BMS communication protocols (CAN bus) in Power Electronics - Step-by-Step Execution

Concept Flow - BMS communication protocols (CAN bus)

Start BMS system

↓

Prepare CAN message

↓

Send message on CAN bus

↓

Other nodes listen

↓

Message received?

No→Wait

Yes↓

Process message

↓

Respond or update

↓

Repeat communication cycle

The BMS uses CAN bus to send and receive messages in a cycle, where messages are prepared, sent, received by other nodes, processed, and then the system updates or responds.

Execution Sample

Power Electronics

1. Prepare CAN frame with battery data
2. Transmit frame on CAN bus
3. Other BMS nodes receive frame
4. Nodes process data and respond if needed

This code simulates a BMS node sending battery status data over the CAN bus and other nodes receiving and processing it.

Analysis Table

Step	Action	Message Content	Bus State	Result
1	Prepare CAN frame	Battery voltage=48V	Idle	Frame ready to send
2	Transmit frame	Battery voltage=48V	Bus busy	Message sent on CAN bus
3	Other nodes listen	Battery voltage=48V	Bus busy	Message detected
4	Message received	Battery voltage=48V	Bus idle	Data stored in node
5	Process message	Battery voltage=48V	Bus idle	Update battery status
6	Respond if needed	Request charge stop	Bus busy	Response sent
7	Cycle repeats	Next data frame	Bus idle	Ready for next message

💡 Communication cycle repeats continuously for real-time monitoring

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	After Step 6	Final
CAN frame	None	Battery voltage=48V	Sent on bus	Received by nodes	Response frame ready	Cycle repeats
Bus state	Idle	Idle	Busy	Idle	Busy	Idle
Node data	Empty	Empty	Empty	Battery voltage=48V stored	Updated with response	Ready for next

Key Insights - 3 Insights

Why does the bus state change from idle to busy during transmission?

When a CAN frame is sent (see Step 2), the bus becomes busy because the message occupies the communication line until fully transmitted.

How do other nodes know when a message is for them?

All nodes listen to the bus (Step 3). They check the message ID in the CAN frame to decide if they should process it (Step 4).

What happens if two nodes try to send messages at the same time?

CAN bus uses arbitration to avoid collisions. The node with the highest priority message continues, others wait and retry, ensuring orderly communication.

Visual Quiz - 3 Questions

Look at the execution table, what is the bus state during Step 3 when other nodes listen?

A Idle

B Busy

C Error

D Disconnected

💡 Hint

Check the 'Bus State' column for Step 3 in the execution_table.

At which step is the battery voltage data stored in the receiving node?

A Step 4

B Step 6

C Step 2

D Step 1

💡 Hint

Look at the 'Result' column in the execution_table for when data is stored.

If the CAN frame was not prepared correctly at Step 1, what would happen next?

A Message would still be sent correctly

B Bus state would remain busy

C No message would be sent, bus stays idle

D Nodes would respond with error immediately

💡 Hint

Refer to Step 1 and Step 2 in execution_table about frame preparation and transmission.

Concept Snapshot

BMS uses CAN bus to communicate battery data.
Messages are prepared as CAN frames and sent on the bus.
Other nodes listen, receive, and process messages.
Bus state changes between idle and busy during communication.
CAN bus ensures orderly message sending with arbitration.

Full Transcript

The Battery Management System (BMS) communicates using the CAN bus protocol by preparing messages called CAN frames containing battery data. These frames are sent on the CAN bus, which becomes busy during transmission. Other BMS nodes listen to the bus and receive these messages if the message ID matches their interest. Upon receiving, nodes store and process the data, possibly responding with commands or status updates. The communication cycle repeats continuously for real-time battery monitoring. The bus state alternates between idle and busy depending on message transmission. CAN bus uses arbitration to avoid message collisions, ensuring reliable communication among multiple nodes.

Practice

1. What is the main purpose of the CAN bus in a Battery Management System (BMS)?

easy

A. To allow different parts of the system to communicate battery information reliably

B. To increase the battery's physical capacity

C. To charge the battery faster

D. To cool down the battery cells

Solution

1. Step 1: Understand the role of CAN bus in BMS

   CAN bus is designed to enable communication between the BMS and other system parts.

2. Step 2: Identify the correct function among options

   Only To allow different parts of the system to communicate battery information reliably describes communication of battery information reliably, which is the main purpose.

