What are the cooling systems of lithium-ion batteries powered by new energy vehicles?

The cooling system of power lithium battery, the cooling performance will directly affect the efficiency of the battery, but also affect the life of the battery and the use of safety.

Power lithium battery in the process of charging and discharging will generate a certain amount of heat, resulting in temperature rise, temperature rise will affect many characteristics of the battery parameters, such as internal resistance, voltage, available capacity, discharge efficiency and battery life and so on. In order to extend the service life of power lithium battery as far as possible and obtain the maximum power, it is necessary to use the battery in the specified temperature range, which involves the cooling system of power lithium battery.

At present, the thermal management of new energy vehicle power lithium battery system can be divided into four categories: natural cooling, air cooling, liquid cooling and direct cooling. Among them, natural cooling is a passive thermal management mode, while air cooling, liquid cooling and DC are active. The important difference between these three is the difference of heat exchange medium.

Natural cooling

Natural cooling does not have additional devices for heat transfer, generally speaking is to rely on natural wind, such as BYD Qin, Tang and Teng use LFP cell models are used in natural cooling. The advantages of natural cooling are simple structure, low cost, small space occupation, and relatively clear disadvantages, low heat dissipation efficiency, which can not adapt to the cooling demand of high-power charging and discharging. Generally, it is only used for electric vehicles with moderate operating conditions and sensitive to cost.

Air cooling

In the new energy electric vehicles sold on the market, the proportion of air cooling is still large, which is the more widely used heat dissipation technology in lithium batteries of new energy vehicles. Air cooling adopts air as heat exchange medium. The principle is to use the cooling fan to draw the air from the inside of the car into the power lithium battery box to cool the power lithium battery and the control unit of the power lithium battery and other components. Models such as the Toyota Prius, Honda Insight, Camry Hybrid and Corolla dual engine all use air-cooled battery cooling systems. Compared with other technologies, air cooling technology is relatively simple, safe maintenance is more convenient, can achieve good heat dissipation performance at low cost.

However, the disadvantages of air cooling technology are also very clear, especially compared with liquid cooling technology, it has a low heat exchange coefficient with the battery surface, the cooling and heating speed is still relatively slow, the average temperature inside the battery box is not easy to control, and the sealing design of the battery box is difficult, and the dustproof and waterproof effect is poor. It has to be said that some electric vehicle fire incidents are due to the poor thermal management performance of air-cooling technology.

Surface

As using the environment more and more high to the requirement of power lithium batteries, liquid-cooled technology also gradually replace air cooling technology as various auto enterprises is preferable, especially in large and medium-sized pure flood, liquid-cooled system utilization rate is very high, in small all-electric cars and plug-in hybrid vehicles, using the liquid-cooled also more and more new vehicles. The principle of liquid cooling technology is to take away the heat generated during the operation of the battery through the coolant inside the battery pack, so as to reduce the temperature of the battery.

In simple terms, the liquid-cooling system passes through a pipe in the battery pack. Cold water is poured into the pipe to cool the battery, which takes away the heat, and hot water is poured into the pipe to warm the battery. The temperature control effect of liquid cooling system on battery pack is better than that of air cooling system. The heat transfer coefficient of liquid medium is high, the heat capacity is large, and the cooling speed is faster.

Straight cold

The structure of the direct cooling system is similar to that of the liquid cooling system, but the direct cooling will directly inject the refrigerant of the automobile air conditioning system into the battery pack. The refrigerant can absorb a lot of heat in the gas-liquid phase transformation process, and take away the heat inside the battery more quickly, and the heat dissipation efficiency is higher. Direct cooling uses refrigerant as heat exchange medium, refrigerant can absorb a large amount of heat in the process. Direct cooling can increase the heat transfer efficiency by more than three times compared with liquid cooling, which takes away the heat inside the battery system more quickly. 

The direct cooling system improves the heat exchange efficiency, but it also has defects, it has higher requirements for the air tightness of the system, higher requirements for the processing and manufacturing process, and then the heat dissipation average of the direct cooling system is not easy to control, and the temperature difference of the electric core is too large. In addition, the direct cooling system can only integrate the function of heat dissipation, does not have the function of heating, to install an independent heating system to deal with the low temperature in winter.