Investigation of Water Ingress Driven Circuit Board Faults As a Contributory Factor to Battery Thermal Runaway

Abstract

Recent high-profile examples of battery fires in electric vehicles (EV’s) during the aftermath of Hurricane Ian [1] have led to increased public visibility of the susceptibility of EV batteries to thermal failure due to water immersion or ingress.Although clearly an issue for manufacturers and users of automotive EV’s, this issue poses a particular challenge in the rapidly increasing micromobility market with nearly 800 million battery powered 2- and 3-wheeled vehicles (such as e-bikes or scooters) expected to be in operation by 2040 [2]. With privately owned micromobility vehicles generally not subject to the rigorous servicing or maintenance schedules of automotive EV’s, and fleets of rented e-bikes / scooters subjected to prolonged rough handling and environmental exposure, the risk of failure in these vehicles due to improper handling, water ingress or mechanical abuse is very real.Testing and failure recreation of thermal runaway in lithium-ion based micromobility battery packs within Exponent’s European laboratory (London) has shown that even small amounts of moisture ingress to sensitive electronic components of the battery pack, such as the battery protection circuitry, can lead to thermal runaway of the cells within the battery pack. Such failures are often highly dependent on battery pack design (and physical condition) and may only manifest in the days or weeks after water immersion or ingress.The study presented here investigated specific conditions and behaviours of saltwater ingress-driven circuit board faults in lithium-ion battery packs, and demonstrated localised temperature increases of ~100 °C, even at relatively low fault currents (mA’s), showing the potential for circuit board faults to propagate to cell thermal runaway.Further studies investigated the effect of full, partial and targeted immersion of lithium-ion battery packs in 5 wt% NaCl solution, demonstrating the influence of the volume of water ingress and ingress location within the battery pack on the potential for battery thermal runaway. This paper will also discuss design aspects and/or mitigation methods to protect from such failure modes.[1] - https://content.govdelivery.com/accounts/USDHSFACIR/bulletins/333744b[2] - https://www.faraday.ac.uk/wp-content/uploads/2023/02/Faraday_Insights_16_FINAL.pdf