Why does Tesla's battery management system work better than other electric vehicles?

1. The importance of thermal management systems

The heat-related problem of the battery is a key factor in determining its performance, safety, life, and cost of use. First, the temperature level of a lithium ion battery directly affects the energy and power performance in use. When the temperature is low, the available capacity of the battery will decay rapidly. If the battery is charged at an excessively low temperature (eg, less than 0°C), an instantaneous voltage overcharge may occur, causing internal lithium to precipitate and short circuiting. . Second, the heat-related issues of lithium ion batteries directly affect the safety of the battery. Defects in the manufacturing process or improper operation during use may cause local overheating of the battery, which in turn may cause chain exothermic reactions. Eventually, serious thermal runaway events such as smoke, fire, or explosion may endanger the lives of vehicle occupants. Safety. In addition, the operating or storage temperature of a lithium ion battery affects its useful life. The suitable temperature of the battery is about 10~30°C, too high or too low temperature will cause the battery life to decay quickly. The large size of the power battery makes the ratio of surface area to volume relatively small, the internal heat of the battery is not easy to dissipate, and problems such as uneven internal temperature and excessive local temperature rise may occur, thereby further accelerating battery attenuation, shortening battery life, and increasing users. The total cost of ownership.

The battery thermal management system is one of the key technologies for coping with the heat-related problems of the battery and ensuring the performance, safety, and life of the power battery. The main functions of the thermal management system include: 1) Effective heat dissipation when the battery temperature is high to prevent thermal runaway accidents; 2) Preheating when the battery temperature is low to increase the battery temperature to ensure charging and discharging performance at low temperatures And safety; 3) reduce the temperature difference within the battery pack, suppress the formation of local hot spots, prevent the battery from rapidly decaying at high temperature locations, and reduce the overall life of the battery pack.

2. Battery Thermal Management System of Tesla Roadster

Tesla Motors' Roadster pure electric vehicle uses a liquid-cooled battery thermal management system. The on-board battery consists of 6,831 18650-type lithium-ion batteries, each of which is connected in groups of sixty-eight (bricks), nine in series, and then stacked in series. The battery thermal management system coolant is a mixture of 50% water and 50% ethylene glycol.

Figure 1. (a) is a thermal management system inside a sheet. The cooling pipes are arranged in a zigzag manner between the batteries. The cooling liquid flows inside the pipes and takes away the heat generated by the batteries. Figure 1. (b) is a schematic diagram of the cooling pipe. The inside of the cooling pipe is divided into four holes, as shown in Figure 1.(c). In order to prevent the temperature during the flow of coolant from gradually increasing and the heat dissipation capacity of the terminal is not good, the thermal management system adopts a bidirectional flow field design. The two ends of the cooling pipeline are both the liquid inlet and the liquid outlet, as shown in the figure. 1(d) shows. Between cells and batteries and pipes filled with electrically insulated but good thermal conductivity material (such as Stycast 2850/ct), the role is: 1) the contact between the battery and the heat pipe from the line contact to surface contact; 2) have It helps to increase the temperature uniformity among the single cells; 3) It is beneficial to improve the overall heat capacity of the battery pack, thereby reducing the overall average temperature.

Figure 1. Battery Thermal Management System Diagram for Roadster

Through the above-mentioned thermal management system, the temperature difference of each single cell in the Roadster battery pack is controlled within ±2°C. A report in June 2013 showed that after traveling 100,000 miles, the capacity of the Roadster battery pack can still be maintained at 80% to 85% of the initial capacity, and the capacity decay is only significantly related to the number of miles traveled, and the ambient temperature There is no obvious relationship between car age. The above results are obtained relying on the strong support of the battery thermal management system.

3. Thermal Management System for Other Electric Vehicles

3.1 Nissan LEAF Thermal Management System

Nissan Motor's LEAF pure electric vehicles use a rare passive battery thermal management system. The battery pack consists of 192 33.1 Ah stacked lithium-ion batteries. The four single cell batteries are composed of two parallel and two series connection modules, and 48 modules are connected in series to form a battery pack. The battery pack adopts a sealed design. The outside is not ventilated. There is no liquid-cooled or air-cooled thermal management system inside, but there are heating options in cold areas. The lithium-ion battery used by LEAF reduces the internal impedance after the electrode design, and reduces the heat production rate. At the same time, the thin layer (single thickness 7.1 mm) structure makes the internal heat of the battery less likely to accumulate, so the complex active type can be avoided. Thermal management system. The battery life guarantee period is 8 years or 160,000 kilometers.

