The power battery is the energy source of the electric vehicle. During the charging and discharging process, the battery itself will generate a certain amount of heat, which will cause the temperature to rise. The temperature rise will affect many operating characteristics of the battery, such as internal resistance, voltage, SOC, and available capacity. , charge and discharge efficiency and battery life.
This article directory:
01. Power battery thermal management necessity
02. Classification and introduction of thermal management systems
03. Thermal Management System Design Process
04. Key technologies in the design process
05. Thermal Management System Performance Evaluation
The battery thermal effect problem will also affect the performance and cycle life of the vehicle. Therefore, it is very important to do thermal management to improve the performance and life of the battery to the mileage of the whole vehicle.
Next, let's talk about it from the aspects of battery thermal management system and design flow, component type and selection, thermal management system performance and verification:
01 power battery thermal management necessity
1, the generation of battery heat
Due to the presence of battery impedance, during battery charging and discharging, current flows through the battery causing heat to be generated inside the battery. In addition, due to the electrochemical reaction inside the battery, it also causes a certain amount of heat generation.
2, the impact of temperature increase on battery life
The increase in temperature has an impact on the calendar life and cycle life of the battery.
As can be seen from the above two figures, the temperature has a great influence on the calendar life of the battery. The same battery, after the ambient temperature of 23 ° C, 6238 days, the remaining capacity of the battery is 80%, but the battery in the environment of 55 ° C, after 272 days, the remaining capacity of the battery has reached 80%. When the temperature rises by 32 ° C, the calendar life of the battery drops by more than 95%. Therefore, temperature has a great influence on the calendar life, and the higher the temperature, the more serious the calendar life decline.
As can be seen from the above two figures, the temperature also has a great influence on the cycle life of the battery. The same battery, when the remaining capacity is 90%, the output capacity is 300kWh at 25°C, and the output capacity at 35°C is only 163kWh. When the temperature rises by 10 ° C, the cycle life of the battery drops by nearly 50%. It can be seen that the temperature has a great influence on the cycle life of the battery.
Therefore, in order to optimize the performance of the battery pack, it is necessary to design a thermal management system to ensure that each battery operates within a reasonable temperature range.
02 Classification and introduction of thermal management system
Different thermal management systems, different types of components, different weights, different system costs and different control methods make the system achieve different performance. There are five main categories:
1, direct cooling system
The direct cooling system has the advantages of compact system, light weight and good performance. However, this system is a dual evaporator system, the system has no battery heating, no condensate protection, the refrigerant temperature is not easy to control, and the refrigerant system has a short life.
2, low temperature radiator cooling system
The low temperature radiator cooling system is a separate system of batteries consisting of a radiator, water pump and heater. The cooling system has the advantages of simple system, low cost, and economical energy saving in a low temperature environment. However, this system has the disadvantages of low cooling performance, high summer water temperature, and weather restrictions.
3. Direct cooling water cooling system
The direct cooling water cooling system has the advantages of compact system, good cooling performance and wide range of industrial applications. However, this system has more components than direct cooling, complicated system, poor fuel economy and high compressor load. This type of cooling system is one of the most commonly used battery thermal management systems.
4, air-cooled / water-cooled mixed cooling system
There are two key components in an air/water cooled hybrid cooling system:
1) water-cooled battery cooler;
2) Air-cooled battery radiator.
The air-cooled/water-cooled hybrid cooling system has the advantages of compact system, good performance and economical energy saving in a low temperature environment. However, this system is complicated, costly, complicated to control, and highly reliable.
5, direct air cooling system
This system uses the low temperature air in the cockpit to cool the battery.
The direct air cooling system has the advantages of simple system, controllable air temperature and low cost. However, this system is not suitable for all types of cells, and the recovery is slow after soaking and there is a risk of contamination inside the battery.
03 thermal management system design process
1. Product development process
The development process of the battery thermal management system should be consistent with the battery pack development process. The design of the thermal management system runs through the design process of the entire battery pack. During the development of the whole vehicle, there are 5 stages of A sample, B sample, C sample, D sample and final product. Battery thermal management participates in each stage. Design, change, trial and verification.
