Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Understanding SAE J2847/2: Communication Protocol for DC Charging and Vehicle-to-Grid/Home
SAE J2847/2 is a surface vehicle standard that defines the digital communication messages and protocols between plug-in electric vehicles (EVs) and off-board DC chargers. Revised in September 2023, this standard extends beyond conventional DC charging to support bidirectional energy transfer applications such as vehicle-to-grid (V2G) and vehicle-to-home (V2H). It provides a comprehensive framework for signal definitions, charging phases, and safety requirements, ensuring robust interoperability between vehicles and electric vehicle supply equipment (EVSE).
Why SAE J2847/2 Matters for DC Charging and V2G/V2H
The transition to electric mobility relies on standardized communication to enable seamless and safe charging. SAE J2847/2 addresses the specific needs of DC fast charging, where high power levels demand precise control and coordination. With the addition of V2G and V2H use cases in the 2023 revision, the standard now covers bidirectional power flow, including pre-charging of the DC EVSE and isolation requirements for home systems. This ensures that energy can flow both into and out of the vehicle battery, enabling grid services and backup power.
Key topics include digital messaging sequences for charging phases, signal definitions for parameters like voltage, current, and state of charge, and the handling of charging profiles. The standard builds on use cases defined in SAE J2836/2 and J2836/3, aligning with industry best practices for DC energy transfer.
Core Technical Elements and Signal Definitions
The standard specifies a detailed set of signals and messages exchanged between the EV and EVSE. These include identifiers, status reports, charge parameters, and error codes. The table below highlights some of the critical signals defined in SAE J2847/2:
| Signal | Description |
|---|---|
DC_EVChargeParameter |
Parameters for DC charging from the EV, such as target voltage and current. |
DC_EVSEChargeParameter |
EVSE charge parameters including maximum current, voltage, and power limits. |
ChargingProfile |
Defines time-based charging profiles with power and current limits. |
EVSEIsolationStatus |
Status of isolation for V2H systems, critical for safety during bidirectional transfer. |
EVTargetCurrent and EVTargetVoltage |
Requested current and voltage by the EV to the EVSE during charging. |
The signal definitions ensure that both entities can negotiate safe operating points. The standard also defines charging phases such as pre-charging, bulk charging, and completion, each with specific message sequences. Engineers must correctly interpret these signals to avoid errors like miscommunication or unsafe charging conditions.
⚠️ Common Implementation Pitfall: A frequent mistake is misinterpreting signal definitions, leading to communication errors between the EV and EVSE. For example, incorrect handling of the ChargingProfile messages can result in incomplete charging cycles or exceeding safety limits. Always verify that your implementation follows the exact sequence and data formats defined in the standard.
Design Insights and Implementation Considerations
From an engineering perspective, SAE J2847/2 provides a robust architecture for DC communication. The structure of charging phases and limits guides the development of charging algorithms and control logic. For V2G/V2H support, additional complexities such as pre-charging the DC EVSE and meeting isolation requirements must be addressed. Pre-charging ensures that the EVSE output capacitors are charged to the correct voltage before contactor closure, preventing inrush currents and damage. Isolation monitoring is crucial for safety in home systems, particularly when power flows from the vehicle to the home.
🛠️ Engineering Design Insight: To ensure interoperability, focus on implementing the full message sequence for each charging phase. Leverage the detailed signal tables and timing diagrams provided in the standard. For V2H systems, prioritize isolation status monitoring and incorporate fail-safe mechanisms in case of isolation faults. Testing with multiple EVSE models can help identify subtle protocol mismatches.
Common mistakes include neglecting isolation requirements for V2H applications, which can compromise safety. Also, engineers sometimes overlook the differences between AC and DC communication protocols, leading to incorrect message mapping. Adhering to the standard’s definitions and sequences is essential for reliable operation.
Frequently Asked Questions
What is the main purpose of SAE J2847/2?
SAE J2847/2 defines the digital communication protocol between plug-in electric vehicles and off-board DC chargers. It covers messages for DC charging and extends to support vehicle-to-grid (V2G) and vehicle-to-home (V2H) applications, ensuring safe and interoperable energy transfer.
How does the standard handle pre-charging for DC chargers?
The standard includes messages and procedures for pre-charging the DC EVSE. This involves raising the voltage of the EVSE output to match the battery voltage before the main contactors close, preventing inrush current and protecting the system.
What are the key signals for ensuring safety in V2H applications?
Key signals include EVSEIsolationStatus and EVSE Notification, which monitor and report the isolation condition of the home system. The standard also specifies requirements for isolation monitoring to prevent ground faults during bidirectional power flow.
How can engineers avoid common mistakes in implementation?
Carefully follow the signal definitions and message sequences outlined in the standard. Pay special attention to charging phases, profile definitions, and error handling. Use the provided tables and diagrams to verify correct data formats and timing.
In conclusion, SAE J2847/2 is essential for engineers developing DC charging systems or bidirectional power transfer solutions. By understanding its signals, phases, and safety requirements, you can design systems that are interoperable, efficient, and safe. For complete details, refer to the full standard document.
