Behind the strengthening of electric vehicle batteries, these advanced technologies are indispensable

For decades, the automotive industry has been slowly consolidating, while technology and brand differentiation have been decreasing. The powertrain is the system that converts energy into motion. It can be said that it is the most valuable intellectual property of automakers and has been perfected for more than a century. In this context, the emergence of new car manufacturers is remarkable because it means that powertrain technology is being challenged.

A typical internal combustion engine (ICE) car has a 15-gallon tank, which is equivalent to nearly 500 kWh of electricity. 15 gallons of gas translates to 375 miles of range for an ICE car; 500 kWh translates to 1,450 miles of range for an EV. This huge efficiency advantage is why EVs will eventually win, but the biggest problem facing today's generation of EVs is that their battery capacity can't match the range of ICE cars.

What is the challenge?

The battery pack of an electric vehicle consists of hundreds of cells working in series, producing voltages of 400 V to 800 V. Overcharging and overdischarging can damage or prematurely age the battery, reducing capacity or life, and ultimately leading to battery failure. The main function of a battery management system is to determine and control the state of charge and health of each cell that makes up the battery pack. Charging any lithium-ion battery to 100% state of charge or discharging it to 0% state of charge will reduce its capacity. Determining the state of charge requires measuring the cell voltage and temperature, and the accuracy of these measurements directly determines how well the state of charge management is done. In summary, the electronics of the battery management system are key to maximizing the operating range, life, reliability, and safety of the battery system in an electric vehicle.

Note: Accurately and continuously measuring all cells in a long, tightly packed high voltage battery string is no mean feat. The measurements need to be immune to high electrical noise from inverters, actuators, switches, relays, etc. The electronics themselves also need to be electrically isolated due to the high voltages of the battery pack. Finally, the electronics need to operate for years despite the effects of wear, weather conditions, vehicle age, and mileage.

The heart of the battery management system

As a leading supplier of integrated circuits (ICs) and solutions, ADI's battery management products focus on several key aspects: individual cell measurements (battery monitors), overall battery pack measurements (battery pack monitors), communication networks that interconnect devices (via wires or wireless networks), and software that controls these devices. The goal of these electronic devices is to allow all cells to be safely charged to the highest possible capacity, ensuring that the entire battery pack obtains the maximum storable energy and fully improves the vehicle's range.

Arguably, the most critical equipment 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. Cell voltage and temperature are key parameters; measurement accuracy and synchronization are key characteristics.

Combining these parameters allows the battery management system to operate the battery within the maximum safe operating range without stressing the battery. Therefore, the performance of these battery monitors is critical for the battery management system to fully optimize vehicle range, cost, weight, and reliability. Measurement errors can lead to inefficient battery management, and ADI's battery management system products provide accurate measurement capabilities.

The ADBMS6815 series of precision battery monitors recently launched by Analog Devices is an ideal combination of functions for achieving safety, performance and cost-effectiveness. The series consists of three basic devices, differentiated by the number of batteries monitored by each device: ADBMS6816 monitors six cells, ADBMS6817 monitors eight cells in series, and ADBMS6815 monitors 12 cells in series. The three different numbers of cell monitors can meet different battery configurations and are suitable for a wide range of battery pack configurations.

Figure 1. Simplified illustration of a multi-cell monitor.

Furthermore, these components can be mixed and matched to create the right number of battery monitoring channels. Because the operating environment includes extreme electrical noise, adjustable low-pass filtering is included to reduce this noise and ensure high-fidelity measurements.

ADI Battery Management System Communication Technology

The ADBMS6815 series battery monitors are designed with daisy-chain interconnection using the isoSPI™ two-wire communication interface. This is a stable, reliable, electromagnetically insensitive, electrically isolated network that can synchronize, poll, and control ADI's battery management system devices from the battery management system microcontroller. Therefore, all cells in the battery pack as well as the battery pack current and battery pack voltage can be measured synchronously through the ADI battery pack monitoring device. This daisy chain can be operated with one path to each device or with dual paths in a loop configuration. The loop supports 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 a 2.4 GHz 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 the most important. Identifying and remediating potential faults within the integrated circuit requires built-in self-test capabilities and redundancy. These features include redundant measurement paths, improved synchronization between input signals, self-test capabilities, and more.

The ADBMS6815 series parts are designed to support ISO 26262 ASIL-D standards.

Note: ISO 26262 is a commonly adopted automotive functional safety standard designed to ensure the safety of automotive electrical devices and systems throughout their lifecycle. ASIL-D is a risk classification in this ISO standard that represents the highest automotive safety level in a system. ADI parts are designed and certified to support ASIL-D, ensuring that automotive manufacturers using ADI parts can achieve this critical 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-autonomous applications.

Low power cell monitoring

In addition to ensuring a steady, predictable, and reliable source of energy for the vehicle, the battery management system must also ensure that the battery cells themselves are always safe. Although rare, defects in the battery cells can cause the battery to lose life over time and lead to thermal runaway, with catastrophic consequences. To do this, the battery management system needs to monitor conditions that could indicate any potential problems.

Battery cells do not become inert because they are not used. As electrochemical devices, they change over time even when they are at rest. In other words, the battery’s state of failure continues to develop even when the vehicle is not in operation. To continuously monitor the cells within the battery pack (even when the vehicle is off), Analog Devices has developed low-power cell monitoring (LPCM) technology. LPCM is an advanced cell monitoring function that automatically checks the key parameters of the battery cells at regular intervals. With the LPCM function, the battery monitor alerts the battery management system to wake up and perform appropriate checks if any potential problems are detected. The battery management system is also alerted if the battery monitor fails to provide regular confirmation signals.

Flexibility, functionality and cost-effectiveness

The ADBMS6815 series offers an ideal combination of features to meet a wide range of requirements and provide some complementary effects on the above safety, reliability and performance. These devices use the same package and pins, allowing designers to build universal designs with different channel counts (each device monitors 6, 8, and 12 single cells), and meet the configuration requirements of more battery packs or battery modules through different option configurations. These products also contain general-purpose I/Os that can operate as digital inputs, digital outputs, or analog inputs. When operating as analog inputs, they can measure any voltage below 5 V with the same measurement accuracy as the original battery. In addition, these auxiliary measurements, such as temperature or current measurements, can be synchronized with cell measurements to obtain a more accurate state of charge.

Figure 2. Overview of a wired battery management system

Figure 3. wBMS replaces communication lines with radio

These I/O pins can also control I2C or SPI daughter node devices to implement 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, which can discharge up to 300mA of current on any battery. This enables system balancing to keep the state of charge 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 long-term balancing even when the cell monitor is in sleep mode.

General Features

►ADBMS6815 (12 channels)

►ADBMS6817 (8 channels)

►ADBMS6816 (6 channels)

■Support 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