Streamline your EV junction box design

Electric vehicles (EVs) are powered by huge battery banks (Fig. 1), constructed of long strings of batteries in series that can achieve operating voltages higher than 800 V and an average current of 40 A. These battery systems are highly complex in terms of isolation, current sensing, charging and discharging logic control, and chassis leakage detection, therefore, require a junction box for high-voltage connections.

Each battery cell voltage is monitored by control modules and appropriate control methods are applied to keep voltage deltas between cells to a tight tolerance. The junction box controls the high-voltage connections for the charging system, inverter/motor, and battery pack. High-voltage connections, currents, and isolation resistance are measured within this module and sent back to the main electronic control unit (ECU) for state of charge (SOC) and power calculations, monitoring vehicle status, and ensuring safety during various vehicle conditions.

Fig. 1: EV car battery pack and wiring (Image: Shutterstock)

In this design solution, we review the structure of a typical EV battery system with the associated junction box. We then introduce a novel junction box design that is streamlined, better integrated into the system, and capable of reporting measurements that are time-aligned with the rest of the system.

Distributed battery system architecture

Fig. 2 illustrates a typical distributed battery system. As an example, on the left of the battery pack, eight supervisory modules (N=8), residing on the high-voltage board, each control 14 rows (K=14) of cells in series with each row comprised of 70 batteries in parallel (a 7,840 Li+ batteries ensemble). Isolation is required between the microprocessor and the first module, and from one module to the next. The data is subsequently passed along to the microcontroller on the low-voltage board.

On the right of the battery pack, the junction box senses six critical voltage nodes (contactors X and isolation ISO_RES) and a Hall sensor measures the current. The data is then passed along to a second microprocessor.

Monitoring the contactor voltage nodes is important to check the health of the batteries when contactors are closed and when they are open. It is safety-critical since it also tells the system when the contactors are in the correct state.

Fig. 2: Typical system architecture (Image: Maxim Integrated)

Streamlined system architecture

In the streamlined implementation in Fig. 3, DC-blocking capacitors (or transformers) are used to isolate daisy-chain devices that operate at different common-mode voltages. Inexpensive capacitors can be used in the daisy chain between modules, which reduces system cost.

Further, the daisy chain can be easily extended to incorporate the junction box data acquisition IC, which eliminates the need for a local microprocessor and enables time alignment between measurements from the junction box and those from the battery modules. Time alignment is important because it gives better correlation for power management and calculations. Finally, the junction box high-voltage data acquisition IC has current-sensing capability that enables flexibility in using either a shunt resistor (shown here) or Hall effect current sensor, or both (for redundancy).

Fig. 3: Streamlined system architecture (Image: Maxim Integrated)

High-voltage data acquisition with current sense

As an example, the MAX17852 with an integrated current-sense amplifier is a flexible data-acquisition system for the management of high-voltage and low-voltage battery modules. The system can measure 14 cell voltage nodes (or seven ground referenced high voltage nodes), one current, and a combination of four temperatures or system voltage measurements with fully redundant measurement engines in 263 μs. It can also poll all inputs solely with the fast ADC SAR measurement engine in 156 μs. By integrating the current sense amplifier, the MAX17852 eliminates multiple system components from a discrete solution, including a bulky Hall-effect current sensor, its bias circuitry and ADC, saving bill-of-materials (BOM) cost and board space.

This highly integrated battery sensor incorporates a high-speed differential UART bus for robust daisy-chained serial communication, designed for maximum noise immunity. Up to 32 devices can be daisy chained. The single daisy chain enables time-alignment between the junction box and the battery supervisory measurements. Accordingly, cell voltage, bus bar measurements, pack voltage, pack current, contactor voltages, and temperature measurements are aligned within 10 µs.

The system uses Maxim’s battery-management UART or SPI protocol for robust communications and supports an I2C master interface for ex­ternal device control. It is optimized to support a re­duced feature set of internal diagnostics and rapid-alert communication through both embedded communication and hardware-alert interfaces to support ASIL D and FMEA requirements.

Battery electrical isolation measurement

The Department of Transportation (TP-305-01) prescribes for this measurement a resistance (in ohms) approximately 500 times the nominal operating voltage of the vehicle (in volts) per SAE  1766, between the negative (positive) side of the propulsion battery and the vehicle chassis, namely 200 kΩ for 400 V. Accordingly, the isolation resistance RLEAK- (RLEAK+) between the chassis and the battery positive (negative) can be sensed with the network shown in Fig. 4 and reported as a voltage to the AUX pin of the data acquisition IC.

Fig. 4: Battery isolation resistance measurement (Maxim Integrated)

Based on the network in Fig. 4, the VAUX equation for the RLEAK- case is:

The graph below reports the curves for both RLEAK+ and RLEAK- and shows that the 200 kΩ RLEAK- isolation resistance produces a sensed voltage VAUX of 2.18 V while a 200 kΩ RLEAK+ isolation resistance produces a sensed voltage of 1.08 V.

Fig. 5: Isolation resistance curve (Maxim Integrated)

Conclusion

Electric vehicles handle high voltages and high currents. Electrical connections, contact resistance, currents, and isolation resistance between high-voltage and low-voltage boards must be monitored to ensure safe operation. By using the MAX17852, a unique data acquisition IC with a low-noise, cost-effective, capacitive-isolation daisy-chain communication architecture, in an EV battery system, it can eliminate the need for a junction box-dedicated microprocessor.

The MAX17852 also eliminates the bulky and more expensive Hall-effect current sensor thanks to the integrated current-sense amplifier. In addition, it enables time alignment between the junction box and the cell voltage measurement, while the fast SAR ADC architecture enables multiple measurements in a minimum amount of time.

About the authors:

Tamer Kira is executive director of business management for the Automotive business unit at Maxim Integrated. His current interests include battery and power management, specifically for electric vehicles, hybrids, and plug-in hybrids. He holds a Bachelor of Science degree in Electrical Engineering.

Nazzareno (Reno) Rossetti is an Analog and Power Management expert at Maxim Integrated. He is a published author with several patents in this field. Reno holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.