High-voltage lithium-ion battery pack management - ensuring safe power supply

At the heart of the Chevrolet Volt sedan is a sophisticated power pack management system that ensures the safety and reliability of the multi-cell lithium-ion battery pack that provides power to the Volt drivetrain.

The battery monitoring board within this management system uses two key subsystems to reliably monitor battery health and provide digital results to the main processor, which then coordinates the overall operation of the system. Separating the two subsystems reveals a signal interface that ensures good isolation between the high-voltage battery detection circuitry and on-board communication devices.

In this teardown report, we review the challenges associated with high-voltage Li-ion battery pack management in automotive applications and discuss how the overall architecture of the Chevrolet Volt battery pack management system meets these challenges. In particular, we discuss the requirements for Li-ion battery monitoring, focusing on the architecture and components used in the battery monitoring subsystem, digital communication subsystem, and isolation interface. We also take a detailed look at the parts selected for this design, including a custom ASIC, Freescale's S9S08DZ32, Avago's ACPL-M43T, and Infineon's TLE6250G. Finally, we discuss the advantages of this specific solution for mission-critical battery pack management and weigh possible alternatives that can meet similar design challenges.

To provide more information on the role of isolation in automotive battery management systems, we also offer a series of three in-depth video interviews.

Part 1: Introduction to the role of isolation in automotive battery management systems;

Part 2: Discusses some considerations when selecting devices for these applications.

Part 3: Exploring the use of isolation devices in the Chevrolet Volt battery management system


Electric vehicle challenges

The Chevrolet Volt is one of the first production battery-powered electric vehicles (EVs) that can travel nearly 40 miles on batteries alone. When the battery reaches low limits, the gasoline engine can be activated to generate additional electricity, extending the car's range to several hundred miles. At the heart of the Volt is a lithium-ion battery pack that is 1.8 meters long and weighs 181 kilograms. It produces 16 kWh of power, enough to start the drive motors, power passenger devices, and power the complex battery management system, which is comparable in complexity to an aircraft system.

Robert LeBlanc, IBM senior vice president, noted that the Volt's software content, at 10 million lines of code, exceeds the 7.5 million lines of code said to fly the U.S. DOD F-35 Lightning II Joint Strike Fighter—which itself is more than three times the size of current jet fighter code, according to the U.S. Government Accountability Office. While LeBlanc may have picked a less controversial system for comparison, the Volt has certainly generated its own share of controversy. Perhaps no other car has received the attention that the Volt has. In fact, when a Volt test vehicle caught fire in a test crash after being parked for weeks, the incident immediately attracted the attention of government agencies and prompted a buyback from GM—even though there were no battery-related fires following "actual crash events," according to the National Highway Traffic Safety Administration.

Ultimately, the Volt's success depends on public acceptance—and its functionality. To that end, while designing the Volt, GM worked with IBM to simulate the performance of the "system of systems" in the Volt. Using detailed models of key systems, IBM software not only verified the behavior but even generated key portions of the software code used in the Volt's systems. This approach to code generation and system modeling was critical to ensuring the performance of the Volt's battery management system, as ensuring optimal lithium-ion battery performance and life requires complex algorithms; in fact, optimizing the performance of such batteries remains a topic of intense research interest in industry, government, and academia. For the Volt, ensuring battery performance enabled the final multi-board design (Figure 1) to integrate the work of multiple embedded systems into a single, complete system that meets the range, safety, performance, and extended life requirements of the Volt's lithium-ion battery pack.

Figure 1: The Chevrolet Volt battery management system divides all functions into multiple subsystems implemented on multiple PCBs. The focus of this teardown is the battery interface control module - the red, blue and green boards in the second column from the right in the figure above. (Courtesy of UBM TechInsights)

Lithium-ion battery characteristics

The complex system required to meet the performance, safety and reliability requirements of the Volt is directly related to the characteristics of lithium-ion batteries. When a lithium-ion battery is discharged, the lithium is ionized in the (typically) graphite anode, and the lithium ions enter the electrolyte and pass through the separator to the cathode, thereby generating charge flow. The charging process is the opposite, with lithium ions flowing from the cathode into the electrolyte and through the separator back to the anode.

