Design Con 2015

Teardown: High-voltage Li-ion battery stack management - the drive for safe power

-July 31, 2012

At the heart of the Chevrolet Volt, a sophisticated battery-stack management system ensures the safety and reliability of the multicell lithium-ion battery stack that delivers power on demand to the Volt drive system.

Within the management system, battery-monitoring boards use two key subsystems to reliably monitor cell health and deliver digital results to a host processor that orchestrates system operation. Separating those subsystems, a signal interface ensures isolation between high-voltage battery-sensing circuitry and communications devices on the boards.

In this teardown, we review the challenges associated with high-voltage Li-ion battery-stack management in automotive applications and discuss how the overall architecture of the Chevy Volt battery-stack management system meets those challenges. In particular, we discuss the requirements for Li-ion cell monitoring and focus on the architecture and components used in the cell-monitoring subsystem, digital-communications subsystem, and isolation interface. We take an in-depth look at parts selected for this design, including a custom ASIC, the Freescale S9S08DZ32, the Avago ACPL-M43T, and the Infineon TLE6250G. Finally, we examine the benefits of this specific solution for mission-critical battery-stack management and consider the trade-offs with possible alternatives available for similar design challenges.
For further information about the role of isolation in automotive battery management systems, we have included a series of three in-depth video interviews.
Part 1 addresses the role of isolation in automotive battery management systems;
Part 2 looks at considerations for parts selection for these applications;
Part 3 examines the use of isolation devices in the Chevy Volt battery management system.

EV Challenges
The Chevrolet Volt is the first production battery EV (electric vehicle), able to run nearly 40 miles solely on batteries. When battery charge reaches its lower limits, a gasoline engine engages to generate additional electricity to extend the vehicle’s range by several hundred miles. At the heart of the vehicle, a lithium-ion battery pack measuring 1.8m in length and weighing 181 kg generates the 16-kWh power needed to turn drive motors, power passenger features, and supply power to a sophisticated battery-management system that rivals avionic systems in its complexity.

IBM senior vice president Robert LeBlanc has noted that with its 10 million lines of code, the Volt’s software content surpasses the 7.5 million lines of code said to fly the US DOD F-35 Lightning II Joint Strike Fighter—a level of software content that itself more than triples the code content of current jet fighters, according to the US Government Accountability Office. While LeBlanc could probably have picked a less controversial system for comparison, the Volt does attract its own share of controversy. Perhaps no other vehicle has faced the same level of scrutiny as the Volt. Indeed, when a Volt test vehicle caught fire after sitting for weeks following a test crash, the incident kicked off a government agency review and a buyback offer from GM—even though no battery-related fires have occurred after "real-world crashes," according to the National Highway Traffic Safety Administration.

Ultimately, the Volt’s success hinges on public acceptance—and its ability to perform. To that end, in designing the Volt, GM worked with IBM to simulate performance of the “system of systems” that power the Volt. Using detailed models of key systems, IBM software verified behavior and even generated key elements of the software code used in the Volt systems. That approach to code generation and systems modeling was crucial for ensuring performance of the Volt battery-management system because of the complex algorithms required to ensure optimal Li-ion cell performance and lifetime; indeed, optimizing such cells remains a highly active focus of research in industry, government, and academia. For the Volt, ensuring battery performance resulted in a final multiboard design (Figure 1) that orchestrates the operation of multiple embedded systems into a single system responsible for meeting the range, safety, performance, and extended-life requirements for the Volt's Li-ion battery pack.

Click for larger image
Figure 1. The Chevy Volt battery management system partitions functions across multiple subsystems implemented in several PCBs. The focus of this teardown is the battery-interface control module—the red, blue, and green boards shown above in the second column from the right. (Courtesy of UBM TechInsights)

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