r03 battery.Analysis of lithium battery safety and monitoring technology

　　In addition to their small size and light weight, lithium-ion batteries have a nominal voltage of up to 3.6 volts and a high energy density (meaning that fewer battery cells can be used to obtain the same output voltage). However, from the perspective of safety and to prevent battery performance degradation caused by excessive charging and discharging, it is necessary to install a subsystem (IC) that monitors the voltage and temperature of each battery cell in the battery pack.

　　I. Introduction

　　In recent years, with the development of various hybrid vehicles and electric vehicles, the performance requirements for on-board batteries have become increasingly higher. This is especially true for plug-in hybrid vehicles (PHEV) and electric vehicles (EV): compared with gasoline hybrid vehicles, they have higher requirements for battery capacity, while charging and discharging losses and self-discharge are required to be as small as possible. Therefore, the status of lithium-ion batteries is becoming more and more important.

　　Research reports show that in 2013, the global lithium-ion battery market size was 567 billion yen. By 2018, its scale increased by 163.8%, reaching 928.2 billion yen.

　　In addition to being small in size and light in weight, lithium-ion batteries have a nominal voltage (NominalVoltage) of up to 3.6 volts and a high energy density (meaning that fewer battery cells can be used to obtain the same output voltage). However, from the perspective of safety and to prevent battery performance degradation caused by excessive charging and discharging, it is necessary to install a subsystem (IC) that monitors the voltage and temperature of each battery cell in the battery pack. At the same time, considering that this subsystem may also fail, an independent parallel system is needed to detect the working status of the system.

　　2. Inherent problems of series battery packs

　　When the number of battery cells in a series battery pack increases from tens to hundreds, a problem in the series battery pack becomes prominent, which is the problem of cell balance.

　　Although lithium-ion batteries are industrially mass-produced products, under the current production environment, it is impossible for all battery units to have the same quality. For example, during the manufacturing process, changes in the tension when the battery cell electrodes are wound will affect the degradation rate of the battery cells. On the other hand, it is not required that all battery packs be used in the same environment when used. During use, battery units that are closer to the heat source deteriorate faster, while battery units that are farther from the heat source deteriorate more slowly.

　　The problem that arises from this is that each unit in the battery pack deteriorates at a different rate as the use time changes, resulting in a deviation in the capacity of the battery unit.

　　The overall performance of the battery pack also follows the "barrel principle (short board principle), that is, the capacity of the barrel depends on the shortest piece of all the wooden boards that make up the barrel, and the capacity of the battery pack also depends on the battery unit with the smallest capacity. During the charging process of the battery, once one of the battery cells reaches a fully charged state, the charger will stop charging. The same is true for the discharge process of the battery pack: when a certain battery unit is discharged, the entire battery pack will also Discharge will stop. As a result, the charging capacity of the entire battery pack decreases and the battery cannot fully utilize its capabilities.

　　Let's take a battery pack consisting of 3 battery cells as an example: Suppose one of the battery cells deteriorates faster. When this battery pack is discharged, the battery unit that deteriorates faster will finish discharging before the other two battery units. If discharge continues, the battery unit is in an over-discharge state. When lithium-ion batteries are over-discharged, they may emit smoke and catch fire. In order to prevent accidents, the discharge can only be stopped at this time, which means that the remaining power in the remaining two battery units cannot be used.

　　On the contrary, when the battery pack starts to charge, the two battery units that deteriorate more slowly are fully charged first; while the battery unit that deteriorates faster is not fully charged at this time. At this time, if charging is continued based on the battery unit that deteriorates faster, the two fully charged battery units that deteriorate more slowly will be in an overcharged state. Overcharging can also cause the battery to burn or explode. Similarly, in order to prevent accidents, the battery pack will end charging when the rapidly deteriorating battery cells are not fully charged.

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