By hmimcu By Greg Zimmer, Marketing Manager, Battery Management Systems Division, Analog Devices
For decades, the auto industry has been slowly consolidating, while technology and brand differentiation has diminished. The powertrain, the system that converts energy into motion, is arguably the most valuable intellectual property of an automaker, having undergone more than a century of refinement. In this case, the emergence of new car-making forces is remarkable, because it means that powertrain technology is being challenged.
A typical internal combustion engine (ICE) car has a 15-gallon fuel tank, which equates to nearly 500 kWh of electricity. 15 gallons of gasoline translates into 375 miles of range for an internal combustion engine vehicle; 500 kWh of electricity translates into 1,450 miles of range for an electric vehicle. This huge energy efficiency advantage is what makes EVs ultimately victorious, but the biggest problem facing today’s generation of EVs is that their battery capacity doesn’t match the range of an internal-combustion vehicle.
What is the challenge?
The battery pack of an electric vehicle consists of hundreds of cells working in series to generate a voltage of 400 V to 800 V. Overcharging and overdischarging can damage or prematurely age batteries, reducing capacity or longevity, and ultimately leading to battery failure. The main function of the battery management system is to determine and control the state of charge and state of health of each cell that constitutes the battery. Any lithium-ion battery charged to 100% state of charge or discharged to 0% state of charge will reduce its capacity. Determining the state of charge requires measuring battery voltage and temperature, and the accuracy of these measurements directly determines how well the state of charge is managed. In conclusion, the electronics of the battery management system are the key to maximizing the operating range, life, reliability and safety of electric vehicle battery systems.
Note: Accurately and continuously measuring all the cells in a long string of high-voltage batteries that are tightly coupled together is no easy task. Measurements need to be immune to high electrical noise from inverters, actuators, switches, relays, etc. Due to the high voltage of the battery pack, the electronics themselves also require electrical isolation. Finally, electronic components need to operate for years under the influence of factors such as wear, climatic conditions, vehicle age and mileage.
The heart of the battery management system
As a leading provider of integrated circuits (ICs) and solutions, Analog Devices’ battery management products focus on several key areas: single cell measurement (battery monitor), overall pack measurement (battery monitor), interconnect The device’s communication network (either by wire or wireless), and the software that controls those devices. The goal of these electronics is to safely charge all cells to the highest possible capacity, ensuring that the entire battery pack receives the maximum amount of energy that can be stored, maximizing the vehicle’s range.
Arguably the most critical device is the high voltage battery monitor IC. Battery monitor ICs measure the voltage and temperature of cells connected in series, typically 12 cells per monitor. Battery voltage and temperature are key parameters; measurement accuracy and synchronization are key characteristics.
Combining these parameters, the battery management system can operate the battery within the maximum safe operating range without stressing the battery. Therefore, the performance of these battery monitors is critical for battery management systems to fully optimize vehicle range, cost, weight, and reliability. Measurement errors can lead to inefficiencies in battery management, and ADI’s battery management system products provide accurate measurement capabilities.
ADI’s recently introduced ADBMS6815 family of precision battery monitors is an ideal combination of features for safety, performance, and cost-effectiveness. The family consists of three basic devices, differentiated by the number of cells each device monitors: the ADBMS6816 monitors six cells, the ADBMS6817 monitors eight cells in series, and the ADBMS6815 monitors 12 cells in series. Three different numbers of cell monitors are available for different battery configurations and are suitable for a wide range of battery pack configurations.
figure 1. Simplified description of the multi-cell monitor
Additionally, these components can be combined in a mix-and-match fashion to form the right number of battery monitoring channels. Since the operating environment includes extreme electrical noise, adjustable low-pass filtering is also included to reduce this noise and ensure high-fidelity measurements.
ADI battery management system communication technology
The ADBMS6815 family of battery monitors is designed with a daisy-chain interconnect using the isoSPI™ two-wire communication interface. This is a robust, EMI insensitive, electrically isolated network capable of synchronous operation, polling, and control of Analog Devices’ battery management system devices from the battery management system microcontroller. Therefore, through the ADI battery pack monitoring device, all cells in the battery pack as well as the battery pack current and battery pack voltage can be measured synchronously. This daisy chain can operate with one path to each device or with dual paths in a loop configuration. This loop enables access to all cell monitoring data in the event of a wire or connector failure.
The ADBMS6815 family also supports operation in a wireless battery management system (wBMS), where the wired daisy chain is replaced by the battery monitor’s 2.4 GHz wireless battery management system node.
Safety
Of all the goals of a battery management system, ensuring the safety of the battery pack is the most important. Identifying and remediating latent failures within integrated circuits requires built-in self-test capabilities and redundancy. These features include redundant measurement paths, improved synchronization between input signals, self-test capabilities, and more.
ADBMS6815 series parts are designed to support ISO 26262 ASIL-D standard.
