BMS BMS Communication Protocols & Interfaces 2 — Questions and Answers
Question 1: Which standard defines the communication protocol between an EV and a DC fast charger (DCFC) in the US?
- SAE J1772 (AC only)
- CHAdeMO
- SAE J1772 Combined Charging System (CCS/SAE Combo) (Correct answer)
- ISO 15118 only
Correct answer: SAE J1772 Combined Charging System (CCS/SAE Combo)
The SAE J1772 Combined Charging System (CCS Combo 1) is the dominant US standard for DC fast charging, using PLC over the pilot pin for communication.
Question 2: What does 'daisy-chain topology' mean in the context of BMS cell monitoring ICs?
- All cell monitors share a single power rail
- Each cell monitor IC passes data to the next in a serial chain (Correct answer)
- Cells are connected in parallel groups
- Monitors communicate wirelessly in a mesh network
Correct answer: Each cell monitor IC passes data to the next in a serial chain
In daisy-chain topology, cell monitoring ICs are linked serially so data propagates from the lowest cell group IC up to the master BMS controller.
Question 3: What is the primary advantage of using CAN FD (Flexible Data-rate) over classical CAN in modern BMS designs?
- Lower cost transceivers
- Higher payload capacity (up to 64 bytes) and faster data rates (up to 8 Mbps) (Correct answer)
- Longer cable lengths
- Wireless capability
Correct answer: Higher payload capacity (up to 64 bytes) and faster data rates (up to 8 Mbps)
CAN FD increases the data frame payload from 8 to 64 bytes and supports bit rates up to 8 Mbps in the data phase, enabling richer BMS telemetry.
Question 4: In an OCPP (Open Charge Point Protocol) context, what role does the BMS data play?
- OCPP directly controls cell balancing
- BMS state (SOC, faults) informs the charger and CSMS about permissible charging parameters (Correct answer)
- OCPP replaces CAN bus inside the vehicle
- BMS transmits payment data via OCPP
Correct answer: BMS state (SOC, faults) informs the charger and CSMS about permissible charging parameters
OCPP governs charger-to-network communication; the BMS feeds SOC and health data to the vehicle controller, which then negotiates charge limits with the charger via OCPP.
Question 5: What is the function of a 'wake-up message' on a CAN bus connected to a BMS?
- It resets all cell voltage measurements
- It transitions the BMS from sleep/low-power mode to active operation (Correct answer)
- It clears all stored fault codes
- It initiates cell balancing
Correct answer: It transitions the BMS from sleep/low-power mode to active operation
A wake-up message (often a specific CAN frame or a local wake line) transitions the BMS from its low-power sleep state to full active mode.
Question 6: Which layer of the OSI model does the CAN bus protocol primarily operate at for physical and data link functions?
- Layers 5 and 6 (Session and Presentation)
- Layers 1 and 2 (Physical and Data Link) (Correct answer)
- Layers 3 and 4 (Network and Transport)
- Layer 7 (Application) only
Correct answer: Layers 1 and 2 (Physical and Data Link)
CAN defines Physical layer (bit encoding, voltage levels) and Data Link layer (framing, arbitration, error detection) — higher-layer protocols like CANopen sit above it.
BMS BMS Communication Protocols & Interfaces 2
Which standard defines the communication protocol between an EV and a DC fast charger (DCFC) in the US?