What is a BMS & What are the Components in Lithium-ion battery BMS?

AI generated Image of BMS

A Battery Management System (BMS) is an electronic system that manages a rechargeable battery (cell or battery pack), by monitoring its state, controlling its environment, and protecting it from operating outside its safe operating area.

1. Battery Monitoring Unit (BMU)

The BMU is the "sensory" part of the BMS. It's typically distributed throughout the battery pack, with individual monitoring circuits for groups of cells or even individual cells in high-precision systems.

These circuits are connected to a central communication bus that relays data to the Battery Control Unit (BCU).

 

Components:

Voltage Sensors: High-precision analog-to-digital converters (ADCs) that measure the voltage of each cell or cell group. Accuracy is crucial here, as even small voltage differences can indicate significant state-of-charge variations.

Current Sensors: Devices that measure the current flowing into and out of the battery pack. Common types include:

Shunt Resistors: Measure voltage drop across a small resistor in the current path. Simple and cost-effective but can generate heat.

Hall Effect Sensors: Measure the magnetic field generated by the current. More accurate and efficient than shunt resistors.

Current Transformers: Measure the current by inducing a current in a secondary winding. Used for high-current applications.

Temperature Sensors: Thermistors or thermocouples placed at strategic locations within the battery pack to monitor temperature.

 

Working:

1. Voltage sensors continuously measure the voltage of each cell or cell group.
2. The current sensor measures the total current flowing in and out of the battery pack.
3. Temperature sensors monitor the temperature at various points in the pack.
4. The BMU converts these analog measurements into digital signals using ADCs.
5. This digital data is then transmitted to the BCU via a communication bus (e.g., CAN, SPI).

2. Battery Control Unit (BCU)

The BCU is the "brain" of the BMS. It's a central processing unit that receives data from the BMU, makes decisions, and controls the battery's operation.

 

Components:

Microcontroller (MCU): A powerful processor that executes the BMS software, performs calculations, and controls the other components.

Memory: Stores the BMS software, battery parameters, and historical data.

Communication Interfaces: Allow the BCU to communicate with the BMU, other vehicle systems (e.g., VCU), and external devices (e.g., chargers).

Gate Drivers: Control the switching of power electronic devices (e.g., MOSFETs) in the charging and discharging circuits.

 

Working:

1. The BCU receives data from the BMU (voltage, current, temperature).
2. The MCU processes this data to:
3. Calculate State of Charge (SOC) and State of Health (SOH).
4. Determine if any protection thresholds have been exceeded.
5. Implement cell balancing algorithms.
6. Control the charging and discharging process.
7. The BCU sends control signals to:
8. Cell balancing circuits: To equalize cell voltages.
9. Charging/discharging circuits: To regulate current and voltage.
10. Cooling/heating systems: To maintain optimal temperature.
11. External systems: To communicate battery status and receive commands.

3. Cell Balancing Circuit

Cell balancing circuits are integrated into the BMU or as separate modules. They are connected to each cell or cell group.

 

Components:

Passive Balancing:

• Resistors: Used to dissipate excess energy from higher-voltage cells.

• Switches (e.g., MOSFETs): Control the connection of the resistors to the cells.

Active Balancing:

• DC-DC Converters (e.g., buck-boost converters): Transfer energy between cells.

• Capacitors or Inductors: Used as energy storage elements in the transfer process.

• Switches (e.g., MOSFETs): Control the flow of energy between cells.

Working:

Passive Balancing: When a cell's voltage exceeds a certain threshold, the corresponding switch is closed, connecting the resistor. The resistor dissipates the excess energy as heat, lowering the cell's voltage.

Active Balancing: Energy is transferred from higher-voltage cells to lower-voltage cells using the DC-DC converter and energy storage elements. This is a more efficient method as it doesn't waste energy as heat.

4. Protection Circuit

The protection circuit is typically implemented within the BCU and uses dedicated hardware and software to ensure fast and reliable response to fault conditions.

Components:

Comparators: Compare measured values (voltage, current, temperature) to predefined thresholds.

Logic Gates: Combine the outputs of the comparators to generate protection signals.

Switches (e.g., MOSFETs, relays): Disconnect the battery from the load or charger in case of a fault.

Working:

1. Comparators continuously monitor voltage, current, and temperature.
2. If any of these parameters exceed their safe limits, the comparators trigger a protection signal.
3. Logic gates combine these signals to generate a final protection command.
4. This command activates the switches, disconnecting the battery to prevent damage.

