For a period of time, as long as the smog is over, the "fine particles" force the family to dare to open the window, the Beijing citizens will inevitably start to eagerly support the government to support new energy vehicles. . What's more, electric cars have become the "hot words" of the moment, but electric vehicles must use lithium-powered batteries to provide power, and the country may still know very little. In fact, this is not surprising. People are more concerned about when new energy vehicles can be truly commercialized and run on the road. It seems that the concern of using what energy to power is still a "slow fever" process.

First, start with the strange circle of new energy vehicle development

Indeed, new energy vehicles carry too much low-carbon effects, save energy and reduce emissions, and improve environmental burdens. In particular, the most valuable "stars" in alternative energy sources have emerged on the international stage, attracting the attention of Chinese people. .

This topic will also start from the successful hosting of the Olympic Games in Beijing in 2008. The wonderful appearance of 50 electric buses in the Olympic Games has greatly inspired the "electric car dream" of the Chinese people. The Chinese people talked about the "same starting line" and "curve overtaking" of new energy vehicles for the experts; so that the national policy supports the "Ten Cities and Thousand Vehicles" and other demonstration projects; or the State Grid is the new energy vehicle operation and the construction of large-scale civil horses The preemptive construction of the predatory charging and replacement stations has conveyed to the people a very clear and gratifying signal to varying degrees, and the new energy era is really coming.

But reality often tends to tease people, and the scene that people in the end want to see is the most unwilling to see. According to the incomplete statistics of the official website of the China Automobile Association, the production and sales volume of new energy vehicles in 2011 were 8,368 and 8,159 respectively; in 2012, the production and sales volume of new energy vehicles were 12,552 and 12,791 respectively; However, it is planned from the "Energy Conservation and New Energy Vehicle Industry Development Plan (2012-2020)" issued by the State Council on July 9, 2012. "To 2015, the cumulative production and sales of pure electric vehicles and plug-in hybrid vehicles will strive to reach 500,000 units; by 2020, the production capacity of pure electric vehicles and plug-in hybrid vehicles will reach 2 million units, and the cumulative production and sales volume will exceed 5 million units. The era of new energy vehicles is not as glamorous as a girl’s imaginary arrival. The large number of charging stations and charging piles that were built in advance seem to have no “cars”, and a large number of investors have flocked from the initial stage. Up to now, the "new door" of the national and local aircraft carrier-class new energy automobile industry alliance has sprung up, and a cold snap has been continually impacting the pace of new energy vehicles. The climax of such a new energy vehicle with the meaning of "Great Leap Forward" has to rise and fall, and it must not give people too much vigilance.

Second, the PK of electric vehicles and fuel vehicles

The marketization process of new energy vehicles is bumpy, and where is the problem? People are eager to pursue the answer. Suspense seems to start from the "new energy" defined by the new energy vehicles. In fact, the new energy sources for vehicles mentioned by people are mainly related to the "power battery" used in electric vehicles. Related "BMS".

Speaking of power batteries, people are no strangers. As early as 1842, the world’s first electric vehicle powered by non-rechargeable batteries was born in the world. It came out even before the oil derivatives – gasoline fuel. Powered gasoline car (1887). The arrival of the oil era did not bring too much development opportunity for the growth of electric vehicles. At that time, the electric vehicles were lost to the gasoline vehicles that were later occupied by the "bulky" mainly using traditional batteries. The popularity of the car culture has added fuel to the take-off wings. At present, almost all power machinery in the world is inseparable from the use of petroleum as a power source. Gasoline has high specific energy, high conversion efficiency and convenient use. It is almost unmatched for power machinery such as automobiles. At the same time, another problem inevitably emerged. The oil history of just over a hundred years, with the greed and excessive exploitation of human beings, has caused oil resources to gradually drift away from humans. With the depletion of oil resources, the oil crisis in the "nightmare" seems to be in front of us. At the same time, the road congestion caused by urban traffic and the automobile exhaust pollution caused by automobiles bring about a global greenhouse effect caused by various respiratory diseases and the resulting air pollution.

