ACAP: Autonomous Charging Alignment Protocol for Driverless Electric Vehicle Fleets

The convergence of autonomous vehicle (AV) fleets and electric vehicle (EV) charging infrastructure presents a critical systems integration challenge: enabling fully driverless vehicles to locate, approach, physically connect to, and transact with charging stations without any human intervention. Current charging protocols—ISO 15118, OCPP 2.0.1, and SAE J3400—assume a human driver is present for physical alignment, connector insertion, and session initiation. This assumption fundamentally breaks for autonomous robotaxis. We propose ACAP—the Autonomous Charging Alignment Protocol—a multi-layer protocol stack that orchestrates the full lifecycle of an autonomous charging session across four phases: Discovery & Reservation Approach & Alignment Connection & Charging Departure & Settlement ACAP introduces Cryptographic Spatial Anchors (CSA) for sub-centimeter docking verification, a 14-state finite state machine with deterministic timeout recovery, a mutual-authentication handshake leveraging TPM 2.0–rooted trust, and a transactional settlement layer compatible with OCPP 2.0.1 billing infrastructure. We formalize the protocol, present detailed sequence diagrams, analyze safety and liveness properties, and discuss integration paths with ISO 15118-20 and CharIN’s Automated Connection Device (ACD) framework. Our analysis demonstrates that ACAP can reduce average autonomous charging session overhead to under 45 seconds from bay entry to energy flow while maintaining fail-safe guarantees under communication loss, mechanical fault, and adversarial conditions.