5. Communication Interface

The communication interface is implemented within the BCU and provides connectivity to external systems.

 

Components:

Communication Controllers: Implement communication protocols such as CAN, LIN, SPI, or UART.

Transceivers: Convert digital signals into signals suitable for transmission over the communication bus.

 

Working:

1. The BCU formats data into messages according to the communication protocol.
2. The transceiver converts these messages into electrical signals.
3. These signals are transmitted over the communication bus to other systems.
4. The receiving system decodes the messages and extracts the data.

Benefits of using a BMS

Using a BMS has many benefits, including:

• Increased battery life: By protecting the battery from operating outside its safe operating area, the BMS can help to extend the battery's life.
• Improved battery performance: By ensuring that all cells in the battery pack are balanced, the BMS can help to improve the battery's performance.
• Enhanced safety: The BMS can help to prevent battery fires and other safety hazards.
• Reduced warranty costs: By protecting the battery from damage, the BMS can help to reduce warranty costs.

Pyrofuse or Pyroswitch and how does it works?

 

Pyrofuse or Pyroswitch 

The "pyro" part refers to the small explosive charge within the device.

When triggered, this charge detonates, breaking a connection within the fuse and interrupting the electrical circuit.

This process happens very rapidly, within milliseconds after the collision and triggered by the VECU of an electric vehicle.

Pyro switch or Pyro fuse is an electrical fuse that uses a small explosive charge to quickly and permanently cut off the flow of current in a circuit.

This is typically used in high-voltage battery systems, such as those found in electric vehicles, to prevent fires or other hazards in the event of a crash or short circuit.

Pyro switch function:

• Initiator: This is the small explosive charge. When an electrical signal is sent, it ignites.
• Gas Generation: The explosion creates a rapid expansion of gas.
• Piston: The force of the expanding gas pushes the piston. This piston is the key to how the switch works.

How this translates to a pyro switch:

1. Enclosure: All of this would be contained within a robust, insulated enclosure (likely plastic or ceramic). This enclosure would be the visible "pyro switch" itself.
2. Terminals: The enclosure would have electrical terminals connected to the circuit that needs to be interrupted.
3. Piston Action: The piston's movement is used to break the electrical connection. There are a couple of ways this could happen:
• Direct Break: The piston could directly separate two electrical contacts, physically opening the circuit.
• Cutting Element: The piston could drive a small cutting element (like a blade) that severs a conductive link within the switch.

Structure of the Pyro switch:

Internal Structure of Autoliv Pyro Saftey switchs

                                                                                      

Imagine a small, rectangular or cylindrical box.

• Casing: The outer casing is made of a dark-coloured, robust plastic or ceramic.
• Terminals: Two or more metal terminals protrude from the casing.
• Internal Mechanism (Invisible): Inside, the initiator, gas generation chamber, and piston are arranged as shown in your image.
• External Indication (Possible): There might be a slight bulge or a line on the casing indicating where the piston moves internally. After activation, this area might be visibly deformed or cracked.

Know about lithium cell models?

 

lithium cell model

Lithium cells come in a wide variety of models, each with its own specific designation. Here are some common examples:

Common Shapes and Sizes

• Cylindrical Cells (like AA or AAA batteries, but bigger):

  • 18650: 18mm in diameter, 65mm long. Common in laptops and flashlights.
  • 21700: 21mm in diameter, 70mm long. Used in electric cars and power tools.
  • 26650 & 32650: Even bigger cylinders, for things that need a lot of power.

• Pouch Cells (flat and flexible):

  • Fit well in slim devices like smartphones and tablets.

• Prismatic Cells (boxy):

  • Rectangular blocks are often used in electric cars and large battery packs.

What those extra numbers and letters mean

• They give you more details about the cell's performance, like how much energy it can store and how quickly it can release. For example, in "18650-30Q", the "30Q" tells you about its discharge rate.

Finding the right battery

• Always check the manufacturer's information for the most accurate details about a specific lithium cell model.

How moisture infiltrate Lithium-ion battery Packs?

Moisture on Lithium-ion Cells

 Moisture can infiltrate lithium-ion battery packs through several primary mechanisms:

• Condensation: When warm, humid air encounters a colder surface within the battery pack (such as the cells or enclosure), the moisture in the air transitions into liquid water. This phenomenon is particularly prevalent in environments where temperature and humidity levels fluctuate significantly.

