18650 lithium battery.Model 3 sets off a craze. Analysis of Tesla’s battery management system

　　At the beginning of this month, the launch of Tesla Model 3 and the subsequent hot booking volume once again aroused global attention. As we all know, the core technology of electric vehicles lies in batteries, but Tesla does not produce batteries itself. Its batteries are provided by Panasonic of Japan. So why does Tesla stand at the head of the industrial chain and create miracles in the field of new energy vehicles again and again? The big reason may be that Tesla has the world's top battery management system (BMS).

　　The electric vehicle battery management system (BMS) is an important link between vehicle power batteries and electric vehicles. Its main functions include: real-time monitoring of battery physical parameters; battery status estimation; online diagnosis and early warning; charge, discharge and precharge control; and balance management. and thermal management, etc.

　　According to experts, Tesla’s pure electric vehicles use a liquid-cooled battery thermal management system. The vehicle battery pack is composed of 6831 18650 lithium-ion batteries, of which every 69 cells are connected in parallel to form a group (brick), and then 9 groups are connected in series to form a layer (sheet), and finally 11 layers are stacked in series. The coolant of the battery thermal management system is a mixture of 50% water and 50% ethylene glycol.

　　The cooling pipes in the management system are arranged zigzag between the batteries, and the coolant flows inside the pipes to take away the heat generated by the batteries. The interior of the cooling pipe is divided into four channels. In order to prevent the temperature from gradually rising during the flow of coolant, which will lead to poor heat dissipation at the end, the thermal management system adopts a bidirectional flow field design. The two ends of the cooling pipe are both the liquid inlet and the cooling pipe. The mouth is also the liquid outlet. Materials with electrical insulation but good thermal conductivity are filled between batteries and between batteries and pipes. Their functions are:

　　1) Change the contact form between the battery and the heat dissipation pipe from line contact to surface contact;

　　2) It is helpful to improve the temperature uniformity between single cells;

　　3) It is helpful to increase the overall heat capacity of the battery pack, thereby reducing the overall average temperature.

　　Through the above thermal management system, the temperature difference of each single cell in the electric vehicle battery pack is controlled within ±2°C. A report in June 2013 showed that after driving 100,000 miles, the capacity of the Tesla pure electric vehicle battery pack can still be maintained at 80% to 85% of the initial capacity, and the capacity attenuation is only significantly related to the mileage traveled. , but the relationship with ambient temperature and vehicle age is not obvious. The achievement of the above results relies on the strong support of the battery thermal management system.

　　From the above analysis, we can see that the thermal management system of Tesla's pure electric vehicles is far more complex than that of other electric vehicles. Tesla's battery pack is composed of 6831 18650 batteries with smaller single capacity. It is very difficult to ensure that the temperature difference of so many batteries does not exceed ±2°C, but Tesla has done it.

　　It is reported that among Tesla’s technical team, engineers who prefer electronics and electricians account for the majority. When the media interviewed Tesla technical director J.B. Straubel, he said without humility that Tesla is more than a little ahead of other companies in the same industry in terms of programming software and electronic engineering.

　　The author believes that because Tesla’s technical team focuses on electronics and electrical engineering, developing a battery management system is much less difficult than developing batteries (which focuses on materials and chemistry) or chassis (which focuses on mechanics). Tesla made full use of its own advantages to develop a top-notch battery management system, allowing it to create a miracle in the new energy automobile industry even though it was neither a battery manufacturer nor a traditional automobile manufacturer.

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