging test for EV batteries to protect the battery from overcharging. To undertake this test, all active safety functions are activated and the charge control limits of the Device Under Test (DUT) must be deactivated. To pass the test, the battery must stop the current flow when it reaches the maximum charge voltage. If the active safety functions do not stop the charging, the test is over when the battery reaches the double  
Figure 3: Example circuit for an CID opening in cell inside battery system. The complete systems voltage will drop at a cell whose internal resistance is high due to the opening of a CID or overcharge protection additive [15]. nominal capacity [1]. To obtain the ISO 6469-1:2019 [19] certification, an overcharge test is vital.