With the strengthening of environmental awareness and the support of national policies, in the next few years, new energy electric vehicles will cover the urban and rural roads with a rapid growth rate. As an electric vehicle, the biggest security risk is the reliability of the power battery. For example, in the high and low temperature weather of 40 ℃ in summer and minus dozens of ℃ in winter, can the power battery work safely and reliably? This requires our R & D engineers to do enough work on power battery cooling.
At present, the main batteries of new energy vehicles are lithium batteries, which are mainly used in logistics vehicles and passenger vehicles, while the passenger cars generally use lithium iron phosphate batteries.
How can we solve the problem of power battery cooling?
There are two kinds of heat dissipation of battery pack, active and passive, which are quite different in efficiency. The cost of passive system is low and the measures are simple. The structure of active system is more complex and needs more additional power, but its thermal management is more effective.
The power battery is in a relatively closed environment in structure. When it works, the current is large and the heat generated is large, which will directly lead to the temperature rise of the power battery. The reason is that the electrolyte in the lithium battery plays the role of charge conduction in the lithium battery. The battery without electrolyte cannot be charged and discharged. Most of lithium batteries are composed of flammable and volatile non-aqueous solutions. Compared with the batteries composed of aqueous electrolyte, this system has higher specific energy and voltage output, which meets the higher energy demand of users. Because the non-aqueous electrolyte itself is inflammable and volatile, and soaks in the battery, it also forms the combustion source of the battery. Therefore, whether it is lithium battery or lithium iron phosphate battery, their working temperature shall not be higher than 60 ℃, but if the outdoor temperature reaches nearly 40 ℃ in summer and the heat generated by the battery itself is large, the working environment temperature of the battery will rise, and if the heat is out of control, the situation will be very dangerous. In order to avoid the similar situation, it is very important to heat the power battery.
Simultaneous interpreting of different heat transfer medium has different cooling effects.
The main advantages of using gas (air) as heat transfer medium are: simple structure, light weight, effective ventilation when harmful gas is produced, low cost; the disadvantages are: low heat transfer coefficient between the battery wall, slow cooling speed, low efficiency. At present, it is widely used.
The main advantages of using liquid as heat transfer medium are: high heat transfer coefficient with battery wall, fast cooling speed; the disadvantages are: high sealing requirements, relatively large quality, complex maintenance and repair, requiring water jacket, heat exchanger and other components, relatively complex structure.
In the actual application of electric bus, due to the large capacity and volume of battery pack, relatively speaking, the power density is relatively low, so air-cooled scheme is often used. But for the battery pack of ordinary passenger car, its power density is much higher. Accordingly, it will also have higher requirements for heat dissipation, so the water-cooling scheme is more common.
Different battery pack structure sensors will be determined according to the temperature measurement point and demand. The temperature sensor will be placed in the most representative position with the largest temperature change, such as the air inlet and outlet position and the middle area of the battery pack. Especially at the highest temperature and the lowest temperature, as well as the area where the heat accumulation in the center of the battery pack is more severe. This will help to control the temperature of the battery in a relatively safe environment and avoid the danger of overheating and supercooling to the battery.
In addition, the function of the battery diaphragm is to separate the positive and negative plates of the battery in a narrow space to prevent the short circuit caused by the contact between the two poles, but to ensure the free passage of ions in the electrolyte between the positive and negative poles. Therefore, the separator becomes the core material to ensure the safety and stability of lithium-ion batteries.
The electrolyte is to isolate the source of combustion, the diaphragm is to improve the heat-resistant temperature, and the sufficient heat dissipation is to reduce the temperature of the battery, so as to avoid excessive heat accumulation leading to out of control of the battery. If the temperature of the battery rises to 300 ℃, even if the diaphragm does not melt and contract, the electrolyte itself, the electrolyte and the positive and negative electrodes will have strong chemical reaction, release gas, form internal high pressure and explode, so it is very important to adopt suitable heat dissipation mode.
Introduction to heat dissipation principle and method:
The comprehensive parameter reflecting the ability to prevent heat transfer. When heat is transferred in the form of heat conduction inside the object, the resistance encountered is called heat conduction resistance. The thermal resistance of heat conduction is L / (k * a) when the cross-sectional area of heat flow is constant. Where l is the thickness of the plate, a is the cross-sectional area of the plate perpendicular to the heat flow direction, and K is the thermal conductivity of the plate material.