The performance and reliability of this chemical process depends on the temperature and voltage of the battery. At low temperatures, the chemical reaction is slow, thus reducing the battery voltage. As the temperature rises, the reaction speeds up until the lithium-ion battery components begin to decompose. When the temperature is above 100°C, the electrolyte begins to decompose and release gases, which will cause pressure to accumulate in the battery without a pressure relief mechanism. At high enough temperatures, lithium-ion batteries may experience thermal runaway, accompanied by the decomposition of oxides and the release of oxygen, which further accelerates the temperature rise.

Keeping lithium-ion batteries in optimal working condition is therefore a key requirement for the Volt's battery management system. The problem for Volt engineers is to ensure reliable data collection and analysis in order to properly monitor and control the state of lithium-ion batteries in the car - a problem made more severe by the characteristics of lithium-ion batteries themselves.

A characteristic of our lithium-ion battery technology is that it can maintain a nearly flat voltage output in the middle of its capacity range for a given temperature and output current value (Figure 2). While this characteristic enhances the advantages of lithium-ion batteries as an energy source, it also complicates engineers' attempts to provide users with a means of maintaining the battery's power level or state of charge (SOC) using simple cell voltage measurements. For Volt drivers, accurate SOC measurement is the key to accurately estimating the vehicle's remaining driving range. In fact, in the emerging electric vehicle market, "range anxiety" is a key factor hindering the adoption and sales of electric vehicles, so accurately describing SOC is very important.

Figure 2: At a given temperature and discharge current value, a lithium-ion battery like the Panasonic CGR18650CG has a nearly flat output voltage in the middle of the discharge range. This is an advantage for energy sources, but adds design complexity for engineers who need to accurately measure state of charge (SOC).

In addition, keeping the SOC within a specific range is also important for extending battery life. A battery with a state of charge that is too low or too high will degrade faster than if it is kept in the middle, and this specific range is generally determined based on experience. If allowed to discharge completely, the performance of the lithium-ion battery components will begin to deteriorate and cause permanent damage. If a lithium-ion battery is allowed to charge above the recommended upper voltage limit, the battery may overheat or suffer permanent deformation of the structure.

In the Volt, GM engineers established a safe SOC window of 58% to 65%, which can be adjusted according to driving mode. In normal driving mode, the lower limit can be set to 30% SOC, and in "mountain driving" mode, the lower limit can be set to a higher 45% to ensure that there is enough power to go uphill and extend driving time. When the Volt reaches the appropriate SOC lower limit, the car's gasoline engine will be activated to extend the driving distance.

Estimated state of charge

Because state-of-charge (SOC) measurements of lithium-ion batteries are not very reliable, engineers can only perform SOC estimation, typically using current-based or voltage-based methods.

Current-based methods provide the most accurate results. Such methods track the change in charge, essentially counting the number of coulombs added to the battery during charging or subtracted during the discharge cycle, and then determine the SOC of the battery relative to the fully charged state. However, self-discharge losses or inefficiencies in the battery itself can make the "coulomb counting" method erroneous. In addition, because continuous monitoring is impractical for many applications, the coulomb counting method requires the use of a sampling method. In automotive applications, this method must be fast enough and automatically track the rapid discharge associated with acceleration and the rapid charging associated with regenerative braking.

Voltage-based methods use the battery's transient voltage output as the basis for further calculations to estimate SOC, accounting for variations in battery temperature, aging, current output, and discharge rate. When used with accurate characterization data for single-cell lithium-ion batteries under a variety of operating conditions, the voltage method can provide accurate SOC estimates. For production vehicles like the Volt, maintenance requires accurate battery characterization and specific tools and procedures that enable the battery management system to learn the capacity of new battery modules—or relearn the battery capacity when necessary.