3. Final Answer:

   To allow different parts of the system to communicate battery information reliably -> Option A

4. Quick Check:

   CAN bus = communication in BMS [OK]

Hint: CAN bus is about communication, not charging or cooling [OK]

Common Mistakes:

• Confusing communication with battery charging
• Thinking CAN bus changes battery capacity
• Assuming CAN bus controls battery temperature

2. Which of the following is the correct way to describe a CAN bus message ID in BMS communication?

easy

A. A unique identifier that helps organize messages and avoid conflicts

B. The battery voltage level

C. The physical address of the battery cell

D. The temperature of the battery pack

Solution

1. Step 1: Understand what a CAN bus message ID is

   Message IDs in CAN bus are unique numbers used to identify and organize messages.

2. Step 2: Match the description to the options

   A unique identifier that helps organize messages and avoid conflicts correctly describes the message ID as a unique identifier to avoid conflicts.

3. Final Answer:

   A unique identifier that helps organize messages and avoid conflicts -> Option A

4. Quick Check:

   Message ID = unique identifier [OK]

Hint: Message ID organizes messages, not battery data values [OK]

Common Mistakes:

• Confusing message ID with battery voltage or temperature
• Thinking message ID is a physical address
• Mixing message ID with sensor readings

3. Consider a BMS using CAN bus where message ID 0x101 sends battery voltage and 0x102 sends temperature. If a device receives a message with ID 0x101 and data 36000, what does this data most likely represent?

medium

A. Battery current of 36000 amps

B. Battery temperature of 36 degrees Celsius

C. Battery voltage of 36.000 volts

D. Battery state of charge at 36%

Solution

1. Step 1: Identify message ID meaning

   Message ID 0x101 is assigned to battery voltage data.

2. Step 2: Interpret the data value

   Data 36000 likely means 36.000 volts (common to send voltage in millivolts).

3. Final Answer:

   Battery voltage of 36.000 volts -> Option C

4. Quick Check:

   0x101 = voltage, 36000 = 36.000 V [OK]

Hint: Match message ID to data type to interpret values correctly [OK]

Common Mistakes:

• Mixing voltage and temperature data
• Assuming raw number is amps or percent without context
• Ignoring message ID meaning

4. A BMS CAN bus message is not received correctly because two devices send messages with the same ID at the same time. What is the most likely cause and how can it be fixed?

medium

A. CAN bus cable is broken; replace cable

B. Battery voltage too high; reduce voltage

C. Temperature sensor failure; replace sensor

D. Message ID conflict; assign unique IDs to each device

Solution

1. Step 1: Identify the problem cause

   Two devices sending messages with the same ID causes conflicts on CAN bus.

2. Step 2: Find the correct fix

   Assigning unique message IDs to each device prevents conflicts and ensures proper communication.

3. Final Answer:

   Message ID conflict; assign unique IDs to each device -> Option D

4. Quick Check:

   Same ID causes conflict; unique IDs fix it [OK]

Hint: Unique message IDs prevent CAN bus conflicts [OK]

Common Mistakes:

• Blaming hardware issues instead of ID conflicts
• Ignoring the importance of unique IDs
• Assuming cable or sensor faults cause ID conflicts

5. In an electric vehicle, the BMS uses CAN bus to send battery status messages every 100 ms. If the system needs to add a new sensor sending temperature data without causing message conflicts, what is the best approach?

hard

A. Use the same message ID as battery voltage to save bandwidth

B. Assign a new unique message ID for the temperature sensor and update all devices to recognize it

C. Send temperature data only when battery voltage changes

D. Increase the message frequency to 200 ms for all messages

Solution

1. Step 1: Understand the need for unique message IDs

   Each sensor must have a unique message ID to avoid conflicts on the CAN bus.

2. Step 2: Choose the best method to add new sensor data

   Assigning a new unique ID and updating devices ensures proper communication without conflicts.

3. Final Answer:

   Assign a new unique message ID for the temperature sensor and update all devices to recognize it -> Option B

4. Quick Check:

   New sensor needs unique ID to avoid conflicts [OK]

Hint: Always use unique IDs for new CAN bus messages [OK]

Common Mistakes:

• Reusing existing message IDs causing conflicts
• Reducing message frequency without solving conflicts
• Sending data conditionally causing delays