The development of electric vehicles can be described as twists and turns. In 1834, the first electric car was born. At that time, dry batteries were used and the cruising range was very short. After Karl Benz invented the internal combustion engine in 1886, electric cars all but disappeared. In 1973, the oil crisis broke out in the Middle East, which caused the world's auto giants to set off a wave of research and development of electric vehicles. However, due to the characteristics of batteries and technical limitations, the development of electric vehicles still failed this time. But we saw 1971 when Toyota introduced its first hybrid car, the Prius. It uses a Ni-MH battery with high current charge and discharge capability, which is safe, but has a low specific capacity and a large volume.

Development status and trend analysis of electric vehicle power battery

The fourth generation Toyota Prius

With the birth of the fourth-generation Prius, in addition to the nickel-metal hydride battery for the HEV version, the PHEV version of the car is also equipped with a lithium battery. The specific capacity has been improved, but the safety has been reduced. At this point, we can't help but ask, which type of battery is the best? What is the standard of a good battery? In this regard, Kuang Dezhi pointed out that any type of battery has its advantages and disadvantages, and their suitable applications are also different.

Analysis of advantages and disadvantages of mainstream power batteries

The current mainstream batteries include supercapacitors, metal hydride nickel batteries, lithium-ion batteries, and fuel cells. Supercapacitors are characterized by being able to withstand instantaneous high-current charge and discharge, but the storage capacity is low, and they cannot be used to drive vehicles for a long time; metal hydride batteries have high-current charge and discharge capabilities, good safety, but low specific capacity and large volume; Lithium-ion batteries have the highest voltage and high specific capacity among these types of batteries, but their safety and low-temperature performance are poor; fuel cells have attracted the attention of more and more people since last year, and have sufficient energy reserves to quickly replenish fuel. But the cost is high, the instantaneous output capability is poor, and the fatal flaw is that energy feedback cannot be performed, so that the driven vehicle cannot only use fuel cells to realize energy recovery during braking.

The current mainstream new energy vehicles in the market include Nissan Leaf, Toyota Prius, Chevrolet Volt, etc., and of course the red-hot Tesla Model S. These four vehicles all use lithium batteries or nickel metal hydride batteries without exception, so it can also be seen that among the above four power batteries, lithium batteries and nickel metal hydride batteries are the two types of batteries with the most intense competition. Many people in the industry have their own opinions on parameters such as power density, energy density, and maximum capacity.

In fact, not all parameters are high is the best (certainly impossible to achieve). Kuang Dezhi pointed out that the power density is directly related to the application field of the battery. If it is used in a field where hybrid vehicles require high power, it must also require a large power density; if it is used in a pure electric vehicle, the power requirement is not high but energy storage requirements In higher fields, the power density requirements can be lowered, but the energy density must be large enough. Kuang Dezhi emphasized that whether it is a nickel-hydrogen battery or a lithium battery, its power density and energy density can be adjusted by the battery designer according to the application.

Development status and trend analysis of electric vehicle power battery

Tesla Model S

For example, the Tesla Model S has a battery life of 480 kilometers, and its 18650 lithium battery has an energy density of 200 Wh/kg; while the Chevrolet Volt also uses lithium batteries, because its requirements for pure electric battery life are not high. Only 60 km, so its energy density is only 88 Wh/kg (16 kWh ÷ 181 kg).

The relationship between the performance of the battery

In addition, the performance of the power battery is mutually balanced, or "contained", and the performance of the battery does not depend on a certain characteristic of a single battery, but on the overall performance. From the figure above, we can see that capacity and durability, high temperature performance and low temperature performance, heat dissipation and insulation, tolerance and cost are four sets of "contradictory" parameters. Therefore, Kuang Dezhi emphasized that only when the relationship between battery performance parameters and the environment is comprehensively considered can it truly have application value.

"Safety, life, cost" is the biggest bottleneck

my country is a big country of automobiles. In 2014, the production and sales both exceeded 20 million vehicles. However, it is not a big country of automobiles. This is true in the field of traditional automobile technology, and it is also true in the field of new energy vehicle technology. The technical strength of the three core elements of new energy vehicles, batteries, motors, and electronic control systems in my country, is not strong, especially in terms of batteries, which mainly rely on outsourcing. Therefore, the so-called "curve overtaking" can't help but make people feel that it is not worthy of the reputation.

Let's look at a set of data first, so that we can have an intuitive understanding of the gap in the development of power batteries at home and abroad. Recently, the official website of PEVE, a well-known Japanese power battery company, released data saying that it has sold 8 million sets of power batteries, and Japanese hybrid vehicles account for 40% of all car sales. In contrast, in 2014, the total sales volume of new energy vehicles in my country was less than 80,000. How many of these batteries come from local Chinese companies? The huge level difference is horrifying. So, where is the gap between my country's power battery technology and the international advanced level? And in what technical bottlenecks?

Kuang Dezhi pointed out that the bottleneck of domestic power battery technology lies in life, safety and cost. These three points are not completely independent, but interrelated. Battery life refers to cycle life and shelf life, and the reduction of cycle life mainly comes from the consistency problem in the process of battery grouping. Low cycle life will increase the need for battery replacement, which will greatly increase the cost of electric vehicles. The shelf life refers to the attenuation of the life of the battery in a static state. In automotive applications, the battery is left on hold most of the time. From the perspective of practical application, the attenuation of the battery is very obvious when the vehicle is exposed to high temperature in summer.

It introduced that when using Ni-MH batteries, especially power-type Ni-MH batteries, it is more guaranteed that the battery can be used in a smaller mileage range, which can prolong its life. In addition, the operating temperature of the battery also has an impact on the lifespan, the higher the temperature, the shorter the lifespan. The Ni-MH battery is limited to 35°C, and the life will be cut in half for every 10°C increase in temperature. Therefore, when designing, install a thermal management system to manage the battery temperature and keep it used at a constant temperature, which can also prolong the battery life.

The most important system related to battery life and safety is the BMS (Battery Management System). Kuang Dezhi emphasized that BMS is the core component to ensure battery application. After the battery has been produced consistently, the main factor that can determine the battery life is the BMS. At present, domestic BMS can basically eliminate potential safety hazards such as overvoltage, short circuit, and extrusion under the conditions of single cells and modules, but the safety of the whole package level has not been completely resolved, which is also one of the bottlenecks of battery technology.

Kuang Dezhi added that both domestic and foreign BMSs can be developed by professional companies. However, battery companies must put forward their own requirements for battery internal control and identification of battery characteristics. Because enterprises have the deepest understanding of their own batteries, only independent development can achieve optimal control of products. This is Mr. Kuang's suggestion to domestic battery companies, and Keba itself does the same.

Summarize:

Like the internal combustion engine technology of traditional automobiles, my country also needs to go through the process from outsourcing to cooperative research and development to independent development in the field of new energy vehicle power batteries. Whether it is for new energy vehicle companies or power battery companies, the pursuit of goals should not be limited to production and sales, but start with small components, concentrate on research and development, and achieve breakthroughs in bottlenecks.

As Kuang Dezhi said, although my country lags behind the international level in terms of power battery technology, we are backed by a vast automobile market, which provides us with an opportunity to catch up with and surpass foreign advanced levels.