How does a car know when to discharge?

It follows the wallbox. After both have selected the bidirectional service and stated their limits, the wallbox drives the current every moment within those limits and reports it; a negative value means out of the car. The deep dive reads one evening of those messages.

Explained in

Bidirectional charging · Deep diveOne evening of messages between car and wallboxThe evening of this path as the car and the wallbox negotiated it over the cable: the ISO 15118-20 messages that select bidirectional transfer, state the limits in both directions, carry the owner's frame and then report the power as the wallbox drives it, decoded from the binary into readable form.
  • The service. The wallbox offers service 6, DC with bidirectional transfer, and the car selects it. A car or a wallbox without the service would never get past this line.
  • The limits. The car states what it can give as well as take: maximum discharge power and current. The wallbox answers with its own.
  • The frame. Departure time, target and floor travel as energy: how far the battery is from full, from the target and from the floor. These are the owner's settings from stop 01, turned into watt-hours.
  • The sign. In the loop nobody sends a target. The wallbox drives the current within the limits both sides stated and reports it; a negative value is discharge.
From “Four moments that make it bidirectional”, stop 09
  1. What is bidirectional charging?Smart charging in which electricity can also flow from the car's battery back to the charging point, to a device, a house or the grid.
  2. What is the difference between V2H and V2G?V2H powers the house behind its meter; V2G feeds the public grid. V2G brings in generator rules, metering in both directions and the electricity market.
  3. Does V2G damage the battery?It adds energy throughput, typically a fraction of what driving adds, at low power. Fast charging above 100 kW was linked to faster ageing in a large study.
  1. Sevdari, Knudsen, Fabbri, Pedersen, Alami and Marinelli (DTU): Demonstration of bidirectional charging using ISO 15118-20, 38th International Electric Vehicle Symposium (EVS38), 2025. https://orbit.dtu.dk/en/publications/d64a7ba2-6860-4006-be7a-d43f9a7a45f1. Checked 11 Oct 2026.