| Lithium ion batteries have a well documented tendency to fail energetically under various abuse conditions. These conditions frequently result in decomposition reactions within the electrochemical components of the battery resulting in gas generation and increased internal pressure, which could lead to an explosive case rupture. The 18650 format cell incorporates a vent mechanism located within a crimped cap to relieve pressure and mitigate the risk of case rupture. However, cell venting introduces additional safety concerns associated with the flow of flammable gases and liquid electrolyte into the ambient environment. Experiments are performed here to quantify key parameters in describing the external dynamics of battery venting. A first experiment measures the vent burst pressure. Burst vent caps are then tested on a second experimental fixture developed to simulate the choked flow present during failures. This experiment allows measurement of the vent opening area and flow discharge coefficient from stag- nation temperature, stagnation pressure, and static pressure measurements combined with a known fixture geometry, compressible-isentropic flow equations, and conservation of mass. Validation experiments are performed to confirm the accuracy of opening area and discharge coefficient measurement. A test series of fifty commercially sourced vent caps describes statistical characteristics of the measurements, and additional trials are performed on vent caps removed from live batteries to demonstrate the potential variation between manufacturers. |