• Leaks: Imperfections in the battery pack's casing, including cracks, faulty breather plugs or inadequate seals, can create pathways for external moisture to penetrate. These vulnerabilities can arise from manufacturing defects, physical damage during transport or usage, or the natural wear and tear associated with the battery pack's lifespan.

• Coolant Leaks: In battery packs equipped with liquid cooling systems, leaks within the coolant lines can introduce moisture into the pack. Coolants often contain water or possess hygroscopic properties, meaning they readily absorb moisture from the surrounding air.

• Manufacturing Process: While less frequent, moisture can be inadvertently introduced during the battery pack's manufacturing process if proper drying and sealing procedures are not strictly adhered to.

The presence of moisture within a lithium-ion battery pack can have detrimental consequences:

• Diminished Performance: Moisture can corrode the internal components of the battery pack, leading to an increase in internal resistance and a subsequent reduction in energy capacity.

• Safety Hazards: Moisture can interact chemically with the battery's constituents, generating heat and potentially culminating in fires or explosions.

• Accelerated Aging: Moisture can expedite the aging process of the battery pack, resulting in a shorter overall lifespan.

To mitigate moisture ingress, manufacturers employ a variety of strategies:

• Hermetic Sealing: Utilizing high-quality seals and gaskets to establish a moisture-tight barrier.

• Desiccants: Incorporating moisture-absorbing materials within the battery pack to minimize the impact of any residual moisture that may enter.

• Regular Inspections: Conducting periodic inspections to identify and address any potential leaks or damage.

• Controlled Manufacturing Environments: Maintaining a low-humidity environment throughout the manufacturing process.

By comprehending the pathways through which moisture can infiltrate lithium-ion battery packs and implementing effective preventative measures, manufacturers and users can contribute to ensuring the safety, performance, and longevity of these critical components.

Thermal adhesives in lithium-ion battery packs

Thermal adhesives - Thermal management in lithium-ion battery packs

Thermal adhesives play a crucial role in lithium-ion battery packs, especially in electric vehicles (EVs), by facilitating efficient heat dissipation and maintaining optimal operating temperatures. Here's a breakdown of their applications in lithium cell connections.

1. Cell-to-Cell Bonding:

• Heat Dissipation: Lithium-ion cells generate heat during operation due to internal resistance. Thermal adhesives between cells create a thermally conductive pathway, allowing heat to spread evenly and dissipate more effectively. This prevents localized hot spots that can accelerate battery degradation and compromise safety.

• Structural Support: Adhesives provide mechanical bonding between cells, enhancing the structural integrity of the battery pack. This is particularly important in cell-to-pack designs where individual cells are directly integrated into the battery pack structure.

• Electrical Insulation: While primarily designed for thermal conductivity, these adhesives offer electrical insulation, preventing short circuits between adjacent cells.

2. Cell-to-Cooling Plate Bonding:

• Efficient Heat Transfer: Thermal adhesives bond cells to cooling plates or heat sinks, which are essential components of the battery thermal management system. The adhesive facilitates efficient heat transfer from the cells to the cooling system, maintaining optimal operating temperatures.

• Uniform Contact: The adhesive ensures uniform contact between the cell and the cooling plate, maximizing the heat transfer area and efficiency.

3. Module-to-Pack Bonding:

• Thermal Management at the Module Level: Battery packs with modular designs, and thermal adhesives are used to bond individual modules to the pack structure. This helps manage heat dissipation at the module level, further enhancing overall thermal management.

• Structural Integrity: Adhesives contribute to the structural integrity of the battery pack by bonding modules together and to the pack housing.

Key Considerations for Thermal Adhesives:

• Thermal Conductivity: The primary requirement is high thermal conductivity to ensure efficient heat transfer.

• Adhesion Strength: The adhesive provides strong bonding to maintain contact between components under various operating conditions.

• Electrical Insulation: The adhesive should be electrically insulating to prevent short circuits.

• Chemical Compatibility: The adhesive must be chemically compatible with the cell materials and other components in the battery pack.

• Long-Term Reliability: The adhesive must maintain its properties over a wide temperature range and throughout the battery's lifespan.

In conclusion, thermal adhesives are essential for effective thermal management in lithium-ion battery packs. They facilitate heat dissipation, enhance structural integrity, and contribute to the overall safety and performance of the battery system.