Humanity calls for low carbon, calls for environmental protection, calls for energy conservation, calls for green energy, and brings new opportunities for new energy vehicles. As the biggest beneficiary of this business opportunity, new energy vehicles will soon form a new market driving force, facing new People are waiting for the major changes in the energy vehicle technology route and technological innovation to adapt to the new demands of marketization.

Third, the safety and life of lithium battery battery

A new power source for new energy vehicles with energy-based power lithium-ion batteries (lithium-powered batteries) was introduced at the beginning of the century. As an energy carrier in the new energy era, it can be said that it is smaller than the traditional lead-acid battery, lighter in weight, higher in specific energy than power, longer in cycle life, and capable of high-rate charging/discharging. , quickly became the first choice for new energy vehicles.

From the analysis of the development of objective things, things are always divided into two. Lithium-powered batteries are no exception. While giving people more joy, their safety is gradually exposed compared with the characteristics of traditional batteries. In recent years, many electric vehicle burning accidents have appeared in the media reports, and the lithium battery fire in a Boeing 787 passenger cabin of Japan's All Nippon Airways on January 16 this year seems to have brought some bitter taste to people. People are constantly asking whether the lithium-powered battery electric car is safe in the end? Therefore, how to evaluate and solve the safety of electric vehicles has become a necessary condition for lithium battery applications. Of course, with the advancement of technology and the introduction of lithium-ion battery safety standards, this distressing problem seems to have been qualitatively improved under the efforts of all parties. From the objective analysis, the safety of lithium-ion battery itself has been A big step forward.

However, a more difficult problem has been placed in front of the world. Electric vehicles are still facing obstacles such as unsatisfactory running, not far away, large volume-to-weight ratio, and high price compared with gasoline vehicles. Rapid development.

In particular, the lithium battery life cycle promised by the battery manufacturers to the user is limited to single cells, and the life of the group is often greatly reduced. One of the reasons is that electric vehicles are not a single battery application like mobile phone batteries, but need multiple series of batteries in parallel, but the battery is only used in groups, and the problem of “consistent reduction” is highlighted. Come out, even in the case of consistency when the battery is shipped, as long as the electric vehicle has a special working condition (the battery is placed at different positions of the electric vehicle and the temperature field is different), it is caused by the parallel connection of the battery pack. The change of electrochemical characteristics, the difference of the battery strings in parallel after grouping will appear, resulting in a single battery in the battery pack failing due to "overcharge" or "overdischarge", etc., forming a "domino effect" "The resulting comprehensive performance of the battery pack and the battery pack being over-used will inevitably affect the safety and cycle life of the battery pack." The long life of the so-called lithium-powered battery has therefore been marked with large quotation marks, which is precisely the phenomenon that people are least willing to see.

If this problem is not solved, it will greatly "stay" the pace of industrialization of new energy vehicles.

Fourth, BMS is the key to solving battery group applications

Where is the problem? After a large number of theoretical analysis and practice by electrochemical experts and battery manufacturers, the industry generally agrees that "the consistency of battery serial-parallel group application is the key to the problem", and the application results will directly lead to the quality of the application. The battery life of the group is reduced. This result is due to the electrochemical characteristics of the battery itself, which should be inevitable in the inevitable characteristics of battery grouping. Is there a way to solve the problem of such "battery electrochemical group consistency characteristics worse"?

Battery Management System: An electronic circuit is used to solve such problems. The industry is accustomed to calling BMS, which is specially designed to manage lithium-ion battery series and parallel group applications.

BMS can prevent the "overcharge" and "overdischarge" of lithium battery by controlling the management methods of the charger and motor controller, and solve the battery safety problem that people are worried about. We simply call it "limited to two ends"; It is also possible to solve the so-called "long life" problem of the battery by using a reasonable management strategy of the electrochemical "inflection point" characteristic of the battery, which is a two-pronged approach.

After several years of demonstration operation of the Beijing Olympic Games and Shanghai World Expo BMS on electric vehicles, its role has been widely recognized by the industry, and more typical cases of BMS participating in the demonstration of electric vehicle operation are numerous.

