Is the battery thermal management system for electric vehicles simple? SES Power analyzes the thermal management of lithium batteries for you

　　As a manufacturer with nearly 20 years of experience in customizing lithium-ion batteries, SES Power focuses on the integration of lithium-ion battery energy storage systems, such as fully intelligent lead-acid replacement products with Bluetooth or RS485 communication ( Lithium iron phosphate 12V100Ah, 12V200Ah), high current (2000A) starting lithium battery, UPS high voltage lithium battery system (up to 860V), 3Kw~20Kw off-grid, grid-connected, island-type lithium battery energy storage system, base station communication backup battery system ( Standard 19 inches), a full set of photovoltaic energy storage components, etc. But we also have a lot of products that are used in power equipment, which are close to the requirements of electric vehicles, such as lithium batteries for large electric forklifts, electric ships, etc., and the knowledge of lithium batteries is the same, so we have the same knowledge about lithium battery systems for electric vehicles. Not unfamiliar.

　　Thermal management is an important part of electric vehicles, which is related to the health of the energy system of lithium batteries. SES Power will give you an in-depth analysis of the battery thermal management system of electric vehicles.

　　A: What is the function of battery thermal management?

　　The most suitable working temperature of lithium battery is between 10-30°C. And the working environment of the car is very broad, -20-+50°C is very common, so what should we do? Then there are only 3 functions of thermal management:

　　The first function is cooling. When the temperature is too high, the capacity decay of the battery will be accelerated, and the risk of thermal runaway will increase. Therefore, when the temperature is too high, heat dissipation is required.

　　The second function is heating: when the temperature is too low, the capacity of the battery will also be attenuated, the performance will be degraded, and even there will be potential safety hazards during charging (the internal short circuit caused by the precipitation of lithium has the risk of thermal runaway). Therefore, when the temperature is too low, heating (or insulation) is required.

　　The third function is to maintain temperature consistency: the purpose is to keep the temperature consistent for each battery in different working environments. Simply put, the power battery needs to reduce the temperature difference in space as much as possible.

　　B: Is it important to maintain temperature consistency?

　　For batteries, maintaining a consistent temperature is just as important as the first two functions and stems from three things:

　　The first is the barrel effect: the performance and reliability of a battery system depend on the weakest cell, and the safety of the system depends on the most unstable cell.

　　To give a practical example in engineering, the following figure is the curve of the maximum discharge current of a certain type of ternary lithium battery with temperature. It can be seen that the performance of the battery has a very large relationship with the temperature. Assuming that the temperature of most cells is 20 degrees, and the temperature of battery B is only 10 degrees because of the slow heating, the entire battery pack must accommodate the B cell, the discharge current is forced to drop from 140A to 100A, and the performance drops by one-third.

　　The second is to reduce secondary inconsistencies: differences in operating temperature are the main cause of secondary inconsistencies. The inherent individual differences of single cells when they leave the factory are called primary inconsistencies, and the differences between single cells that gradually increase during use are called secondary inconsistencies.

　　The primary inconsistency mainly reflects the manufacturing and testing level of the cell manufacturer. Its causes are very complex, involving factors such as material system, variety, manufacturing process, and on-site control.

　　The secondary inconsistency mainly reflects the system integration level of the pack factory or integrator, especially the thermal management design level. That is to say, no matter how good the battery cells are, if the system integration capability of the pack factory is poor, the differences between the cells will gradually expand during use, which will bring performance degradation and safety risks.

　　The third is to avoid the chain reaction: the expansion of battery differences is a gradual process, perhaps only the case where the capacity is 5% smaller and the internal resistance is 10% larger; but the thermal runaway of the battery pack caused by the weakest link is a sudden process ,Why is this so? The reason is the chain reaction.

　　The side reaction of the battery is usually exothermic. If the heat dissipation conditions are not good, the thermal runaway of the single battery will release a lot of heat. If the thermal insulation/heat dissipation conditions between the single cells are not good, the heat may cause thermal runaway of adjacent cells, which in turn causes thermal runaway of the entire module (module) or even the entire battery pack (Pack).