Calculation formula: R thermal resistance = l heat transfer distance / (s heat transfer area XK thermal conductivity)
Introduction to the cooling structure of the power battery pack:
1: install a cooling fan at one end of the battery module, and leave a vent at the other end to accelerate the flow of air between the gaps of the cell and take away the high heat generated when the cell is working;
2: xk-p series thermal conductive silica gel gasket is added at the top and bottom of the electrode end respectively, so that the heat that is not easy to be dissipated at the top and bottom is transmitted to the shell through the thermal conductive silica film for heat dissipation. Meanwhile, the high electrical insulation and puncture resistance of xk-p series thermal conductive silica gel gasket have a good protection effect on the battery pack.
(temperature rise simulation of air cooling structure of power battery module, surface temperature difference and sectional temperature difference diagram)
The air cooling mode is very effective and fast to dissipate the heat generated by the battery module when it works, so as to ensure the stable and effective operation of the battery module.
Introduction to the cooling mode of the liquid cooling structure of the power battery pack:
1: the heat of the battery core is transferred to the liquid cooling pipe through the conduction silicon film. The heat is taken away by the free circulation of the expansion and contraction of the coolant, so that the temperature of the whole battery pack is unified. The strong specific heat capacity of the coolant absorbs the heat generated during the operation of the battery core, so that the whole battery pack operates in a safe temperature.
2: the conductive silica gel sheet has good insulation performance and high resilience, which can effectively avoid the vibration, friction and damage between the electric cores, and the hidden danger of short circuit between the electric cores. It is the best auxiliary material for the water-cooling scheme.
In this liquid cooling scheme, S-type heat-conducting aluminum tube is adopted, and special-shaped heat-conducting silica gel gasket is pasted on the aluminum tube (convex stripes are added on the contact surface between the heat-conducting silicon film and the electric core), so that the contact surface between the electric core and the heat-conducting tube is larger, and the heat-conducting effect and damping effect are better.
(temperature rise simulation of liquid cooling structure of power battery pack, sectional temperature difference distribution diagram)
The core temperature is balanced, the temperature difference is small, and the core working temperature can be well controlled within the safe temperature.
Introduction to natural convection heat dissipation mode of power battery pack:
This kind of battery pack has large space and good contact with air. The exposed part can exchange heat naturally through air. The part at the bottom that can't exchange heat naturally can dissipate heat through the radiator. The heat-conducting silicon film fills the space between the radiator and the battery pack, conducting heat, damping and insulating.
Soft pack battery stack structure - heating module
1: this scheme is applied to the battery module of new energy vehicles. When the battery temperature is too low, the heater is started to preheat the battery first, and the heat is transferred to the heat-transfer aluminum plate in the battery group through the heat-transfer interface material to preheat the battery module uniformly and efficiently; when the battery is running overheated, the heat of the battery core is transferred to the heat-transfer aluminum plate through the heat-transfer interface material, and then transferred to the metal shell for rapid heat dissipation and protection Ensure the stable operation of the battery module.
2: glpoly xk-p10ld series low density lightweight power battery thermal conductive silicon film has good insulation performance and wear resistance, which can effectively protect the friction between battery pack, heating plate and shell, and avoid the wear, short circuit and other related safety issues.
Heat dissipation of lithium iron phosphate hard shell battery pack:
1: this scheme is also applicable to the natural heat dissipation scheme and heating module scheme of the battery module of new energy vehicles. When the battery temperature is too low, the side heating element heats up and preheats the battery, and the heat is transferred to the heat conduction interface material filled in the battery group through the heat conduction interface material, and then to the electric core, uniformly and efficiently preheat the battery module, so that the new energy vehicles can quickly enter the stable operation state When the electric core is running overheated, the heat of the electric core is transmitted to the metal shell through the heat conduction interface material, which can quickly dissipate heat and ensure the safe and stable operation of the battery module.
2: xk-p series low density lightweight power battery thermal conductive silicon film has good insulation performance and wear resistance, which can effectively protect the friction between battery pack and heating plate, metal shell, avoid the wear, short circuit and other related safety issues.
Source: glpoly
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