Tesla electric car battery thermal management system

Tesla currently offers two electric vehicles, the Roadster and the Model S. The Roadster is currently collecting more information, so this article focuses on the Roadster battery management system.

The main task of the Battery Management System (BMS) is to ensure that the battery pack operates within a safe interval, provide the necessary information for vehicle control, respond to the response in the event of an anomaly, and respond to ambient temperature, battery status, and vehicle requirements. The battery is charged and discharged, etc. The main functions of BMS include battery parameter monitoring, battery status estimation, online fault diagnosis, charging control, automatic equalization, and thermal management. This article will focus on the Battery Thermal Management System (BTMS).

The importance of thermal management systems

The heat-related issues of a battery are key factors in determining its performance, safety, longevity, and cost of use.

First, the temperature level of a lithium-ion battery directly affects the energy and power performance of its use. At lower temperatures, the available capacity of the battery will rapidly decay. Charging the battery at too low a temperature (such as below 0 °C) may cause an instantaneous voltage overcharge, causing internal lithium and causing a short circuit. . Secondly, the heat related problems 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 a chain exothermic reaction, eventually causing serious thermal runaway events such as smoke, fire or even explosion, threatening the life of the vehicle occupants. Safety. In addition, the working or storage temperature of a lithium ion battery affects its service life. The proper temperature of the battery is between 10 and 30 ° C. Too high or too low temperature will cause a rapid decay of battery life. 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 be dissipated, and the internal temperature unevenness and local temperature rise are more likely to occur, thereby further accelerating battery attenuation, shortening battery life, and increasing users. Total cost of ownership.

The battery thermal management system is one of the key technologies to deal with the heat related problems of the battery and to ensure 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, increasing the battery temperature, ensuring charging and discharging performance at low temperatures And safety; 3) reduce the temperature difference in the battery pack, inhibit the formation of the local hot zone, prevent the battery from decaying too fast at the high temperature position, and reduce the overall life of the battery pack.

1. Tesla Roadster's battery thermal management system

Tesla Motors' Roadster pure electric vehicle uses a liquid-cooled battery thermal management system. The vehicle battery pack consists of a 6831 section 18650 lithium-ion battery, in which each 69 sections are connected in parallel as a brick, and then 9 sets are connected in series as a sheet, and finally 11 layers are stacked in series. The coolant in the battery thermal management system is a mixture of 50% water and 50% ethylene glycol.

Figure 1. (a) is a thermal management system inside the sheet. The cooling duct is arranged in a zigzag between the batteries, and the coolant flows inside the duct to remove the heat generated by the battery. Figure 1. (b) is a schematic view of the structure 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 from gradually increasing during the flow of the coolant, the heat dissipation capacity of the terminal is not good. The thermal management system adopts the flow field design of the two-way flow. The two ends of the cooling pipe are both the liquid inlet and the liquid outlet. 1(d). Materials that are electrically insulated but have good thermal conductivity between batteries and pipes (such as Stycast 2850/ct) are used to: 1) convert the contact between the battery and the heat pipe from line contact to surface contact; 2) It is beneficial to increase the temperature uniformity between the cells; 3) It is beneficial to increase the overall heat capacity of the battery pack, thereby reducing the overall average temperature.

Figure 1. Schematic of the Batteryster thermal management system

Through the above thermal management system, the temperature difference of each unit cell in the Roadster battery pack is controlled within ±2 °C. A June 2013 report showed that after driving 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 reduction is only significantly related to the mileage, and the ambient temperature. The relationship between car age is not obvious. The achievement of the above results relies on the strong support of the battery thermal management system.

Second, the thermal management system of other electric vehicles

1. Nissan LEAF thermal management system

Nissan Motor's LEAF pure electric vehicle uses a rare passive battery pack thermal management system. The battery pack consists of 192 33.1 Ah stacked lithium-ion batteries. The four-cell battery consists of two and two strings of connected modules, and the 48 modules are connected in series to form a battery pack. The battery pack is sealed, the outside is not ventilated, and there is no thermal or air-cooled thermal management system inside, but there are heating options in cold areas. The lithium-ion battery used in LEAF has been designed to reduce the internal impedance and reduce the heat generation 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 it is possible to avoid complicated active Thermal management system. The life guarantee period of the battery pack is 8 years or 160,000 kilometers.