　　(The thermal runaway of the single battery causes the thermal runaway of the related monomer)

　　It can be seen that, as a representative function of battery thermal management, temperature consistency can be used as one of the core indicators to measure the level of battery management technology.

　　C: How difficult is it to ensure temperature consistency?

　　Reducing the temperature difference between single cells mainly depends on the design of the heat dissipation runner. This is a very challenging content, and there are different solutions in the industry.

　　Consider the simplest one-dimensional design first. Figure (a) is the simplest design. The cooling carrier (air-cooled as air, liquid-cooled as water or coolant) flows from left to right, which brings a problem: the right side The temperature of the coolant is higher and the heat dissipation effect is poor, and the temperature of the rightmost single battery will be significantly higher than the leftmost one.

　　Figure (b) has made some improvements. The wedge-shaped flow channel accelerates the flow rate of the cooling carrier on the right side, which offsets the factor of higher coolant temperature, so that the effect is better than that in Figure (a). The problem is that the inside of the battery pack is very limited, and the angle of the wedge cannot be designed to be very large, so the effect of this design will not be much better than Figure (a).

　　Figure (c) is designed with a convection flow channel, where the coolant periodically changes its flow direction, thereby attenuating the temperature difference by half. However, this solution also has a price. If it is air-cooled, it can be achieved by using fans alternately blowing air, but if it is liquid-cooled, it is difficult to achieve in engineering.

　　One-dimensional design does not seem to be difficult to understand, so let's look at the two-dimensional situation: the left side is aligned and the right side is staggered.

　　So can we intuitively answer the following questions?

　　-Which one has higher temperature rise during discharge?

　　-Which temperature difference is greater during discharge?

　　-During the discharge process, are the downstream single cells higher in temperature?

　　In fact, this is difficult to answer intuitively, and you must resort to computational fluid dynamics and heat transfer (simulating with a bunch of partial differential equations, SES Power engineers will also have a headache).

　　Let's look directly at the conclusions of the relevant data:

　　In the case of the same flow, the heat dissipation of the staggered arrangement is better and the temperature rise is lower; but the energy consumption of driving the cooling carrier is greatly increased. (Aligned arrangement can only blow the side, and staggered discharge can blow the front, so the heat dissipation effect is better, but the flow resistance is greater)

　　As the radial distance increases, the temperature rise of the aligned arrangement decreases, while the temperature rise of the staggered arrangement is the opposite. This is counterintuitive.

　　The temperature inconsistencies of the two arrangements are similar, but the temperature distribution laws are quite different. In the staggered arrangement, the temperature rise of the midstream single cell is the smallest, and the temperature rise of the upstream and downstream single cells is larger, which is counterintuitive.

　　The actual situation is actually three-dimensional, which is much more complicated than the two-dimensional situation, because it is necessary to consider the influence of the layout of the whole vehicle, the shape of the cells after grouping may not be a complete cuboid, each car will have different encounters, etc. These situations will bring greater challenges to cell thermal management.

　　SES Power has analyzed it for you. I believe you can understand the difficulty of battery thermal management. Some engineers even start to be glad that they are not in this position, because it is really difficult!

　　D: What is the difference between the thermal management of cylindrical lithium batteries and square lithium batteries?

　　Generally speaking, a qualified battery pack should control the temperature difference within ±5°C. If it can be achieved within ±2°C, it can be called excellent. Those who can achieve this standard include the traditional car giant General Motors and Tesla, the leader of the new car-making forces.

　　d1: 18650 cylindrical battery used by Tesla

　　The contact surface is relatively small and the monomer capacity is low. With a large number of used, these factors increase the difficulty of controlling the battery temperature to be consistent. Therefore, Tesla has put a lot of thought into thermal management design, and some design ideas can be seen from the published patents.

