lithium ion batteries of bms

Exploring the BMS Core Technology Behind Electric Vehicle Batteries

Electric vehicles are a major trend of the future, and battery management system technology is a key enabler.

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) vehicle 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.

1. What is the challenge of electric vehicle batteries?

The battery pack of an electric vehicle consists of hundreds of cells working in series to generate a voltage of 400V-800V. Overcharging and over-discharging 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 makes up the battery pack. 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.

2. 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 areas: single cell measurement (battery monitor), overall pack measurement (battery monitor), interconnection device communication network (via wire or wireless network), and software to control these 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.

3. ADI battery management system communication technology

The ADBMS6815 family of battery monitors is designed with a daisy chain to 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.

tesla BMS charge

  • 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.

The ADBMS6815 series of parts are designed to support the 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.

  • 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.

4. 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 meet the configuration needs of more battery packs or battery modules. These products also contain general purpose I/O that can operate as digital input, digital output or analog input. When operating as an analog input, they can measure any voltage up to 5V with the same accuracy as a galvanic cell.

In addition, these auxiliary measurements, calculations of these I/O pins can also control I2C or SPI sub-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 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.

In conclusion

In the next 30 years, the world will shift from internal combustion engines to electric passenger cars. Gasoline, as a finite resource, is extremely inefficient in its use and is bound to drive this shift. Geopolitical and environmental concerns will also 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 offer 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; effectively meeting the changing and challenging requirements of electric fleets and large-scale energy storage systems.

Measurements such as temperature or current can be synchronized with cell measurements for a more accurate state of charge.

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