BMS communication protocols (CAN bus) in Power Electronics - Mini Project: Build & Apply

Understanding BMS Communication Protocols: CAN Bus

📖 Scenario: You are working with a Battery Management System (BMS) that uses the CAN bus protocol to communicate battery status to other vehicle systems. Understanding how data is structured and sent over CAN bus is essential for monitoring and controlling battery health.

🎯 Goal: Build a simple representation of CAN bus messages used in BMS communication. You will create a data structure for CAN messages, configure message identifiers, extract key data fields, and finalize the message format for transmission.

📋 What You'll Learn

Create a dictionary representing CAN messages with specific message IDs and data fields

Add a configuration variable for the standard CAN message ID format

Extract battery voltage and temperature from the CAN message data using dictionary comprehension

Complete the CAN message structure by adding a checksum field

💡 Why This Matters

🌍 Real World

BMS systems in electric vehicles use CAN bus to send battery voltage and temperature data to controllers for safe operation.

💼 Career

Understanding CAN bus communication is essential for engineers working on battery management, automotive electronics, and embedded systems.

1

Create CAN messages dictionary

Create a dictionary called can_messages with these exact entries: 0x100 mapped to {'voltage': 360, 'temperature': 25}, and 0x101 mapped to {'voltage': 355, 'temperature': 27}.

Power Electronics

# Create the can_messages dictionary with specified message IDs and data
# Your code here

Hint

Use hexadecimal keys for message IDs and dictionaries for data fields.

2

Add CAN ID format configuration

Add a variable called can_id_format and set it to the string 'standard' to represent the standard CAN message ID format.

Power Electronics

can_messages = {
    0x100: {'voltage': 360, 'temperature': 25},
    0x101: {'voltage': 355, 'temperature': 27}
}
# Define the CAN ID format
# Your code here

Hint

This variable helps specify the type of CAN ID used in communication.

3

Extract voltage and temperature data

Use dictionary comprehension to create a new dictionary called battery_data that extracts 'voltage' and 'temperature' from each message in can_messages. Use msg_id and data as loop variables.

Power Electronics

can_messages = {
    0x100: {'voltage': 360, 'temperature': 25},
    0x101: {'voltage': 355, 'temperature': 27}
}
can_id_format = 'standard'
# Extract voltage and temperature into battery_data
# Your code here

Hint

Use dictionary comprehension with msg_id and data to extract fields.

4

Add checksum to CAN messages

Add a new key 'checksum' with value 0xAB to each dictionary in can_messages using a for loop with variables msg_id and data.

Power Electronics

can_messages = {
    0x100: {'voltage': 360, 'temperature': 25},
    0x101: {'voltage': 355, 'temperature': 27}
}
can_id_format = 'standard'
battery_data = {msg_id: {'voltage': data['voltage'], 'temperature': data['temperature']} for msg_id, data in can_messages.items()}
# Add checksum 0xAB to each CAN message
# Your code here

Hint

Use a for loop to add the checksum key to each message dictionary.

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