Lithium-ion battery chemistry

Lithium-ion batteries include a variety of chemistries, each with different characteristics in terms of energy density, efficiency, durability and nominal cell voltage. The battery manufactured by LG Chem for the Volt uses the company's manganese spinel cathode lithium-ion chemistry and a proprietary safety-enhanced separator - a ceramic-coated semipermeable membrane. Across the industry, lithium-ion batteries are manufactured in a variety of forms, including the familiar cylindrical form; the flat pack used in mobile phones; and the hard plastic prismatic form. The original LG Chem battery used in the Volt uses a prismatic form.

As described by analysts at UBM TechInsights and Munro & Associates, the entire Chevrolet Volt battery pack consists of 288 prismatic lithium-ion cells, which are packaged into 96 battery cell groups, ultimately providing the 386.6V DC system voltage measured by the analysts. These battery cell groups are also combined with temperature sensors and cooling units to form four main battery modules. The voltage sense wires connecting each battery pack are terminated at the connector on the top of each battery module, and the voltage sense harness connects the connector to the battery interface module located on the top of each battery module. There are four color-coded battery interface modules that operate at different locations in the battery pack, corresponding to the low, medium and high voltage ranges of the DC voltage offset of the four module groups.

Data from the battery interface module is transmitted upward to the battery energy control module. This control module then transmits fault conditions, status and diagnostic information to the hybrid transmission control module, which acts as the master controller to complete vehicle-level diagnostics. At any time, the entire system will run more than 500 diagnostics every 0.1 seconds. 85% of the diagnostics are focused on battery pack safety, and the remaining diagnostics are used for battery performance and life.


Multilayer Circuit Board

Subsequent analysis of battery performance begins with a focused teardown of the battery interface control module (Figure 3). This module uses a four-layer PCB with most of the components mounted on the top layer, along with the orange battery connector and black data communication connector. The top layer has a ground plane and some signal traces, some of which are connected to the layers below through multiple vias. In layer 2, power and ground planes are laid under the high-voltage areas of the PCB. Layer 3 contains signal traces that pass under these areas. The other side of the PCB, layer 4, is used for the ground plane and signal traces, and contains a few auxiliary components.

Figure 3: There are four battery interface control module PCBs in the Chevrolet Volt, each of which integrates multiple detection circuits and CAN communication circuits and is isolated by optocouplers located at the edge of the communication subsystem. (Courtesy of UBM TechInsights)

The black ATLPB-21-2AK PCB mount connector carries the 5V reference, low voltage reference, signal ground, CAN bus high speed serial data, CAN bus low speed serial data, and high voltage fault signal. The orange battery connector carries the battery module temperature signal, low voltage reference, and voltage sense line from the battery cell group.

Detection subsystem

The core of the battery interface control system is a complex sensing subsystem - a complete embedded system circuit that monitors the output voltage of each lithium-ion battery pack and the temperature of the battery pack. The battery voltage passes through the battery connector to the L9763, an ASIC jointly developed by STMicroelectronics and LG Chem.

The L9763 ASIC can monitor up to 10 individual Li-ion battery packs, with cell-load-current monitoring through on-chip current-sense amplifiers and cell voltage monitoring through on-chip analog multiplexers and sample-and-hold circuits (Figure 4). The device's differential inputs ensure millivolt-accurate measurements under large offset voltage conditions, depending on the location of the battery cells in the battery pack. In addition, PCB designers can use a combination of trace layout techniques, isolation techniques, and the ground planes mentioned above to ensure signal integrity in this challenging environment.

Figure 4: The L9763 ASIC includes on-chip circuitry for measuring the voltage and current of a Volt battery pack and balancing the charge in those cells using passive resistor cell balancing techniques. (Courtesy of STMicroelectronics)

Based on these measurements, the L9763's on-chip circuitry switches individual battery groups to external resistor networks to selectively discharge the cells, reducing stress caused by large voltage differences. This simple passive technique provides a simple, low-cost solution for cell balancing, but loses efficiency because energy is lost as heat on the discharge resistors (Figure 5). An alternative cell balancing technique uses an active method to store the charge from the highest voltage cell and redistribute it to the lowest cells. This technique requires switching between each cell sequentially and using capacitors, inductors, or transformers to store or redistribute the charge. Although active methods have the advantage of saving energy over passive methods, they increase system cost and complexity.

