TIDUF46 October   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Multiplexer Network and Switch Strategy
      2. 2.2.2 Cell Balancing
      3. 2.2.3 Stacked AFE Communication
      4. 2.2.4 Isolated UART Interface to MCU
    3. 2.3 Highlighted Products
      1. 2.3.1 BQ79616
      2. 2.3.2 TMUX1308
      3. 2.3.3 TMUX1574
      4. 2.3.4 TMUX1102
      5. 2.3.5 TPS22810
      6. 2.3.6 ISO7742
      7. 2.3.7 TSD05C
      8. 2.3.8 ESD441
      9. 2.3.9 ESD2CAN24-Q1
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Cell Voltage Accuracy
      2. 3.3.2 Temperature Sensing Accuracy
      3. 3.3.3 Cell Voltage and Temperature Sensing Timings
      4. 3.3.4 Cell Balancing and Thermal Performance
      5. 3.3.5 Current Consumption
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

1 System Description

Currently, the battery energy storage systems (BESS) play an important role in residential, commercial and industrial, grid energy storage, and management. A BESS has various high-voltage system structures. Commercial and industrial and grid BESS contain several racks that each contain packs in stack. Residential BESS only contains packs.

A pack is a basic module composing the BESS. A pack consists of battery cells in a matter of series and parallel connection. The number of cell channels varies from 12 to 64. Since the battery cells require a proper working and storage temperature, voltage range, current range for lifecycle and safety, the designer must monitor and protect the battery cell in the pack level.

A battery management unit (BMU) is a controller that monitors the voltage and temperature of each battery cell in the pack for a complete lifecycle. High measurement accuracy for voltage and temperature monitoring is required for the BMU. The information collected by the BMU is transmitted to the rack-level controller battery control unit (BCU) for safety and charging management. A robust and fast-speed communication is also required between the BMU and the BCU.

Safety, regulations, and cost concerns drive the need for a LiFePO4 battery in a BESS. The LiFePO4 battery charge or discharge curve remains fairly linear for the approximately 85% to 100% state of charge (SOC) range, but the curve abruptly changes in slope in the approximately 10% to approximately 85% SOC range. This becomes significant when choosing what voltage accuracy is acceptable in a BESS design. For most conditions, measuring accuracy in 3 mV to 5 mV is required to calculate a high SOC accuracy and a wide depth of discharge (DOD).

For a communication interface, a controller area network (CAN) is traditionally and widely used for robustness of communication. A CAN structure controller needs a microcontroller unit (MCU), a digital isolator, and an isolated power module to operate the CAN communication function.

A daisy chain can replace a CAN design. Compared with the CAN interface, only a couple of transformers are needed in the BMU. Thus, a daisy chain design shows an advantage in cost over a CAN especially in high-capacity battery pack applications since cost is a concern for a CAN structure in large-capacity BESS which consist of many BMU nodes and CAN interface devices. Insulation requirements also raise cost because the reinforced insulation required between the BMU and BCU communication interface necessitates a digital isolator and isolated power module.

This design focuses on large capacity battery pack applications and applications that can be applied in residential, commercial and industrial, grid BESS, and so forth. The design uses two BQ79616 devices (battery monitor, balancer, and integrated hardware protector) to monitor each cell voltage, the temperature of a 32s battery pack, and to protect the pack against situations that include cell overvoltage, cell undervoltage, and overtemperature. The design contains four TMUX1308 devices for a GPIO expansion ratio of 8:1 to measure up to 32s cell temperature and one TMUX1574 device for serial peripheral interface (SPI) expansion to restore pack information in an external electrically-erasable programmable read-only memory (EEPROM). The design uses an internal cell balancing (CB) to get 100-mA balancing current per cell channel and reserves an external CB circuit for a potential larger balancing current.

The onboard communication between two BQ79616 devices uses capacitor-isolated daisy chain. The offboard communication between the BMU and BMU or BCU uses transformer-isolated daisy chain. The design also reserves an isolated UART interface to the offboard MCU which can be used in the CAN structure.