One particular area of interest is improving battery management systems, which work in real time to monitor the performance of individual battery cells within the EV. By effectively monitoring each battery cell, an EV’s microcontroller (MCU) can ensure the proper operation of all battery cells and balance load sharing. This white paper examines the differences between wired and wireless BMS solutions to help you choose the best option for your EV design.

In electrified automotive applications, internal battery packs can extend up to 800 V and beyond to support the demanding loads of the AC motor. This translates into potentially 100 or more lithium-ion cells stacked together in series inside the vehicle chassis. These high-voltage packs are increasingly requiring more sophisticated technologies to report cell diagnostics in a safe, timely and reliable manner. One common design technique is to implement a distributed battery pack system, which supports high-cell-count packs by connecting multiple high-accuracy battery monitors on separate printed circuit boards (PCBs).

In a wired BMS solution, connecting these monitors in a daisy chain with twisted-pair cabling enables the propagation of data acquired for each module of battery cells. The difference between a wired and wireless BMS solution is that the latter uses a wireless communications interface rather than daisy-chain cabling. Figure 1 displays a typical distributed battery pack system for 400-V to 800-V EVs.

In Figure 1, there is a subsystem containing the host MCU, which interfaces with the control unit of the vehicle through a Controller Area Network bus. Then the MCU processor drives the battery monitor devices connected out to the battery modules to sense voltage and temperature. Depending on how many channels the battery monitors support, there can be any number of devices stacked to support high-voltage packs, which all need to communicate back to the host MCU quickly. Other common aspects of the system requiring monitoring and communications include high-voltage relay controls to ensure safe disconnection of the high voltage when the vehicle is not in use, and current sensing to calculate the state of charge and state of health of the battery pack.

The focus in this paper will be on the communications interface between each battery monitor device connected in the pack and the host MCU. Both examples will use the BQ796xx family of monitors. The typical wired solution connects battery monitors in a daisy-chain cable with twisted-pair cabling between battery modules. The wireless communication method uses the CC2642R-Q1 wireless MCU for transmitting data.

In Figure 2, the wired solution displays a battery management or monitor unit (BMU) board on the left, which holds the host MCU and BQ79600-Q1 communications bridge device. This BMU acts as an interface between the MCU and other BQ796xx monitoring devices on the cell monitoring unit (CMU), which connects to the actual battery cells. These CMUs are interconnected through a twisted-pair daisy-chain cable on both the high and low sides of each battery monitor device, and there is an optional ring cable that provides the ability to transmit in either direction in the event of a cable break. The wired solution does require isolation components on either side of the daisy-chain cabling to ensure robust communication in noisy environments that can withstand strict automotive electromagnetic interference (EMI) and electromagnetic compatibility (EMC) limits.

The wireless solution uses a wireless interface to transmit universal asynchronous receiver-transmitter (UART) data from the battery monitor to the host MCU via a wireless transceiver device.

Figure 3 uses a more simplified representation of a CMU than Figure 1, but adds a wireless receiver node to show that the CMU has an additional device to transmit cell data wirelessly back to the host. This enables the two CMUs shown in Figure 2 to be naturally isolated from one another.

The significant difference between both solutions comes down to the replacement of the twisted-pair cabling in the wired solution with a CC2642R-Q1 device on each BMU in the wireless solution.

It may appear that adding an extra device may create more complexity and cost more than cabling, but consider the cost and weight of the cabling and the need to place high-performance isolation components on either side of the twisted-pair interface to ensure communication robustness. Table 1 outlines further considerations for wired vs. wireless battery manangement solutions.