But the technical debate about BMS has never stopped. Due to the wide variety of lithium-powered batteries, the domestic mainstream battery manufacturers are fighting each other, which makes it difficult to form a unified technical standard in China. (It is worth mentioning that small-rolled cylindrical batteries, the technical standards for external dimensions at home and abroad are Uniform). It is thus difficult for BMS to form technical standards for different batteries. A large amount of repetitive work is wasted on custom development, which makes the lithium-powered battery and BMS have a lot of controversy due to the lack of mature technology. The relevant technical standards are difficult to introduce and try. This cannot be said to be a pity.

At the same time, lithium-ion batteries are still in the early stage of industrialization, limited to various process routes, and the automation of lithium-ion battery production equipment is not high enough. The quality of many small and medium-sized battery factories still stays at a relatively primitive stage. The initial consistency of the power battery at the factory is difficult to guarantee. Not to mention the battery consistency after the application of the electric vehicle. For the expansion of the BMS function, it has to increase the various balancing methods that have emerged to solve the problem of battery consistency.

V. Comparison of several solutions

Many of the original intentions of BMS to solve the "consistency" scheme of lithium-powered batteries are still good. The effect of solving the problem simply from the function is also different. For example, various balancing methods for solving the difference in lithium-powered batteries are rapidly emerging as the industrialization of lithium-ion batteries continues to rise. But whether these methods can really solve the above problems remains to be discussed. To this end, we analyze the current mainstream solutions.

1. Passive equalization method (energy diversion method):

In the traditional energy-consuming BMS system, the equalization mode is mainly based on passive equalization, and the single-cell parallel shunt energy consumption resistance is adopted, and only the equalization work can be done in the charging process, and the excess energy cup is consumed to the energy consumption resistance. On, the efficiency is zero. At the same time, the equalization current is small, usually less than 100 mA, the effect on the large-capacity battery is negligible, and the SOC estimation accuracy is also low.

The working principle of the passive equalization method is to measure the voltage difference of the single cells in series in the whole battery system. The voltage is collected by the BMS, based on the “upper threshold voltage” of the preset charging voltage. The single battery only needs to reach the "upper threshold voltage" when charging and detects the difference from the battery in the adjacent group, that is, the battery with the highest cell voltage in the group, through the parallel resistance of the single cell. The discharge current is applied, and so on, until the single cell with the lowest voltage reaches the "upper threshold voltage" for an equilibrium period. The purpose is to make the battery voltage in the battery pack consistent through discharge equalization.

Features:

1) The principle is simple and easy to implement;

2) When the equalization current is small, the device cost is relatively low.

Question:

1) Resisting energy consumption discharge, wasting energy and generating heat;

2) Since the discharge resistance cannot be selected too small, at the end of charging, the voltage of the small-capacity battery is the highest according to the characteristics of the battery. In the static equalization, the power of the small-capacity battery is discharged, but the battery is increased. The mutual difference.

2. Active Balance Method (Dynamic Equilibrium Method):

The active equalization method is a method for actively equalizing the individual differences in the capacity of the battery during use and the voltage difference generated by the self-discharge rate. Its main function is to actively balance the difference between battery cells in the battery pack during charging, discharging or placing, to eliminate the battery itself and the various processes generated during use. Inconsistency. However, since the energy transfer in the equalization process will cause heat loss due to the balance of the power supply characteristics of the power supply, and the change of the polarization internal resistance in the electrochemical characteristics of the battery, the problem cannot be solved by a simple energy equalization method.

Active balancing method works: The energy transfer device is used to replenish the energy of the high-energy battery into the low-energy battery. The essence is that the battery energy in the battery pack can be single/bidirectional transfer, so that the battery voltage (capacity) of the battery pack is high, or the total circuit battery in the group, or a separate one. For the independent battery used for balancing, the electromagnetic energy is used, or the single or two-way DC/DC method is used to add the rich energy to the other batteries with lower battery voltage (capacity) in the group. The lower battery, this method can be performed when the battery is charged, discharged, or when the battery is stationary. In order to achieve the purpose of improving battery group differences.

Features:

1) The DC/DC bidirectional active equalization circuit adopts high equalization efficiency;

2) Balanced during charging, discharging and static processes;

3) The balance current is large and the equalization speed is faster