Russell Vare, VP of vehicle-grid integration (VGI), The Mobility House
Vehicle-to-grid (V2G) technology turns an electric vehicle (EV) from an electrical load into a potential energy resource. Sending power back to the grid, however, requires more than a bidirectional charger. The vehicle and charging system must coordinate power conversion while meeting grid interconnection requirements.
Broader V2G deployment also requires engineers to consider how these systems respond to changes in communications and grid conditions.
To discuss the engineering behind V2G, we spoke with Russell Vare, VP of vehicle-grid integration (VGI) at The Mobility House. In this Q&A, Vare discusses how bidirectional power flows between the vehicle and grid and what it takes to integrate V2G into existing electrical infrastructure.
Here’s what he has to say…
Where does the DC-to-AC conversion occur in this deployment, and what drove that architecture choice?
All of these systems are DC-V2G, meaning the power conversion happens at the bidirectional charging station. The primary reason for this architecture is that it reflects what is currently available on the market for bidirectional vehicles.
One driver is the existing interconnection rules for distributed energy resources (DERs), which require compliance with established standards. Today, that interconnection pathway is built around the bidirectional charging station.
New standards are now being published to support AC-V2G, where the power conversion occurs onboard the vehicle rather than at the charging station. This approach requires specialized equipment and vehicle certification, but these options are beginning to enter the market.
In DC-V2G systems, the bidirectional charging station handles power conversion between the vehicle battery and the home or grid. New AC-V2G standards shift that conversion onboard the vehicle.
Which standards govern the bidirectional power path, and where are custom interfaces still required?
Multiple standards enable the flow of electricity from a vehicle battery to the grid. Between the EV and the charging station, most vehicles use ISO 15118, which governs communication between EVs and charging infrastructure. However, some vehicles still use SAE J2847/2 for V2G communication, custom implementations, or, in the case of the Nissan LEAF, CHAdeMO.
Between the charging station and the charge management system (backend cloud), commercial fleet charging primarily uses the Open Charge Point Protocol (OCPP), while residential systems currently rely on custom implementations.
What thermal load does sustained export place on the battery pack while the vehicle is parked, and which parts of the cooling system remain active?
This varies by vehicle manufacturer and is difficult to generalize. However, it’s worth noting that the power rating for residential bidirectional chargers is typically relatively low. For example, the Kia EV9 is available with either a 76 or 99 kWh battery and is compatible with the Wallbox Quasar 2, which provides up to 12.4 kW of power. The resulting C-rate is relatively low, especially when compared with DC fast charging.
How is exported energy counted against battery throughput, and what state-of-charge window is used during a dispatch event?
The state-of-charge (SOC) window for V2G is initially defined by the vehicle manufacturer and can then be adjusted by the customer based on personal preference. For example, a manufacturer may set 20% SOC as the minimum limit, while a customer may choose to increase that limit to 40% to account for unplanned trips.
How does the system detect a grid outage and prevent islanding, including when a home has solar or stationary storage?
Bidirectional charging stations are certified to the same standards required for other distributed energy resources (DERs), such as solar and battery storage systems. They comply with IEEE 1547 requirements for detecting grid outages and preventing islanding.
In Massachusetts, all systems are certified to UL 1741 SB and must complete the utility’s standard interconnection process before receiving permission to operate (PTO).
Residential V2G systems can allow an EV to support home energy needs during grid outages, provided the charging equipment meets anti-islanding and interconnection requirements.
How are charging and export commands authenticated between the vehicle and the external control platform?
Residential chargers often use application programming interfaces (APIs) to communicate, each with its own authentication methodology. Commercial chargers use OCPP 2.1, which includes a series of authentication messages exchanged at the beginning of a charging session. A session begins when communication is established between the charger and the charge management system (CMS).
Authentication failures typically occur when a user plugs in the vehicle and initiates a session. This is useful because the user is usually present and can immediately see whether charging has started, such as through a green indicator light or a change in the user interface. If the session fails, the user can often resolve the issue by unplugging and reconnecting the charger. One common reason a charging session fails to start is that the connector was not fully inserted into the vehicle.
What happens if communication between the charger and the control platform is lost?
While system behavior during a communications outage varies by manufacturer, OCPP includes mechanisms that allow a configurable amount of charging to continue if communication between the charger and the CMS is lost. This helps ensure drivers can continue charging and reach their destination.
Communication loss is a design consideration that engineers take very seriously. Commercial fleet charging sites often incorporate local controllers, hardwired connections, or redundant communication networks to improve resilience. Residential installations, by comparison, are less likely to justify those additional costs.
Filed Under: Charging, Featured Contributions, Q&As, Vehicle-to-Grid (V2G)