BMS handshake is the communication and verification process between an EV’s Battery Management System (BMS) and the charging equipment to confirm that charging can start safely and to set the initial charging parameters. It ensures both sides agree on limits like voltage, current, temperature conditions, and safety interlocks before significant power is delivered.
What Is a BMS Handshake?
A BMS handshake is the “negotiation” phase that happens at the start of a charging session (and continues dynamically during charging). It typically includes:
– Identifying the connection and confirming readiness to charge
– Exchanging allowed voltage/current limits and charging mode information
– Verifying safety conditions (isolation, connector status, interlocks)
– Establishing control authority (how the charger will follow BMS requests)
– Confirming that the session can proceed without fault conditions
The term is most commonly associated with DC charging, where the charger must follow the vehicle’s requested power profile. For AC charging, a simpler handshake occurs via the connector signaling and onboard charger control.
Why BMS Handshake Matters in EV Charging
The handshake is essential because it prevents unsafe charging and avoids failed sessions. It matters for:
– Safe power delivery (avoid overvoltage, overcurrent, overheating)
– Reliable session starts and fewer “can’t start charging” errors
– Optimal charging speed based on real battery conditions
– Protection against compatibility issues between vehicle and charger
– Better diagnostics: handshake failures point to specific fault categories
Many real-world “charger doesn’t work” cases are actually handshake or communication mismatches, not hardware power limits.
How BMS Handshake Works
The exact handshake depends on the charging type:
– AC charging (IEC 61851 control pilot)
– The charger and vehicle use control pilot signaling to agree that a vehicle is connected and how much current is available
– The vehicle’s onboard charger and BMS decide how much current to draw within that limit
– Safety checks confirm earthing, RCD behavior, and connector status
– DC charging (CCS/CHAdeMO high-level communication)
– A digital communication link is established between EV and charger
– The BMS communicates battery limits and requests (max current, max voltage, target power)
– The charger confirms capability and sets output accordingly
– Continuous updates occur during the session as the battery warms, SoC rises, or limits change
– The session ends with an orderly ramp-down and disconnect sequence
Modern CCS systems can also use higher-level protocols (e.g., ISO 15118) for extended features like Plug & Charge and advanced energy services.
Common Causes of Handshake Failure
– Communication issues (PLC signal quality, connector wear, cable damage)
– Protocol compatibility problems (vehicle vs charger software versions)
– Safety interlocks not satisfied (connector not fully latched, proximity signal issues)
– Insulation/ground faults or protection device trips
– Battery conditions preventing charging (too cold/hot, SoC too high, BMS lockout)
– Backend/authentication delays causing timeout before handshake completes (site-dependent)
Handshake failures often appear to users as “session failed,” “vehicle not detected,” or “unable to start.”
Key Benefits of a Successful BMS Handshake
– Safe, controlled start of charging
– Faster and more stable sessions with fewer interruptions
– Better utilization and higher availability rate for operators
– Cleaner session data for billing and automated reconciliation
– Improved interoperability across vehicle brands and charging networks
Limitations to Consider
– Different standards and implementations can create edge-case compatibility issues
– Environmental conditions (cold, moisture, contamination) can affect connector communication
– Handshake timing varies; strict timeouts can increase failed starts
– Some issues are vehicle-side and cannot be fixed by the charger alone
– Advanced handshake features may depend on ISO 15118 support in both EV and charger
Related Glossary Terms
Battery Management System (BMS)
ISO 15118
IEC 61851
CCS (Combined Charging System)
Control Pilot (CP)
Proximity Pilot (PP)
Plug & Charge
Charging Session Failure Rate
Fault Diagnostics
Interoperability