“As electric vehicles (EVs) become more popular, the challenge for automakers is to eliminate driver “range anxiety” while making cars more affordable. This means lower cost and higher energy density for battery packs, where battery storage and retrieval technology is critical for extended driving range.
As electric vehicles (EVs) become more popular, the challenge for automakers is to eliminate driver “range anxiety” while making cars more affordable. This means lower cost and higher energy density for battery packs, where battery storage and retrieval technology is critical for extended driving range.
Accurate measurement of voltage, temperature and current is critical to achieving the state of charge or state of health of each battery in the system.
The primary function of a battery management system (BMS) is to monitor cell voltage, pack voltage, and pack current. Figure 1a shows the battery pack in a white box with multiple cells stacked. The battery monitoring unit includes a battery monitor to check the voltage and temperature of the battery.
Figure 1: Traditional BMS Architecture (a); BMS Architecture with Smart Battery Junction Box (BJB) (b)
In Figure 1a, you rely on the battery management unit (BMU). The BMU usually has a microcontroller (MCU) that manages all functions within the battery pack. The light blue block is the BJB, which is a relay box or switch box with a large contactor that connects the entire battery pack to the load inverter, motor and even the charger.
Figure 1a shows a conventional BMS. In this configuration, there is no active electronics inside the junction box; it’s just passive contactors and fuses. All measurements in the BJB are measured at the BMU. Wires connect the BJB to the analog-to-digital converter (ADC) terminals.
Figure 1b shows a smart BJB. In this concept, there is a dedicated battery pack monitor inside the box that measures all voltages and currents and communicates the information to the MCU using simple twisted pair communication.
Benefits of Smart BJB:
• Eliminate wire and cable harnesses.
• Improved voltage and current measurements with lower noise.
• Simplified hardware and software development. Because the Texas Instruments (TI) battery pack monitor and battery monitor are from the same device family, their architecture and register maps are very similar.
• Enables system manufacturers to synchronize battery pack voltage and current measurements. Latency reduction enhances state-of-charge estimation.
Voltage, temperature and current measurement
Figure 2 shows the different high voltages, currents and temperatures measured by the battery pack monitor inside the BJB enabled by the BQ79631-Q1 battery pack monitor.
Figure 2: High voltage measurement inside the BJB.
Voltage: Voltage is measured using a voltage divider resistor string. These measurements check whether the switch is on or off.
Temperature: The 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 measurement is based on a shunt resistor. Since the current in an EV can be up to several thousand amps, these shunt resistors are very small – in the range of 25 µOhms to 50 µOhms.
Based on Hall effect sensors. Its dynamic range is usually limited, so sometimes there are multiple sensors in the 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 is the time delay in sampling the voltage and current between the battery pack monitor and the battery monitor. These measurements are mainly used to calculate state of charge and state of health through electrical impedance spectroscopy. The impedance of the battery is calculated by measuring the voltage, current and power on the battery, enabling the BMS to monitor the instantaneous power of the car.
Cell voltage, pack voltage, and pack current must be synchronized to provide the most accurate power and impedance estimates. Sampling within a certain time interval is called a synchronization interval. The smaller the synchronization interval, the more accurate the power estimate or impedance estimate. The error for asynchronous data is proportional. The more accurate the state-of-charge estimate, the greater the range.
Next-generation BMSs will require simultaneous 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 acquired samples are not inherently synchronous.
• Each cell monitor can measure 6 to 18 cells; each cell has a data length of 16 bits. There is a large amount of data that needs to be transferred over the daisy-chain interface, which can consume the timing budget allowed by voltage and current synchronization.
• Any filters such as voltage filters or current filters can affect the signal path, causing voltage and current synchronization delays.
TI’s BQ79616-Q1, BQ79614-Q1, and BQ79612-Q1 battery monitors maintain time relationships by issuing ADC start commands to the battery monitor and battery pack monitor. These battery monitors also support delayed ADC sampling to compensate for delays when the ADC start command is transmitted down the daisy-chain interface.
The large-scale electrification efforts underway in the automotive industry drive the need to reduce BMS complexity by adding electronics to the junction box, while improving system safety. The battery pack monitor can locally measure the voltage before and after the relay, the current through the battery pack. Improvements in the accuracy of voltage and current measurements will directly lead to optimal battery utilization.
Effective voltage and current synchronization enables accurate state-of-health, state-of-charge, and electrical impedance spectrum calculations to optimize battery utilization, extend its life, and increase driving range.