BMS: Optimizing Electric Vehicle Charging, Discharging, and Thermal Management for Enhanced Safety and Efficiency

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The Battery Management System (BMS) holds a pivotal position in the world of electric vehicles, transcending its conventional role of overseeing charging and discharging processes. Its significance is further emphasized by its crucial involvement in thermal management. This article delves into the nuanced responsibilities of BMS in battery management, stressing the importance of heightened monitoring in charging, discharging, and thermal operations. This meticulous scrutiny aims to elevate the safety and efficiency of electric vehicles while maximizing battery utilization.

In the ever-expanding realm of electric mobility, as electric vehicles become more commonplace, BMS emerges as the vigilant guardian and manager of the battery. Its impact on the stability and functionality of the entire electric vehicle ecosystem is profound. BMS achieves an advanced level of safety and efficiency by not only monitoring the battery's charge, discharge, and thermal management status but also by integrating custom BMS solutions tailored to specific needs. This adaptability ensures a more personalized and optimized approach to battery management, meeting the unique demands of diverse electric vehicle applications. Its multifaceted role extends beyond conventional monitoring, actively contributing to the overall health and performance of electric vehicles, with custom BMS solutions adding a layer of tailored precision to the mix.


Larger capacity, higher voltage, and longer battery life required for EVs and ESS

Batteries, once confined to toys and flashlights, have become an indispensable power source for portable devices like laptops and smartphones. However, with the gradual electrification of machines and devices that historically burned fossil fuels, coupled with the increasing adoption of renewable energy, new applications for batteries are rapidly expanding. There is a growing demand for batteries that offer high performance, reliability, and safety, particularly in applications like electric vehicles (EVs), electric ships, and aircraft, where high-power engines are electrified. Advanced batteries are required to meet high-level criteria, including achieving large capacity for longer continuous usage, high input/output for fast charging/discharging across a range of power levels, long cycle life to withstand repeated charging and discharging without degradation, and high safety under various conditions such as temperature variations, vibrations, and impacts.
 

However, even for batteries used in novel applications like electric vehicles, the basic structure and materials used are not significantly different from traditional batteries used in smartphones. From various perspectives, such as capacity, power, and lifespan, the most commonly used battery – the lithium-ion rechargeable battery – has been in use without major improvements.

Each cell (the smallest unit of a battery) in a lithium-ion rechargeable battery operates at a working voltage of about 4V when fully charged and around 2V after discharge. This working voltage is consistent with lithium-ion batteries used in smartphones. Moreover, the capacity achieved by each cell in batteries installed in newly introduced electric cars is approximately 26Ah. While the batteries for electric vehicles are somewhat larger compared to those in smartphones, they are still relatively small when considering their role in powering heavy machinery like electric cars.


In reality, electric vehicle motors operate on a high-voltage power supply ranging from 400V to 800V. To achieve practical driving range, a large battery capacity exceeding 50kWh is required. The battery specifications for electric vehicles are achieved by combining over 1000 cells in series and parallel configurations. A high-voltage, large-capacity battery composed of a certain number of cells combined is called a module, and combining multiple modules forms a battery pack.

To adopt the approach of combining small cells into larger batteries, a challenge must be addressed. Generally, individual cells have variations in capacity and characteristics due to differences in materials and manufacturing. Additionally, as repeated charging and discharging occur, there are individual differences in their ability to withstand environmental stresses from charge and discharge cycles, leading to an increasing individual difference between cells. These individual differences significantly impact the overall lifespan and output characteristics of modules and battery packs composed of many cells. This is because the characteristics of modules and battery packs are determined by the cells with the poorest performance and stress resistance. Generally, the surrounding environmental temperature, voltage, and current during charging and discharging fluctuate (referred to as "stress intensity"). Therefore, cells with lower resistance to stress are more prone to degradation. In particular, if insufficient capacity or power loss occurs due to factors like overcharging, overheating, internal short circuits, it may lead to uncontrollable or inoperable vehicles and even accidents.

In the context of the increasing importance of batteries in our daily lives and the evolving landscape of electric transportation, addressing these challenges becomes crucial to ensure the safety, reliability, and performance of battery-powered systems.

What is the critical system "BMS" for efficient battery utilization?

Against this backdrop, in order to ensure the long-term performance and safe operation of battery systems composed of multiple cells, it is essential to establish a highly precise environment capable of minimizing the degradation of each individual cell. To achieve this goal, a control system tasked with closely monitoring and controlling the actions and states of each cell has been developed, known as the Battery Management System (BMS).

Within the BMS, there is a continuous and high-precision monitoring of the actions and states of each cell. This involves the use of sensors to monitor parameters such as voltage, current, temperature, and leakage. Additionally, control over the charging and discharging processes is maintained to ensure balance, aiming to compensate for slight mismatches and imbalances between cells and modules. This process is designed to improve the lifespan and performance of modules and battery packs, ensuring safety. Through software control within a microcomputer, the BMS compares battery specifications and design criteria with collected data. It executes multiple control tasks, including prevention of overcharging and overdischarging, prevention of hazardous overcurrent during charging and discharging, achieving safe and stable temperature management, calculating State of Charge (SOC), and performing cell voltage equalization (known as battery balancing) to enhance range and lifespan.

One of the crucial functions of the BMS is cell balancing technology, which can be approached in two ways. The passive method forces high-voltage cells to discharge by using discharge switches, converting the capacitance difference between cells into heat to achieve voltage equalization. The active method, on the other hand, balances the charging states of adjacent cells by allowing a current to flow between them. The active method is often more effective in maximizing the potential of the battery.

The performance of the BMS relies on the diversity and precision of its built-in control functions. However, to achieve high performance, it is imperative to use sensors and electronic components with high precision in the BMS circuit. Moreover, due to the necessity of monitoring a large number of cells, the BMS circuit itself becomes highly complex, requiring smaller and lighter sensors and components. This complexity and precision in design are essential to ensure that the BMS achieves optimal effectiveness in battery management.

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