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2023

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Introduction to Lithium Batteries

Classification:


【概要描述】Since the 1990s, significant progress has been made in the research and production of lithium-ion batteries, and their applications in various fields have become increasingly widespread. In recent years, lithium-ion batteries have also been used by researchers as power sources in electric vehicles, becoming a new trend in the development of electric vehicles. Below, we will first introduce the basic concepts related to batteries, and then introduce the characteristics and applications of lithium-ion batteries in electric vehicles.

Since the 1990s, significant progress has been made in the research and production of lithium-ion batteries, and their applications in various fields have become increasingly widespread. In recent years, lithium-ion batteries have also been used by researchers as power sources in electric vehicles, becoming a new trend in the development of electric vehicles. Below, we will first introduce the basic concepts related to batteries, and then introduce the characteristics and applications of lithium-ion batteries in electric vehicles.

Basic concepts of charging and discharging

Single battery, single battery, and battery pack: Single battery (Cell) refers to a battery with an electromotive force of around 2V (lead-acid), 1.2V (nickel hydrogen), or 3.6V (lithium battery), which is the basic unit that constitutes a single battery; Several individual batteries are packaged to form a single battery, abbreviated as a Battery; A battery pack is composed of several batteries connected in series.

Battery capacity: refers to the amount of electricity that can be obtained from a battery under certain discharge conditions. A battery has theoretical capacity, actual capacity, rated or nominal capacity, and rated reserve capacity. Expressed in Ah (ampere hour) and mAh (milliampere hour).

Theoretical capacity: Theoretical capacity refers to the amount of electricity given by assuming that all active substances participate in the flow forming reaction of the battery. It is calculated based on the mass of the active substance according to Faraday's law. In order to compare the concept of specific capacity commonly used by different series of batteries, that is, the theoretical amount of electricity that can be provided by a unit volume or unit mass battery, often expressed in Ah/Kg or Ah/L.

Actual capacity: Actual capacity refers to the actual discharge capacity of a battery under certain discharge conditions, which is equal to the product of discharge current and discharge time. Its value is less than the theoretical capacity. The calculation method is:

computing method

In the formula, C is the actual capacity, U is the discharge voltage, I is the discharge current, R is the discharge resistance, and T is the time from discharge to termination voltage.

Rated capacity: Rated capacity refers to the lower limit of electricity that a battery should discharge under certain discharge conditions, as stipulated or guaranteed by national or relevant departments when designing and manufacturing batteries. As specified for electric vehicles, the capacity released at a C/3 discharge rate.

Rated reserve capacity: According to the International Electrotechnical Commission (IEC) standard, the capacity of automotive batteries can be expressed in terms of both rated capacity and reserve capacity. China adopts rated capacity. It refers to the discharge time in minutes at a current of 25A, regardless of the size of the battery, until the termination voltage of 1.75V. Specify different discharge times for batteries with different specifications.

The energy of a battery refers to the electrical energy it can provide under certain discharge conditions, usually expressed in Wh.

The power of a battery refers to the amount of energy given by the battery per unit time under certain discharge conditions, in watts or kilowatts. The power that a unit weight battery can provide is called specific power, in units of W/kg or KW/kg

State of Charge (SOC): It is an important parameter that describes the state of charge of a battery. Usually, the state where the battery is charged to a certain temperature and cannot absorb any more energy is understood as a state of charge (SOC) of 100%, while the state where the battery can no longer release energy is understood as a state of charge (SOC) of 0%

SOC

In the formula, Cr is the remaining electricity, and CT is the nominal capacity of the battery, which is the capacity that can be discharged when it is in an ideal state under the specified current and temperature. Qe has consumed electricity. ω I is the power weighting coefficient under different discharge currents and temperatures;

Depth of Discharge (DOD): DOD=Qe/CT, DOD=1-SOC.

Depth of Charge (DOC): The ratio of the potential discharge of the battery to the actual battery capacity.

DOC=(Ct Qe)/Ct

In the formula, Ct is the actual battery capacity, which is related to the discharge current and temperature. The value of DOC is not only related to the current state (SOC, temperature, current, etc.) but also to the future discharge situation of the battery, so DOC can better reflect the actual situation of the battery than SOC.

Charge acceptance: The ratio of the current used for charging reaction to the total charging current during battery charging.

Namely: a=IA/I

A - Charging acceptance capacity

IA - The portion of current used for charging reactions

I - Total charging current

The voltage inflection point of battery discharge: Through battery discharge experiments, it was found that when the voltage of the battery drops to a certain point, the voltage will sharply decrease and the dv/dt value is large, which is called the inflection point. This point indicates that the battery level has run out, and working below the inflection point can cause damage to the battery life. As shown in Figure 1.1. The actual capacity of a battery is the amount of electricity that can be released before the voltage drops to the inflection point.

Discharge rate: Refers to the discharge rate of a battery expressed in terms of discharge time, expressed by the formula: discharge current, battery capacity

Discharge rate (h)=TV capacity/discharge current

Aging: During the initial period of use, the battery capacity increases by approximately 5% to 15%. In the following period of time, the battery capacity will remain basically unchanged. Then it gradually decreases. When the battery capacity decays to 80% of the rated capacity, it can be considered that the battery's lifespan is over.

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Characteristics of New Energy Vehicle Batteries

Lithium batteries are a general term for chemical power sources that use metallic lithium or lithium containing substances as negative electrodes. It is a new type of high specific energy battery system that has been developed in the past decade. The idea of using active metals such as lithium and sodium as negative electrodes for batteries was first proposed by a researcher born in 1958 at the University of California in the United States. In the 1970s, Yasuro Fukuda of Panasonic Electric Company in Japan first invented and applied lithium fluoride carbon batteries. From then on, lithium batteries gradually moved from experimental research to practicality and commercialization. Due to the excellent performance of lithium batteries, various countries are competing to develop various new types of lithium batteries to meet military and consumer needs, such as lithium iodine batteries (1972), lithium chromate batteries (1973), lithium sulfur dioxide batteries (1974), lithium sulfite chloride batteries (1974), lithium copper oxide batteries (1975), lithium manganese dioxide batteries (1976), lithium molybdenum disulfide batteries (1989), lithium-ion batteries (1991), and lithium manganese dioxide batteries (1994). Especially in the early 1990s, the high specific energy and long life lithium-ion batteries invented and launched by Sony Energy Technology in Japan greatly promoted the development of the lithium-ion battery industry.

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