Figure 1 shows a typical BMS architecture that includes a battery management unit (BMUs), a cell monitoring unit (CMUs), and a battery junction box (BJBS). The BMUs typically contain a microcontroller (MCU) that manages all functions in the battery pack. A traditional battery junction box is a relay box or switch box with a power contactor that connects the entire battery pack to a load inverter, motor, or battery charger.

 

Figure 1a shows a traditional BMS. There are no active electronics in the terminal box. All measurements in the battery terminal box are performed on the battery management unit (BMU). The battery terminal box is connected to the ADC terminal by cable.

 

 Figure 1b shows the smart battery terminal box. The junction box has a dedicated battery pack monitor inside that measures all voltage and current and transmits the information to the MCU via simple twisted pair communication. This helps eliminate wiring and wiring harnesses and improves voltage and current measurements with lower noise.

 

Figure 2 shows the different high voltages, currents, and temperatures measured in the battery terminal box by the battery pack monitor with BQ79731-Q1 enabled.

                                                                        

                                                                          

                                                                                               Figure 2: High voltage measurement inside the battery junction box 

 

● Voltage: High voltage measurement is achieved using a voltage division resistor string. This type of voltage measurement can monitor the operating status of high voltage components in the system.
● Temperature: The temperature measurement monitors the temperature of the shunt resistor so that the MCU can apply compensation, in addition, the temperature of the contactor is also monitored to ensure that it is under load within normal operating conditions.
● Current: Current measurement is based on one of the following two devices:
-   Shunt resistor - Because the current in electric vehicles can be up to thousands of amps, the shunt resistor value is very small, 25µOhms to 50µOhms;
-   Hall Effect sensor - Used to measure electric vehicle high voltage circuit current, this measurement is isolated. Typically, the dynamic range is limited, so multiple sensors may be used in the system to measure the entire range.