A detailed look at the BJB

An intelligent BJB helps measure high voltages in the battery directly through a voltage, current and insulation resistance pack monitor. There are multiple voltage and current measurement channels available in a typical pack monitor, which can measure voltage across fuses and contractors and check the isolation voltage in the BJB. Figure 6 is a simplified system diagram.

Since the pack monitor can measure the battery pack current, overcurrent protection is incorporated in the system. Some pack monitors such as the BQ79731-Q1 include a Coulomb counting feature for state-of-charge calculations, too.

Voltage and current synchronization implemented in the BQ79731-Q1 enables the measurement of battery pack current and voltage at the same moment as the battery cell voltages are measured in the CSU. It is possible to poll all information captured through either one of the daisy-chained communication interfaces to the BCU.

Differentiation between the intelligent BJB and the traditional BJB is the digital control of contactor drivers and pyrofuses to disconnect the battery pack to the EV system during a crash. The BQ79731-Q1 includes Serial Peripheral Interface (SPI) controller channels that can control contactor drivers and pyrofuses, reducing the need for extra SPI resources from the BCU.

The battery pack uses mechanical contactors controlled by the pack monitor to connect or disconnect subsystems throughout the vehicle. It’s important to prevent any potential failure of these contactors or exposure to high-voltage connections to protect drivers from life-threatening injuries.

Mechanical high-voltage contactors can weld or be damaged through arcing and pitting in the event of uncontrolled inrush current. Why Pre-Charge Circuits are Necessary in High-Voltage Systems explains the use of the TPSI3050-Q1 isolated switch driver to form a reliable solid-state relay for pre-charging in an automotive BJB. With no moving parts, the TPSI3050-Q1 can improve system-level reliability or failure-in-time rates when replacing mechanical pre-charge contactors.

Both the positive and negative terminals of the high-voltage battery pack must be sufficiently separated from the chassis of the vehicle in order to protect the driver or a technician from potential electrical shock. Periodic monitoring of this separation is known as isolation check or insulation resistance monitoring. Solid-state relays such as the TPSI2140-Q1 connect and disconnect a known resistance value (such as 1 MΩ) in parallel to the unknown resistance value (between a battery terminal and chassis ground). By measuring the combined resistance using a pack monitor such as the BQ79731-Q1, you can determine whether the battery separation is within tolerance (at least 500 Ω/V per the Federal Motor Vehicle Safety Standards No. 305 specification) or potentially harmful.