■ Junction box charging system, inverter/motor, battery pack high-voltage connection control

Electric vehicles (EVs) are powered by large battery banks (Figure 1) consisting of long strings of batteries connected in series that can achieve operating voltages exceeding 800 V and average currents of 40 A.

Each cell voltage is monitored by a control module, and appropriate control methods are applied when necessary to maintain the voltage difference between cells within a very small tolerance.

The junction box controls the high-voltage connections to the charging system, inverter/motor, and battery pack.

High voltage connections, currents and insulation resistances are measured within these modules and sent back to the main ECU for use in SOC and power calculations and vehicle health monitoring, ensuring safety under a variety of vehicle conditions.





This design solution examines the architecture of a typical EV battery system along with its associated junction box.

We then present a novel junction box design that is streamlined and seamlessly integrated into the system, allowing it to report measurements that are time-aligned with the rest of the system.

■ Distributed Battery System Architecture

Figure 2 illustrates a typical distributed battery system. For example, on the left side of the battery pack, a high-voltage board contains eight supervisory modules (N=8), each controlling 14 rows (K=14) of cells connected in series, each row consisting of 70 batteries connected in parallel (7,840 Li+ batteries in series and parallel).

Isolation is required between the microprocessor and the first module, and between modules. Data is then transmitted to the microcontroller on the low-voltage board.

The junction box on the right side of the battery pack detects six critical voltage nodes (contact X and isolation ISO_RES) and a Hall sensor measures the current. The data is then passed to a secondary microprocessor.

Monitoring the relay (contactor) voltage node is important to determine battery condition when the relay closes and opens.

Additionally, this monitoring is essential for safety as it notifies the system when the relay is in a normal state.





■ Simplified system architecture

In the simplified implementation of Figure 3, a DC blocking capacitor (or transformer) is used to isolate daisy-chained devices operating at different common-mode voltages.

System cost is reduced by allowing the use of inexpensive capacitors in daisy chains between modules.

Moreover, this daisy chain can be easily expanded to integrate a junction box data acquisition IC, eliminating the need for a local microprocessor and enabling time alignment between measurements at the junction box and measurements at the battery module.

Time alignment is important because it provides more accurate correlation between power management and computation.

Finally, the junction box high-voltage data acquisition IC has a current sensing capability that provides the flexibility to use either a shunt resistor (shown in the figure), a Hall-effect current sensor, or both (dual configuration).





■ High voltage data collection with current detection function

For example, the MAX17852 is a flexible data acquisition system for managing high-voltage and low-voltage battery modules.

The system has a fully dual-configuration measurement engine capable of measuring 14 cell voltage nodes (or 7 ground-referenced high-voltage nodes), 1 current, and 4 temperature or system voltage combinations in just 263 μs.

Additionally, the high-speed ADC SAR measurement engine alone can poll all inputs within 156 μs.

This highly integrated battery sensor integrates a high-speed differential UART bus for robust daisy-chain serial communication and is designed for maximum noise immunity. Up to 32 devices can be daisy-chained, and a single daisy-chain provides time alignment between the junction box and battery monitoring measurements. This ensures cell voltage, bus bar measurements, pack voltage, pack current, relay voltage, and temperature measurements are aligned within 10 s.

The system uses Analog Devices' battery management UART or SPI protocols for robust communication and supports an I2C master interface for external device control.

The system is optimized to support a streamlined feature set of internal diagnostics and rapid warning communications via both embedded communications and hardware warning interfaces to support ASIL D and FMEA requirements.

■ Battery electrical insulation measurement

The U.S. Department of Transportation (DOT) regulates these measurements as follows (TP-305-01): The resistance (in ohms) between the negative (positive) side of the electric propulsion battery and the vehicle chassis should be approximately 500 times the nominal operating voltage (in volts) of the vehicle, according to SAE 1766. That is, for 400 V, it should be 200 kΩ.

Accordingly, the insulation resistance RLEAK-(RLEAK+) between the chassis and the battery positive (negative) pole can be sensed by the network shown in Figure 4 and reported as a voltage to the AUX pin of the data acquisition IC.





According to the network in Figure 4, the VAUX equation for the RLEAK- case can be expressed as follows.





In this formula:





Here:





The graph below shows the curves for both RLEAK+ and RLEAK-. Here it is shown that the 200kΩ RLEAK- insulation resistor produces a sensed voltage VAUX of 2.18V, while the 200kΩ RLEAK+ insulation resistor produces a sensed voltage of 1.08V.





■ Monitoring of electrical connection contact resistance, current, and insulation resistance between high-voltage and low-voltage boards

Electric vehicles utilize high voltages and currents, so monitoring electrical connection contact resistance, current, and insulation resistance between high- and low-voltage boards is essential to ensure safe operation. This article examines the structure of a typical electric vehicle battery and junction box system and explores its complexity.

Next, we introduce a new and unique data acquisition IC that is a cost-effective, low-noise, capacitively isolated daisy-chain communication architecture that does not require a dedicated microprocessor in the junction box.

This IC enables time alignment between the junction box and cell voltage measurement. It also integrates current sensing, eliminating the need for a separate Hall-effect current sensor.

The high-speed SAR ADC architecture enables a variety of measurements in minimal time.


※ About the author
Tamer Kira is a director of business management for Analog Devices' Automotive business unit. His current focus is on battery and power management for electric, hybrid, and plug-in hybrid electric vehicles. He received a bachelor's degree in electrical engineering.

Nazzareno (Reno) Rossetti is an analog and power management expert at Analog Devices. He has published books and holds several patents in this field. He received his Ph.D. in Electrical Engineering from the Polytechnic University of Turin, Italy.