Ultrasonic inspection of lithium-ion batteries to determine battery safety

Electric vehicles and energy storage systems are becoming increasingly viable from an operational and financial perspective. The most popular choice for the power source for these applications is the lithium-ion battery due to its high volumetric energy density and long cycle life. While lithium-ion batteries have been used in low-power applications for many years, electric vehicles and energy storage systems are high-power applications which put additional operating stresses on the battery cells and the battery management system (BMS) that ensures operational safety. Along with this change in application, lithium-ion batteries are also being subjected to market pressures to increase single-cycle life and to reach full charge faster. BMS have not kept up with these changing applications and market pressures, which has lead to some battery cells failing catastrophically. This thesis explores the viability of ultrasonic inspection to determine battery safety in the event of an overcharge. Two detection criteria were investigated in this thesis: whether an overcharge event could be detected while it was occurring and whether ultrasonic inspection could detect that a battery had previously undergone an overcharge event. The results of these tests showed two consistent indicators of overcharge while it occurred, one around 105% nominal voltage and one around 114% nominal voltage. Both of these indicators signal overcharge well before a catastrophic event. Results also indicated that overcharge can be detected during a normal cycle after the overcharge event occurred