2. General Volt's thermal management system

GM's Volt plug-in hybrid uses 288 45 Ah stacked lithium-ion batteries. The electrical connection of the battery pack can be equivalent to 96 pieces of cells connected in series, and 3 sets in parallel. The thermal management system uses a liquid-cooled design with a 50% water and 50% ethylene glycol mixture as the cooling medium. A metal heat sink (thickness of 1 mm) is arranged between the cells, and a runner groove is engraved on the heat sink. The coolant can flow away in the runner tank to remove heat. In a low temperature environment, the heating coil can heat the coolant to warm the battery.

Figure 2. Volt's thermal management system

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

Third, the characteristics of Tesla Roadster compared to other electric vehicles in thermal management

As can be seen from the above analysis, the Tesla Roadster is far more complicated in thermal management systems than other electric vehicles. Tesla's battery pack is made up of 8831 cells with a small cell capacity of 6831. It is very difficult to ensure that the temperature difference between so many batteries does not exceed ±2 °C, but Tesla does it, which also highlights Tesla's advanced and unique battery management. However, there is another new problem: Since LEAF and Volt can achieve design goals by using a large-capacity stacked lithium-ion battery to match a simpler thermal management system, why does Tesla use 18650 batteries and a complicated battery management system? The author believes that there are the following reasons:

a. Advantages of the 18650 battery: The 18650 battery has been widely used in consumer electronics. The manufacturer has accumulated a lot of technical experience to control costs and improve performance (especially safety, consistency, etc.). When choosing a battery manufacturer, Tesla chose companies that are actively investing in reducing product defects.

b. Tesla's comparative advantage: Among all electric vehicle 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 from battery and switched to electric vehicles. Japan's Nissan is a traditional car manufacturer. Later, it cooperated with NEC to develop batteries and enter the electric vehicle market. Where is the technical advantage of Tesla? I think the battery management system is definitely a very important part of it. In Tesla's technical team, engineers who prefer electronics and electricians should be the majority, so developing battery management systems is much less difficult than developing batteries (biasing materials, chemicals) or chassis (biasing machinery).

In an interview with Tesla technical director JB Straubel, he said, “Is Tesla always tied to the 18650 battery? Will Tesla choose any other battery?” The answer to this question is: “Believe me, we will be in the near future. Seeing the 18650 is the most convincing. I really don't know why the 18650 will cause so much controversy. No one cares about the shape and size of your fuel tank, but what shape and size are used to put electrochemical energy on the electric car. It has caused so much controversy. What people should really discuss is what kind of chemicals are put in them. The nature of these substances determines the cost and performance. At present, our batteries are actually deeply customized, and we have done a lot with Panasonic. Customized work. We are doing car-grade batteries, and it is absolutely impossible to find such batteries on any notebook according to the automotive-grade standards. We use 18650 batteries and shapes and sizes mainly for production and In terms of cost efficiency, any large battery can't meet the price level we need. We think it is for electricity. For your car, there are some key safety and performance indicators for your product. 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 structure is more cost-effective, we are always listening. But so far we have Haven't found a company that proves to be more cost effective than our battery architecture."

JB Straubel's attitude toward Tesla's long-term partners Daimler and Toyota is: "Toyota is very helpful to us in improving production and supplier quality issues. They are the best companies in the world in large manufacturing companies. They built a science to track production defects and helped us in many places. The key knowledge we learned from Daimler was product validation and testing, and they brought a lot of high-intensity rigor in these areas. What we have to do is really a very high quality product. Daimler’s products and Toyota are different in terms of production price. Therefore, it is really cool for us to build an electric car and absorb the experience from these two hybrids. The cooperation is mutual promotion, they are eager to listen and understand how we innovate, software and solve problems. I have to say that we are leading them more than a little in software and electronic engineering."