Lithium-ion (Li-Ion) batteries are a common energy storage method for electric vehicles. These batteries offer exceptionally high energy densities compared to other existing battery technologies. However, to maximize performance, a Battery Management System (BMS) is essential to manage charge and discharge cycles more safely, thereby extending battery life. This article will introduce the architecture and operation of a BMS, as well as the features and advantages of BMS devices offered by Analog Devices (ADI).
BMS can improve the operating efficiency of electric vehicle batteries.
Advanced BMS can help electric vehicles extract a large amount of charge from the battery pack during operation and accurately measure the battery's state of charge (SOC) to extend battery runtime or reduce weight, and improve battery safety by avoiding electrical overloads such as deep discharge, overcharge, overcurrent, and thermal overstress.
The main function of a Battery Management System (BMS) is to keep all individual cells in the battery pack within their safe operating area (SOA) by monitoring the physical quantities during battery operation. This includes monitoring the charging and discharging currents of the battery pack, the voltage of individual cells, and the temperature of the battery pack. Based on these values, not only can the battery operate safely, but the State of Charge (SOC) and State of Health (SOH) can also be calculated.
Another important function provided by a BMS is battery balancing. In a battery pack, individual cells can be connected in parallel or series to achieve the desired capacity and operating voltage (up to 1 kV or higher). Battery manufacturers attempt to provide identical cells for the entire pack, but this is physically impractical. Even small differences can lead to varying charge or discharge levels, and the weakest cell in the pack can significantly impact the overall performance of the pack. Precise battery balancing is a crucial function of a BMS, ensuring that the battery system operates safely at its maximum capacity.
Wireless BMS eliminates communication cabling and reduces complexity
Electric vehicle batteries consist of several cells connected in series. A typical battery pack (with 96 cells in series) will produce a total voltage of over 400 V when charged at 4.2 V. The more cells in the battery pack, the higher the voltage will be. All cells have the same charging and discharging current, but the voltage on each individual cell must be monitored.
To accommodate the large number of batteries required for high-power automotive systems, multiple battery cells are typically divided into several modules and distributed throughout the available space of the vehicle. A typical module contains 10 to 24 cells and can be assembled in different configurations to suit multiple vehicle platforms. Modular design serves as the basis for large battery packs, allowing them to be placed in a larger area, thus making more efficient use of space.
To support distributed modular topologies in the high-EMI environments of electric/hybrid vehicles, robust communication systems are essential. Isolated CAN buses are suitable for module interconnection in such environments. While CAN buses provide a robust network for interconnecting battery modules in automotive applications, they require numerous additional components, increasing cost and board space. Furthermore, wired connections in modern BMSs present significant disadvantages, as routing wires to different modules becomes a thorny issue, adding weight and complexity. Wires are also prone to generating noise, necessitating additional filtering.
Wireless BMS is a novel architecture that eliminates communication cabling. In a wireless BMS, each module is interconnected wirelessly. While wireless connectivity offers advantages such as lower wiring complexity, lighter weight, lower cost, and higher security and reliability for large, multi-cell battery packs, it presents a challenge for wireless communication due to harsh EMI environments and signal propagation barriers created by RF shielding metal.
Embedded wireless networks can improve reliability and accuracy.
ADI's SmartMesh embedded wireless network has been field-proven in industrial Internet of Things (IoT) applications. It achieves redundancy through path and frequency distribution, providing connectivity with a reliability of over 99.999% in industrial, automotive, and other harsh environments.
In addition to improving reliability by creating multiple redundant connection points, wireless mesh networks extend the capabilities of a BMS. SmartMesh wireless networks enable flexible placement of battery modules and improve the calculation of battery SOC and SOH. This is because more data can be collected from sensors installed in locations previously unsuitable for cabling. SmartMesh also provides time-correlated measurements from each node, enabling more accurate data collection.
ADI has also integrated the LTC6811 battery pack monitor with ADI SmartMesh networking technology, a major breakthrough that promises to improve the reliability of large multi-cell battery packs in electric/hybrid vehicles while reducing cost, weight, and wiring complexity.
The LTC6811 is a multi-cell battery pack monitor capable of measuring the voltage of up to 12 cells connected in series with a total measurement error of less than 1.2 mV. Measurements of all 12 cells can be completed within 290 μs, and a lower data acquisition rate can be selected for high noise suppression. The LTC6811 has a battery measurement range of 0 V to 5 V, suitable for most battery chemistry applications, and multiple devices can be connected in series to simultaneously monitor long, high-voltage battery packs. The device includes passive balancing of each cell; data is exchanged across an isolation barrier and compiled by a system controller responsible for calculating SOC, controlling cell balancing, checking SOH, and keeping the entire system within safe limits.
