Accurate measurement of voltage, temperature, and current is crucial for determining the state of charge or health of each battery in the system.
The primary function of a Battery Management System (BMS) is to monitor battery voltage, battery pack voltage, and battery pack current. Figure 1a shows a battery pack in a white box, where multiple batteries are stacked. The battery monitoring unit includes a battery monitor for checking battery voltage and temperature.
Figure 1: Traditional BMS architecture (a); BMS architecture with intelligent battery junction box (BJB) (b)
In Figure 1a, you rely on the Battery Management Unit (BMU). The BMU typically has a microcontroller (MCU) that manages all the functions within the battery pack. The light blue block is the BJB, which is a relay box or switch box with large contactors that connects the entire battery pack to the load inverter, motor, or even the charger.
Figure 1a shows a conventional BMS. In this configuration, there is no active electronics inside the junction box; it consists only of passive contactors and fuses. All measurements in the BJB are taken at the BMU. Wires connect the BJB to the analog-to-digital converter (ADC) terminals.
Figure 1b shows the smart BJB. In this concept, there is a dedicated battery pack monitor inside the box that measures all voltages and currents and transmits the information to the MCU using simple twisted-pair communication.
Benefits of Smart BJB:
Eliminate wire and cable bundles.
Improve voltage and current measurements with lower noise.
Simplify hardware and software development. Because Texas Instruments (TI) battery pack monitors and battery monitors come from the same device family, their architectures and register mappings are very similar.
This enables system manufacturers to synchronize battery pack voltage and current measurements. Reduced latency enhances state-of-charge estimation.
Voltage, temperature and current measurements
Figure 2 shows the different high voltages, currents, and temperatures measured by the battery pack monitor within the BJB when the BQ79631-Q1 battery pack monitor is enabled.
Figure 2: High-voltage measurement inside the BJB.
Voltage: Voltage is measured using a series of voltage divider resistors. These measurements check whether the switch is on or off.
Temperature: Temperature measurement monitors the temperature of the shunt resistors so that the MCU can apply compensation, as well as the temperature of the contactors to ensure their health.
Current:
Current measurements are based on a shunt resistor. Since the current in an EV can reach thousands of amperes, these shunt resistors are very small—ranging from 25 µOhms to 50 µOhms.
Hall effect sensors are based on this principle. Their dynamic range is typically limited; therefore, sometimes multiple sensors are used in a system to measure the entire range. Hall effect sensors are inherently susceptible to electromagnetic interference. However, these sensors can be placed anywhere in the system, and they inherently provide isolated measurements.
Voltage and current synchronization
Voltage and current synchronization addresses the time delay in sampling voltage and current between the battery pack monitor and the battery monitor. These measurements are primarily used to calculate the state of charge and state of health via impedance spectroscopy. By measuring the voltage, current, and power on the battery, the battery impedance is calculated, enabling the BMS to monitor the vehicle's instantaneous power.
Battery voltage, battery pack voltage, and battery pack current must be synchronized to provide the most accurate power and impedance estimates. Sampling at specific time intervals is called the synchronization interval. The smaller the synchronization interval, the more accurate the power or impedance estimate. The error of asynchronous data is proportional. The more accurate the state-of-charge estimate, the greater the driving range.
Synchronization requirements
The next generation of BMS will need to perform synchronous voltage and current measurements in less than 1 ms, but meeting this requirement presents challenges:
All battery monitors and battery pack monitors have different clock sources; therefore, the samples collected are not synchronized in nature.
Each cell monitor can measure 6 to 18 cells; each cell has a data length of 16 bits. A significant amount of data needs to be transmitted via daisy-chain ports, which may consume the timing budget allowed for voltage and current synchronization.
Any filter, such as a voltage filter or a current filter, will affect the signal path, resulting in a delay in voltage and current synchronization.
TI's BQ79616-Q1, BQ79614-Q1, and BQ79612-Q1 battery monitors can maintain timing relationships by issuing ADC start commands to both the battery monitor and the battery pack monitor. These battery monitors also support delayed ADC sampling to compensate for the latency when transmitting the ADC start command down the daisy-chain interface.
in conclusion
The ongoing large-scale electrification efforts in the automotive industry have driven the need to reduce BMS complexity while improving system safety by adding electronics to the junction box. Battery pack monitors can locally measure the voltage across the relays and the current flowing through the battery pack. Improved accuracy in voltage and current measurements will directly lead to optimal battery utilization.
Effective voltage and current synchronization enables accurate calculations of battery health, state of charge, and impedance spectrum, thereby optimizing battery utilization, extending its lifespan, and increasing driving range.