NOTE: ISO 26262 is a generally adopted automotive functional safety standard designed to ensure the safety of automotive electrical equipment and systems throughout their life cycle. ASIL-D is a risk classification in this ISO standard and represents the highest level of automotive safety in the system. ADI components are designed and certified to support ASIL-D, ensuring that automakers using ADI components can achieve this critical milestone.
In addition, by meeting the ISO 26262 standard, designers can meet other functional safety standards, such as IEC 61508, and thus also meet the standards for non-automatic applications.
Low power cell monitoring
In addition to ensuring a stable, predictable, and reliable energy source for the vehicle, the battery management system must also ensure that the cells themselves are always safe. Although relatively rare, defects in the cells can cause the battery to shorten its life over time and lead to thermal runaway with catastrophic results. To do this, the battery management system needs to monitor for conditions that could signal any potential problems.
Cells are not inert because they are not in use. As electrochemical devices, they change over time even at rest. In other words, the failure state of the battery continues to develop even when the vehicle is not running. To continuously monitor the cells in the battery pack (even when the vehicle is turned off), Analog Devices has developed Low Power Battery Monitoring (LPCM) technology. LPCM is an advanced cell monitoring feature that automatically checks key cell parameters on a regular basis. Through the LPCM function, the battery monitor will alert the battery management system to wake up and perform appropriate checks if any potential problems are detected. The battery management system can also be alerted if the battery monitor fails to provide a regular acknowledgment signal.
Flexibility, functionality and cost-effectiveness
The ADBMS6815 family provides an ideal combination of functions to meet a wide range of requirements and complement the above mentioned security, reliability and performance. These devices use the same package and pinout, allowing designers to build common designs with different channel counts (each device monitors 6, 8, 12 cells), configured with different options to accommodate more packs or cells The configuration requirements of the module. These products also contain general purpose I/O that can operate as digital input, digital output, or analog input. When operated as analog inputs, they can measure any voltage up to 5 V with the same accuracy as a galvanic cell. Additionally, these auxiliary measurements, such as temperature or current measurements, can be synchronized with cell measurements for a more accurate state of charge.
Calculating these I/O pins can also control I2C or SPI subnode devices for more complex functions such as adding multiplexers to expand analog inputs or EEPROM to store calibration information. Finally, these products also include cell balancing capabilities that can draw up to 300mA on any cell. This achieves system balancing, keeping the states of charge of all cells in the pack equal. The equalization process can be set for a specific time period and automatically stops when a pre-programmed threshold is reached. This enables long-term equalization even when the cell monitor is in sleep mode.
General characteristics
? ADBMS6815 (12 channels)
? ADBMS6817 (8 channels)
? ADBMS6816 (6 channels)
■ Supports Automotive Safety Integrity Level: D
■ Maximum service life total measurement error: 1.5 mV
■ Stackable architecture for high voltage battery packs
■ All cell voltage measurements in the system can be completed within 304 μs
■ 16-bit ADC with programmable noise filter
■ 300 mA passive cell balancing per channel with programmable PWM control
■ 2 Mbps Galvanically Isolated Serial Communication
■ Use only 2 wires and capacitors or transformers
■ Reversible communication supports ring topology; communication is possible even if there is a failure on the communication path
■ 7 general purpose interface pins can be used as analog or digital input or digital output; supports temperature sensor, can be configured as I2C or SPI master
■ SLEEP mode supply current: 5.5 μA
■ 48-pin 7 mm × 7 mm LQFP package
in conclusion
In the next 30 years, the world will shift from internal combustion engines to electric passenger cars. Gasoline is a product from a limited resource, and its use is extremely inefficient, which is bound to drive this shift. Geopolitical and environmental concerns will only accelerate this trend. Electric vehicles are the future, and battery management system technology is a key enabler.
Leading battery management system products, such as the ADBMS6815 family, are driving the future. These ICs are certified to ISO 26262 ASIL-D and provide industry-leading cell voltage and temperature measurement accuracy. The ADBMS6815 family features road-proven, multi-generation battery monitoring ICs designed to exceed the environmental, reliability, and safety requirements of automotive and industrial applications. They can effectively meet the changing and challenging requirements of electric fleets and large-scale energy storage systems. Designers can choose ADI products with confidence that ADI’s market-leading technology can provide today’s outstanding battery management systems, and continue to expand innovation based on the development of future cutting-edge systems.
About Analog Devices
ADI is the world’s leading high-performance analog technology company dedicated to solving the toughest engineering design challenges. With outstanding detection, measurement, power, connection and interpretation technologies, build intelligent bridges between the physical and digital worlds, thereby helping customers re-understand the world around them.
About the Author
Greg Zimmer is Marketing Manager in the Battery Management Systems Group at Analog Devices and has extensive experience in product marketing of a wide variety of high performance signal conditioning ICs. Greg has a background in marketing, technical marketing, applications engineering and analog circuit design. Greg holds a BA in Electrical Engineering/ Computer Science from UC Berkeley and a BA in Economics from UC Santa Cruz.