3.2 General Volt Thermal Management System

GM's Volt plug-in hybrid vehicles use 288-cell 45 Ah stacked lithium-ion batteries. The electrical connection of the battery pack can be equivalent to 96 pieces of monomers in series and 3 in parallel. The thermal management system uses a liquid-cooled design scheme with a mixture of 50% water and 50% ethylene glycol as the cooling medium. The metal fins (thickness is 1 mm) are arranged between the cells and the runner grooves are engraved on the fins. Coolant can flow away in the flow channel to take away heat. At low temperatures, the heating coil heats the coolant to warm the battery.

Figure 2. Volt thermal management system

The temperature difference in the Volt's battery pack can be controlled within 2°C, effectively supporting the 8-year battery pack life guarantee period.

4. Thermal management characteristics of Tesla Roadster relative to other electric vehicles

From the above analysis, it can be seen that the Tesla Roadster is far more complex than other electric vehicles in thermal management systems. Tesla's battery pack consists of 6831 cells with 18650 cells of relatively small capacity. It is very difficult to ensure that the temperature difference of such cells does not exceed ±2°C. However, Tesla did, and it also highlighted Tesla is advanced and unique in battery management.

However, a new problem has emerged: Since LEAF and Volt use a large-capacity stacked lithium-ion battery with a simpler thermal management system to achieve design goals, why does Tesla also use 18650 batteries and a complex battery management system?

I think there are the following reasons:

a. Advantages of the 18650 battery: The 18650 battery has been widely used in consumer electronic products, and the manufacturer has accumulated a large amount of technical experience for controlling costs and improving performance (especially safety, consistency, etc.). Tesla chose companies that have actively invested in reducing product defects when selecting battery manufacturers.

b. Tesla's comparative advantage: Among all electric car manufacturers, Tesla is a magical one. It is neither a battery manufacturer nor a traditional car manufacturer, but it has succeeded. China's BYD started with batteries and switched to electric cars. Nissan in Japan is a traditional car manufacturer. Later, it cooperated with NEC to develop batteries and entered the electric vehicle market. Where is Tesla's technological advantage? I think the battery management system is definitely a very important part of it. In Tesla's technical team, engineers with a preference for electronics and electrical engineering should be the majority. Therefore, it is much harder to develop a battery management system than to develop batteries (for materials, chemistry) or chassis (for machines).

In an interview with Tesla Technical Director JB Straubel, he said, “Is Tesla always bound to 18650 batteries? Will you choose any other batteries?” The answer to this question is:

"Believe me, in the near future we will see that the 18650 is the most convincing. I really don't know why the 18650 will cause so much controversy. No one will care about the shape and size of your fuel tank, but use it on electric vehicles. What shapes and sizes put into electrochemical energy has caused so much controversy that what people really should be discussing is what kind of chemicals are put inside, and the nature of these substances determines the cost and performance.

At present, our batteries are actually deeply customized, and we have done a lot of custom work with Panasonic. What we do is a car-grade battery, which is absolutely impossible to find on any laptop, according to automotive-grade standards. The reason why we use 18650 batteries of this shape and size is mainly due to production and cost efficiency considerations. Any large battery can not meet the price level we need.

We think that for electric vehicles, your product has some key safety and performance indicators. This is a must, but the most important thing is the cost efficiency of your product's energy storage. If a company feels that its battery architecture is more cost-effective, we will listen to it at any time. But so far we have not found a company that can prove to be more cost-effective than our battery architecture. ”

His attitude toward Daimler and Toyota, Tesla's long-term partners, is:

“Toyota is very helpful to us in improving the quality of production operations and suppliers. In large manufacturing companies, they are the best companies in the world. They have established a science to track down production defects and have helped us in many places.

The key knowledge we have learned from Daimler is the validation and testing of products. They bring a lot of rigor to these fields. What they want to do is really a very high quality product. Daimler's products and Toyota are all different in terms of output price.

So for us to build electric cars, it is really cool to be able to absorb the experience from these two families. Cooperation is mutually reinforcing. They are eager to listen and understand how we innovate, make software, and solve problems. I have to say that we have led them more than a little in software and electronic engineering. ”