Recently, media reports have stated that Tsinghua University has achieved a new breakthrough in its research on all-solid-state lithium batteries, reaching the longest cycle life to date.
Moreover, the new technology is expected to fundamentally solve battery safety issues and further improve the energy density and cycle life of lithium batteries.
It is reported that this new technology was developed by the team of Academician Nan Cewen and Professor Shen Yang of Tsinghua University. It uses electrospinning-permeation-hot pressing method to prepare a thin and flexible composite solid electrolyte (CSE) composed of Li6PS5Cl and polar polyvinylidene fluoride-copolytrifluoroethylene (P(VDF-TrFE)) skeleton.
At room temperature, this all-solid-state battery (ASSB) with composite solid electrolytes (CSEs) has an ultra-long cycle life of 92% capacity retention after 1,000 cycles at 1.61C and 71% capacity retention after 20,000 cycles at 1.61C, making it the all-solid-state battery with the longest cycle life at room temperature reported to date.
Finally, to expand on this point, a solid-state battery, simply put, is a battery without gas or liquid inside; all materials exist in a solid form, and a solid electrolyte replaces the separator and electrolyte solution (traditional lithium batteries are made by filling a positive electrode, separator, negative electrode, and then filling in an electrolyte solution).
Its energy density can easily reach about twice that of traditional lithium batteries, and the battery design freedom is increased, making the battery lighter and more compact for the same capacity. This means that electric vehicles with a range of over 1,000 kilometers have become possible.
Furthermore, the solid electrolyte used in solid-state batteries is non-flammable, non-volatile, non-corrosive, and does not leak, even at high temperatures.
A typical internal combustion engine (ICE) car has a 15-gallon fuel tank, equivalent to nearly 500 kilowatt-hours of electricity. 15 gallons of gasoline translates to a range of 375 miles for an ICE car; 500 kilowatt-hours of electricity translates to a range of 1450 miles for an electric car. This significant energy efficiency advantage is what ultimately led to the success of electric vehicles, but the biggest problem facing this generation of electric vehicles is that their battery capacity cannot match the range of ICE cars.
What is the challenge?
Electric vehicle battery packs consist of hundreds of cells operating in series, generating voltages from 400V to 800V. Overcharging and over-discharging can damage the battery or cause premature aging, reducing capacity or lifespan and ultimately leading to battery failure. The primary function of the battery management system (BMS) is to determine and control the state of charge (SOC) and health of each cell in the battery pack. Charging any lithium-ion battery to 100% SOC or discharging it to 0% SOC will reduce its capacity. Determining SOC requires measuring battery voltage and temperature, and the accuracy of these measurements directly determines the quality of SOC management. In short, the electronics of the battery management system are crucial for maximizing the operating range, lifespan, reliability, and safety of electric vehicle battery systems.
Note: Accurately and continuously measuring all the cells in a long, tightly packed string of high-voltage batteries is no easy task. The measurement needs to be unaffected by the high electrical noise generated by inverters, actuators, switches, relays, etc. Due to the high voltage of the battery pack, the electronic components themselves also require electrical isolation. Finally, the electronic components need to operate for years under the influence of wear and tear, climatic conditions, vehicle age, and mileage.
The core of the battery management system
As a leading supplier of integrated circuits (ICs) and solutions, Analog Devices' battery management products focus on several key aspects: individual cell measurement (battery monitors), overall battery pack measurement (battery pack monitors), communication networks for interconnected devices (via wired or wireless networks), and the software that controls these devices. The goal of these electronics is to safely charge all cells to the highest possible capacity, ensuring the entire battery pack achieves maximum storable energy and maximizes vehicle range.
Arguably, the most critical device is the high-voltage battery monitoring IC. This IC measures the voltage and temperature of cells connected in series, typically with each monitor monitoring 12 cells. Battery voltage and temperature are key parameters; measurement accuracy and synchronization are crucial characteristics.
By combining these parameters, the battery management system can operate the battery within its maximum safe operating range without stressing it. Therefore, the performance of these battery monitors is crucial for the battery management system to fully optimize vehicle range, cost, weight, and reliability. Measurement errors can lead to reduced battery management efficiency, and Analog Devices' battery management system products provide accurate measurement capabilities.
Analog Devices' recently launched ADBMS6815 series of precision battery monitors offers an ideal combination of safety, performance, and cost-effectiveness. This series consists of three basic devices, differentiated by the number of cells each monitors: the ADBMS6816 monitors six cells, the ADBMS6817 monitors eight cells in series, and the ADBMS6815 monitors twelve cells in series. These three different cell monitor counts cater to various battery configurations and are suitable for a wide range of battery pack setups.
Solid-state batteries will not catch fire, which is expected to completely solve the battery fire problem that is currently troubling the industry.
However, the development cost of solid-state drives is currently quite high, and it is unlikely that they will be able to be put into large-scale commercial use in the short term.
Figure 1. Simplified explanation of multi-cell monitor
Furthermore, these components can be combined in a mixed and matched manner to form an appropriate number of battery monitoring channels. Because the operating environment includes extreme electrical noise, an adjustable low-pass filter is also included to reduce this noise and ensure high-fidelity measurements.
