SOH Illusion: Misunderstandings of EV Battery State of Health and Methods to Promote Understanding

Legislation in the USA will soon require that electric vehicles (EVs) display battery degradation in the instrument cluster as "state of health" (SOH), the percentage of the battery's original capacity. However, the extent to which consumers understand SOH degradation patterns is not known. In an initial study with vehicle owners, we find preliminary evidence for a 'SOH illusion', wherein people expect linear rates of EV battery degradation over time even though batteries degrade non-linearly. Additionally, a third of participants incorrectly conflated SOH with a battery's remaining usable life, demonstrating some misunderstanding of SOH among vehicle owners. In a follow-up study we find that framing SOH information with different chart types and legends reduces linear degradation assumptions and aligns people's expectations. We discuss implications for the design of SOH representations in user interfaces that vehicle UI designers could employ to promote better EV battery understanding.