Furthermore, multiple LTC6811 devices can be cascaded, enabling simultaneous monitoring of batteries in long high-voltage battery strings. Each LTC6811 has an isoSPI interface for high-speed, RF-resistant remote communication. When using the LTC6811-1, multiple devices are daisy-chained, and all devices share a single master processor connection. When using the LTC6811-2, multiple devices are connected in parallel to the master processor, allowing for individual addressing of each device.
The LTC6811 can be powered directly from a battery pack or an isolated power supply, and features passive charge balancing for each battery cell, as well as individual PWM duty cycle control for each cell. Other features include a built-in 5V regulator, five general-purpose I/O lines, and a sleep mode (in which current consumption is reduced to 4μA).
Battery balancing to optimize battery capacity and performance
Battery balancing has a significant impact on battery performance because even with precise manufacturing and selection, subtle differences will still exist between cells. Any capacity mismatch between cells will lead to a reduction in the overall capacity of the battery pack. Clearly, the weakest cell in the pack will dominate the overall performance. Battery balancing is a technique that helps overcome this problem by balancing the voltage and state of charge (SOC) among the cells when they are fully charged.
Battery balancing technologies can be divided into passive and active types. In passive balancing, if a battery is overcharged, the excess charge is dissipated into a resistor. Typically, a shunt circuit is used, consisting of a resistor and a power MOSFET acting as a switch. When the battery is overcharged, the MOSFET turns off, dissipating the excess energy into the resistor. The LTC6811 uses a built-in MOSFET to control the charging current of each battery cell, thereby balancing each monitored cell. The built-in MOSFET allows for a compact design and meets a 60 mA current requirement. For higher charging currents, an external MOSFET can be used. The device also provides a timer to adjust the balancing time.
On the other hand, active balancing redistributes excess energy among the other batteries in the module. This allows for energy recovery and generates less heat, but the downside of this technology is that the hardware design is more complex.
Analog Devices (ADI) introduces an architecture using the LT8584 for active battery balancing. This architecture addresses the problems of passive shunt balancers by actively shunting charging current and returning energy to the battery pack. Energy is not lost as heat but is reused to charge the remaining cells in the pack. The architecture also solves the problem of reduced runtime when one or more cells in the battery pack reach a lower safe voltage threshold before the entire pack is depleted. Only active balancing can redistribute charge from strong cells to weak cells. This allows weak cells to continue powering the load, thus extracting a higher percentage of energy from the battery pack. The flyback topology allows charge to travel back and forth between any two points within the battery pack. Most applications return charge to the battery module (12 cells or more), some return charge to the entire battery pack, and others return charge to the auxiliary power rail.
The LT8584 is a monolithic flyback DC/DC converter designed for active balancing of high-voltage battery packs. The high efficiency of the switching regulator significantly increases the achievable balancing current while reducing heat generation. Furthermore, active balancing enables capacity recovery in unbalanced battery packs, a feature unattainable with passive balancing systems. In typical systems, over 99% of total battery capacity can be achieved.
The LT8584 includes an integrated 6A, 50V power switch, reducing the design complexity of application circuitry. The device operates entirely from its discharged battery, eliminating the need for the complex biasing circuitry typically required when using an external power supply. Its enable pin (DIN) is designed for seamless coordination with the LTC680x series battery voltage monitoring ICs. Furthermore, when used with LTC680x series devices, the LT8584 provides system telemetry capabilities, including current and temperature monitoring. When the LT8584 is not in use, it typically draws less than 20nA of total quiescent current from the battery.
Conclusion
Electrification is key to low-emission vehicles, but intelligent management of energy (lithium-ion batteries) is also crucial. Improper management can render battery packs unreliable, significantly reducing vehicle safety. Active and passive battery balancing enables safe and efficient battery management. Distributed battery modules are easy to support and can reliably transmit data to the BMS controller (whether wired or wireless), enabling reliable SOC and SOH calculations. Analog Devices (ADI) offers a comprehensive portfolio of BMS devices to help customers accelerate BMS development and more efficiently manage the operational efficiency and safety of electric vehicle batteries.
