Battery state characterization based on a contactless electromagnetic ultrasound testing method

Nondestructive ultrasonic detection technology has been increasingly applied to battery diagnostics. However, currently used ultrasonic transducers must be affixed to the tested battery very tightly through coupling agents, thereby lowering the test efficiency and convenience. In this paper, a contactless electromagnetic ultrasonic testing technology is proposed for characterizing the states of lithium-ion batteries. The relationship between the ultrasonic frequency and wave velocity is analyzed via a finite element model. Meanwhile, the influence of battery boundary characteristics on ultrasonic signals is investigated. Based on the propagation characteristics of ultrasonic guided waves in the battery, contactless electromagnetic acoustic transducers (EMATs) are designed and optimized. Three features, including the time of flight, signal amplitude, and energy integration, are extracted from the received signals to indicate the variation in battery state during charging and discharging cycles. Experimental results demonstrate that the ultrasonic guided wave features obtained from EMATs change consistently and regularly with the charging and discharging states of the battery. The proposed method can be used to analyze the aging of batteries and the inconsistency of electrolyte infiltration inside batteries, which is expected to improve the battery safety.