EV BATTERY COMMUNICATIONS Wired vs. Wireless Communications in EV Battery Management
Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs) depend on large numbers of battery cells to provide high voltages for their electric motors – and these cells’ health must be continuously monitored to assure vehicle safety and reliability. This article compares the relative merits of using wired or wireless solutions for battery monitoring systems.
As vehicle emission control legislation becomes tighter worldwide, the case for Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs) becomes stronger. However, the EV market is still relatively new and faces some challenges before it becomes entirely dominant. Development is still ongoing to maximize vehicle operating range, duration, safety, and reliability while minimizing cost, size, and weight.
These factors relate in large part to the batteries used, which are most commonly lithium-ion (Li-ion) and lithium polymer types. Li-ion batteries in particular demand special attention because each cell’s characteristics change in a different way . Therefore, each cell must be managed individually to avoid complete discharge and the resulting power loss – and an automotive battery pack typically comprises a large number of cells, which can be distributed.
The EV’s AC motor may demand up to 800 V or more; this translates into potentially 100 or more Li-ion cells stacked together in series inside the vehicle chassis. These high-voltage packs are increasingly requiring more sophisticated technologies to report cell diagnostics for each cell 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 using multiple high-accuracy battery monitors.
The EV battery communications requirement
The battery monitors, which are mounted one on each cell, must connect with a central Battery Management Unit (BMU) to make up a distributed Battery Management System (BMS). Accordingly, the BMS can operate in real time to monitor the performance of each cell within the EV. This way, the BMS can ensure that all cells are operating correctly, while load sharing is properly balanced.
However, designers must decide on which type of communication channel to use: the monitors can be connected back to the BMU either with wires, or wirelessly. There are several design considerations which must be traded off to decide on the best option for any particular project. We can look a little more closely at both approaches to better understand their relative merits.
In a wired system, the monitors are connected in a daisy chain of twisted-pair cabling, which carries cell temperature and voltage data back to the BMU. Current sensing on the power line is also used to calculate the battery pack’s state of charge and state of health. For extra reliability, a ring cable can be installed, which can transmit data in either direction in the event of a daisy chain cable break. The BMU in turn interfaces to the vehicle’s control unit through a Controller Area Network bus (CAN bus).
A high voltage battery pack has a large number of monitors, which all need to communicate back to the BMU quickly; these can all be connected to the daisy chain. The system will also include high-voltage relay controls to ensure safe disconnection of the high voltage when the vehicle is not in use.
The wired solution requires isolation components on either side of the daisy-chain cabling to ensure robust communications in electrically noisy environments, and to comply with strict automotive Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) limits.
TI offers a solution for both wired and wireless architectures, comprising hardware and protocols. Wired interfaces are supported by their BQ796XX devices with an RX topology similar to RS-485 – but with added design mechanisms to attenuate high common-mode voltages caused by the noisy conditions typical in vehicle environments.
TI’s wireless solution is based on their CC2642R-Q1 devices and their associated proprietary wireless BMS protocol. This is based on Bluetooth Low Energy technology operating at 2.4 GHz. Its star network configuration supports up to 32 monitors per BMU, and provides high throughput, low-latency data transmission. It also features a functional safety-rated protocol.
The interface transmits Universal Asynchronous Receiver-Transmitter (UART) data from the battery monitors to the BMU. This design provides natural isolation between the distributed BMS components.
Wired or wireless - what are the factors?
It may appear that adding an extra device for wireless communications creates extra cost and complexity. However, eliminating the cabling and high-performance isolation components necessary for wired communications robustness provides significant cost, weight and space savings. The wireless solution decreases overall vehicle weight and complexity.
Also, while wiring is reliable, and meets functional safety standards, it can break over time – and if it does, it can be complex to repair. Nevertheless, this situation can be somewhat mitigated by using a ring architecture, which features built-in redundant cables in both directions. Although the wireless solution means no wires to maintain, the design must overcome harsh automotive radio-frequency environments and line-of-sight challenges.
Without cables and isolation components, the wireless solution has a smaller footprint; this improves design flexibility, and provides more location options within the vehicle. It is also easier to service.
Additionally. the wireless solution can improve measurement performance, as its star network configuration naturally supports time-synchronized measurements. This can be further improved by adding more synchronized sensing capabilities. By contrast, in a wired system, time-synchronized voltage, temperature, and current measurements must travel the length of the daisy chain, causing delay between different monitor readings. However, a delayed measurement feature can improve performance.
Security is another important concern. A wired solution means contained and fully secure communications, while it is possible to breach poorly designed wireless systems that lack security protocols.
The above considerations show that adopting a wireless design for an EV battery management system can offer advantages over a wired solution – but only if the wireless design is built to overcome the electrical noise and security issues found within a vehicle environment.