　　The first is that Tesla uses serpentine heat sinks. In this way, the curvature of each heat sink is consistent with the cylindrical shape, which can achieve a contact area of more than half a circle, which can promote heat conduction between the cell and the outside world and reduce thermal resistance; at the same time, each metal sheet will be connected to the main heat sink on the left and right sides. channel connection.

　　Judging from the actual disassembly diagram, there is a large cooling strip between every two layers, probably mainly to save space and weight. The actual arrangement and direction of the heat dissipation belts are not as regular as in the schematic diagram of the patent, but in a ring shape.

　　The second is that Tesla adopts a method similar to the "two-way cooling" mentioned above, that is, the direction of the left and right heat dissipation channels is reversed, the left side flows from the bottom to the top, and the right side flows from the top to the bottom to prevent the unbalanced temperature between the upper and lower sides.

　　Judging from the actual dismantling pictures, it is true that each cooling unit has four water pipe interfaces, two in and two out.

　　As can be seen from another picture, although the principle is very simple, the winding and arrangement of the two opposite circuits is quite complicated. The purpose of this is to ensure that the heat dissipation/heating of each cell is relatively balanced.

　　Universal Volt for d2 square cells

　　Compared with the aggressive Tesla, GM's Volt uses a more stable square cell, but it also puts a lot of effort into thermal management.

　　Because the contact surface of the square cells is relatively large, GM directly adds a heat dissipation ratchet between each two cells, and transfers the heat to the cooling circuit below through the ratchet.

　　It may be due to the fact that the effect of the heat dissipation tabs is larger than the serpentine contact surface of Tesla, and the thermal resistance is smaller. GM did not design two-way flow to control the temperature difference, which may not be as much as Volt's battery pack. body, the overall balance is relatively good.

　　E: How to really do a good job in thermal management of lithium batteries?

　　The first is to control the initial inconsistency of the cells

　　Before the cells are formed into a module, the voltage difference between the cells is required to be less than 25mV and the voltage difference between the modules is less than 30mV to ensure that the internal battery capacity of the battery pack is balanced in the initial state.

　　The second is to have a deep thermal management theory and simulation system

　　With the advancement of the times, the lithium battery systems of electric vehicles and energy storage systems can be said to be larger and more complex, and there are more and more factors affecting thermal management.

　　Therefore, there must be sufficient theoretical depth and an appropriate simulation system. Only in this way can the initial design be completed theoretically, and then the actual test can be carried out. In fact, there is no absolutely correct theory and simulation system. The progress of theory and simulation is based on basic knowledge and a large number of experimental data.

　　The third is the need for enough experimental data

　　As mentioned in the previous point, without sufficient experimental data, the correct theory and results will not be obtained. Of course, a large number of experiments must be based on reasonable, scientific and accurate measurement and simulation links.

　　This requires a lot of investment in R&D personnel, capital, and equipment. SES Power believes that such investment is necessary and must be effective.

　　The fourth is to use new technologies as much as possible

　　For example, an independent liquid cooling circuit is used to reduce the influence of the motor and DCDC system operating conditions on the thermal management of the battery. The aluminum water-cooling plate arranged at the bottom of the battery module has high thermal conductivity and low flow resistance, so as to achieve higher heat dissipation efficiency; while the aluminum plate and the battery are covered with thermal grease (this is what DIY computers dissipate heat from the CPU), improving the efficiency of heat dissipation. The thermal conductivity between the cell and the heat dissipation plate is improved, and the heat dissipation efficiency is improved, which is more conducive to the control of the temperature consistency of the cell.

　　(Aluminum water cooling plates arranged in parallel)

　　In SES Power's point of view, thermal management of electric vehicles and lithium battery energy storage systems is a very important part, but the two are not exactly the same, because their uses, usage environments, requirements, etc. are different. But in any case, there is only one purpose for effective thermal management of lithium batteries, that is, to make lithium batteries work in a comfortable state, so as to achieve the best performance ratio.