Engineering Bidirectional Charging With ISO 15118-20: A Developer's Playbook

This is a practical, step-by-step look at implementing ISO 15118-20 bidirectional power transfer (BPT) — from choosing a control mode to integrating with your back office.

Step 1 — Choose a BPT control mode

• Dynamic control mode. The charger issues frequent real-time power set-points — ideal for fast-response services such as frequency regulation.
• Scheduled control mode. Time-indexed power schedules suit day-ahead energy markets and predictable optimization.

Step 2 — Map the message sequence

During service discovery, the charging station advertises its bidirectional power transfer service. The EV then requests service details and negotiates parameters such as the BPT channel and the vehicle's mobility and generator needs.

Step 3 — Handle schedule exchange

The ScheduleExchange request carries the departure time, target energy, and — optionally — the maximum energy the vehicle is willing to export, letting the charger optimize state-of-charge targets within grid limits.

Step 4 — Implement charge control

Track the requested dynamic set-point accurately; sloppy tracking causes conformance failures. Bidirectional DC converters with sufficient control bandwidth are recommended so the system can follow set-point changes cleanly.

Step 5 — Integrate the back office

OCPP 2.1 adds messages aligned with the ISO 15118-20 schedule model. Make sure your back office maps schedule identifiers correctly and handles negative power values (discharging) throughout.

Testing & Field Checklist

• Confirm the EV's discharge capability and measure round-trip efficiency.
• Validate anti-islanding and grid-protection behavior against the applicable interconnection standard.
• Use an established e-mobility test suite (for example, Vector CANoe's E-Mobility tooling) to exercise the ISO 15118-20 and TLS stack.

Why It Matters

Grid programs increasingly value distributed battery capacity. Utilities and regulators — for instance, California's vehicle-grid-integration efforts — are building toward large-scale V2G participation, and ISO 15118-20 compliance is what positions a charger to take part.