Figure 5: Passive cell balancing (left) switches high-voltage cells to discharge resistors; active cell balancing can sequentially accumulate charge on capacitors (right) or inductors, or use transformers to distribute charge to lower-voltage cells. (Courtesy of STMicroelectronics)

To charge or discharge a multi-cell Li-ion battery pack, designs typically use a constant current or constant voltage approach, where the charging system uses a pair of MOSFETs to reduce the charge current when the desired charge voltage is reached, or to increase the current during a discharge operation. The L9763 provides a charge pump to drive the power MOSFET device. The L9763 transmits the measured data of the monitored Li-ion battery to Freescale's S9S08DZ32 MCU via an SPI interface. The L9763 also provides a 5V LDO output to the MCU. For overall battery management functions, the individual L9763 devices are linked via an on-chip interface and are individually addressed by the main control unit via vertical daisy-chain communication.

Detection Circuit MCU

As mentioned above, SOC estimation of lithium-ion batteries is a complex task that requires sufficient processing power. In this design, each detection subsystem has an L9763 ASIC and a Freescale S9S08DZ32 40-MHz HCS08 MCU, which integrates 32kB flash, 2kB RAM, and 1kB E2PROM. An external 4MHz oscillator provides the reference frequency for the MCU clock operation.

In the GM-LG Chem design, the MCU is required to perform the operations required to estimate the SOC based on the voltage and current measurement data provided by the L9763. Although the SOC algorithms are proprietary, the hardware configuration and maintenance procedures suggest that these estimation algorithms can combine voltage-driven estimates using stored battery characterization data with more direct charge measurements for temporary recalibration during the charging process. The use of a detailed system modeling environment described by IBM provides an ideal platform to help find the right data set for optimizing the SOC calculation and to validate the method under a wide range of sampled operating conditions.

The HCS08's safety features, such as a computer-healthy watchdog timer, help ensure reliable operation and automatically generate a reset signal in the event of an unrecoverable application software failure. Of particular importance in this application is the S9S08DZ32's sophisticated on-chip CAN controller, which can be selectively powered down or put into sleep mode when not in use (Figure 6). To help ensure predictable real-time performance, the on-chip controller integrates five receive buffers organized into a FIFO buffer, and three transmit buffers that allow outgoing messages to be prioritized.

Figure 6: The on-chip CAN controller is a key factor in choosing the Freescale S9S08DZ32 MCU to build the battery interface control module detection subsystem. (Courtesy of Fairchild Semiconductor)

Signal Isolation

In the Chevrolet Volt's system of systems, communication and control are the basis for the operation of the vehicle, and the Volt provides multiple networks to isolate and protect each subsystem. The above complex algorithms need to manage each lithium-ion battery pack and monitor the battery pack within each detection subsystem on the specific battery interface control module. However, the key data required for the final overall battery management is contained in the CAN bus signal interface and high-voltage fault signal. At the same time, system safety and reliability depend on the safe isolation of the CAN bus network and the high-voltage detection circuit. Although isolation can be achieved with a variety of methods and components, the harsh environment and multiple safety regulations make optocouplers the preferred solution for such applications.

Next-generation systems

The Chevrolet Volt is certainly one of the most complex distributed embedded system applications put into production in the commercial market, and its design is at the forefront of many areas. Among the most important systems that will affect the success of the Volt and the popularity of the electric vehicle market, the vehicle's lithium-ion battery and related battery management system illustrate the increasing importance of software and circuits in automotive applications. According to a recently released McKinsey market research report, by 2025, emerging lithium-ion technologies have the potential to increase battery capacity by 80% to 110%, while prices will drop accordingly, making the total cost of ownership of electric vehicles competitive with traditional vehicles powered by internal combustion engines. For engineers, the challenge remains to explore the full potential of emerging lithium-ion battery systems in the face of higher DC voltages, battery capacities, data rates, and consumer expectations.