ADI Battery Management System Communication Technology
The ADBMS6815 series battery monitors employ a daisy-chain interconnect design using an isoSPI® two-wire communication interface. This is a stable, reliable, electromagnetically insensitive, and electrically isolated network that enables synchronous operation, polling, and control of ADI's battery management system devices from the battery management system microcontroller. Therefore, all cells in the battery pack, as well as the pack current and voltage, can be measured synchronously through ADI battery pack monitoring devices. This daisy-chain can operate via a single path to each device or via a dual-path configuration in a loop. The loop allows access to all cell monitoring data even in the event of a wire or connector failure.
The ADBMS6815 series also supports operation in a wireless battery management system (wBMS), where the wired daisy chain is replaced by a 2.4GHz wireless battery management system node for the battery monitor.
Safety
Of all the goals of a battery management system, ensuring the safety of the battery pack is paramount. Identifying and remediating potential faults within integrated circuits requires built-in self-testing capabilities and redundancy. These features include redundant measurement paths, improved synchronization between input signals, self-testing capabilities, and more.
The ADBMS6815 series parts are designed to support the ISO 26262 ASIL-D standard.
Note: ISO 26262 is a widely adopted automotive functional safety standard designed to ensure the safety of automotive electrical equipment and systems throughout their entire lifecycle. ASIL-D is a risk classification within this ISO standard, representing 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 key milestone.
Furthermore, by meeting the ISO 26262 standard, designers can meet other functional safety standards, such as IEC 61508, thereby also meeting the standards for non-automatic applications.
Low-power cell monitoring
In addition to ensuring a stable, predictable, and reliable energy supply to the vehicle, the battery management system must also ensure the safety of the battery cells themselves at all times. While rare, cell defects can shorten battery life over time and lead to thermal runaway, resulting in catastrophic consequences. Therefore, the battery management system needs to monitor for conditions that may indicate any potential problems.
Battery cells do not become inert simply because they are not in use. As electrochemical devices, they change over time, even when at rest. In other words, the battery's state of failure continues to evolve even when the vehicle is not running. To continuously monitor the cells within the battery pack (even when the vehicle is off), Analog Devices (ADI) developed Low Power Battery Monitoring (LPCM) technology. LPCM is an advanced cell monitoring function that automatically checks key parameters of the cells periodically. Through LPCM, the battery monitor alerts the battery management system to wake up and performs appropriate checks if any potential problems are detected. The battery management system also receives an alert if the battery monitor fails to provide a periodic acknowledgment signal.
Flexibility, functionality and cost-effectiveness
The ADBMS6815 series offers an ideal combination of features to meet a wide range of requirements and provides additional benefits to the aforementioned safety, reliability, and performance. These devices use the same package and pinout, allowing designers to build universal designs with different channel numbers (6, 8, or 12 individual cells per device) and to meet the configuration needs of more battery packs or modules through various configuration options. These products also include general-purpose I/O that can operate as digital inputs, digital outputs, or analog inputs. When operating as analog inputs, they can measure any voltage below 5V with the same accuracy as primary cells. Furthermore, these auxiliary measurements, such as temperature or current measurements, can be synchronized with cell measurements for a more accurate state of charge.
Figure 2. Overview of the wired battery management system
Figure 3. wBMS replaces communication lines with radio.
These I/O pins can also control I2C or SPI child devices to achieve more complex functions, such as adding multiplexers to expand analog inputs or EEPROMs to store calibration information. Finally, these products also include battery balancing capabilities, capable of delivering up to 300mA of current to any battery. This achieves system balancing, keeping the state of charge (SOC) of all cells in the battery pack equal. The balancing process can be set for a specific time period and automatically stops when a pre-programmed threshold is reached. This allows for extended balancing even when the cell monitor is in sleep mode.
General characteristics
ADBMS6815 (12-channel);
ADBMS6817 (8 channels);
ADBMS6816 (6-channel);
Supported vehicle safety integrity level: D;
Total measurement error for maximum service life: 1.5mV;
Stackable architecture for high-voltage battery packs;
The voltage measurement of all cells in the system can be completed within 304μs;
A 16-bit ADC with a programmable noise filter;
Passive battery equalization of 300mA per channel with programmable PWM control;
2Mbps electrically isolated serial communication;
Only two wires and a capacitor or transformer are used;
Reversible communication supports ring topology; communication can continue even if there are faults in the communication path.
Seven general-purpose interface pins can be used as analog or digital inputs or digital outputs; supports temperature sensors and can be configured as an I2C or SPI master;
SLEEP mode power supply current: 5.5μA;
48-pin 7mm×7mm LQFP package;
in conclusion
Over the next 30 years, the world will shift from internal combustion engines to electric passenger vehicles. Gasoline, a product from a finite resource, is extremely inefficient, thus inevitably driving this transition. Geopolitical and environmental issues will only accelerate this trend. Electric vehicles are the future, and battery management system technology is a key driving factor.
Leading battery management system products, such as the ADBMS6815 series, are driving the future. These ICs are certified to ISO 26262 ASIL-D standards, offering industry-leading accuracy in cell voltage and temperature measurement. The ADBMS6815 series utilizes road-proven, multi-generational battery monitoring ICs designed to exceed the environmental, reliability, and safety requirements of automotive and industrial applications. They effectively meet the evolving and challenging requirements of electric vehicle fleets and large-scale energy storage systems. Designers can confidently choose Analog Devices (ADI) products, trusting in ADI's market-leading technology to provide today's superior battery management systems and, through continuous innovation, advance the development of cutting-edge systems for the future.