2, thermal management development process
A well-designed battery pack thermal management system requires a systematic design approach. The process of designing the battery pack thermal management system includes the following seven steps:
04 key technologies in the design process
1. Determine the optimal operating temperature range for battery operation
Since the climate and vehicle operating conditions have a large impact on the battery, it is necessary to determine the optimal operating temperature range of the battery pack when designing the BTMS. At present, batteries for electric vehicles mainly include lead-acid batteries, nickel-hydrogen batteries and lithium-ion batteries.
1) Lead acid battery
It has been found that the life of lead-acid batteries decreases linearly with increasing temperature, and the charging efficiency increases linearly. As the battery temperature decreases, the charging capacity decreases, especially below 0 °C. The temperature gradient between modules reduces the capacity of the entire battery pack. It is recommended to keep the temperature distribution within the battery pack evenly and to control the temperature of the existing lead-acid battery between 35 and 40 °C. Efficiency and maximum operating power increase in the range of -26 to 65 °C.
2) Hydrogen nickel battery
When the temperature exceeds 50 ° C, the battery charging efficiency and battery life will be greatly attenuated. At low temperatures, the battery's discharge capacity is also much smaller than the normal temperature. The figure below shows the discharge efficiency of a battery at different temperatures of a 80Ah hydrogen-nickel battery. It can be seen from the figure that the discharge efficiency of the battery is significantly reduced when the temperature is higher than 40 °C or the temperature is lower than 0 °C. If only based on this limitation, the operating range of this battery should be between 0 and 40 °C.
3) Lithium-ion battery
Compared with hydrogen-nickel batteries and lead-acid batteries, the energy density is higher, resulting in more heat generation, so the heat dissipation requirements are higher. The optimal operating temperature of a lithium ion battery is between -20 and 75 °C.
The necessity of thermal management of lead-acid batteries, nickel-hydrogen batteries, and lithium-ion batteries depends on their respective heat generation rates, energy efficiency, and sensitivity to temperature. Hydrogen nickel batteries have the highest heat generation, low efficiency and prone to thermal runaway accidents at high temperatures > 40 °C. Therefore, nickel-hydrogen batteries are in great need of thermal management, and many efforts to liquid-cool nickel-hydrogen batteries have highlighted this point.
2. Battery thermal field calculation and temperature prediction
The battery is not a good conductor of heat. The surface temperature distribution of the battery cannot fully explain the thermal state inside the battery. Calculating the temperature field inside the battery through mathematical model and predicting the thermal behavior of the battery is an indispensable part of designing the thermal management system of the battery pack. It is usually calculated using the following formula:
In the formula:
a, T is the temperature;
b, Ï is the average density;
c, Cp is the specific heat of the battery;
d, kx, ky, and kz are the thermal conductivity of the battery in the x, y, and z directions, respectively;
e, q are the heat generation rate per unit volume.
3, battery heat rate
The reaction heat generation during battery charging can be divided into two stages.
Phase 1:
Before the charging side reaction occurs, the heat generation mainly comes from: battery chemical reaction heat generation, battery polarization heat generation, internal resistance Joule heat.
Phase 2:
After the occurrence of over-charge side reactions, the heat generation mainly comes from: battery chemical reaction heat generation, battery polarization heat generation, over-charge side reaction heat generation, internal resistance Joule heat. Most of the heat generated comes from the over-heating side reaction. Overcharge side reactions begin to occur at the end of charging and overcharging.
The heat generated during the discharge of the battery mainly comes from: battery chemical reaction heat generation, battery polarization heat generation, internal resistance Joule heat. It should be pointed out that the chemical reaction of the nickel-hydrogen battery is an endothermic reaction, which can absorb a part of the heat, so the heat generation problem is not very serious.
The internal resistance of the battery is a key indicator affecting the heat generation rate of the battery. It changes with the SOC of the battery. After the internal resistance of the battery is obtained, the heat generation of the battery can be obtained by calculation. The following figure shows the different SOC of a 12V-80Ah hydrogen-nickel battery module. The internal resistance value.
A specially designed calorimeter can directly measure the heat generation of the battery and measure the heat capacity of the battery.
4, the main factors of battery heat generation
5, heat dissipation structure design
The temperature difference between different battery modules in the battery box will aggravate the inconsistency of the internal resistance and capacity of the battery. If accumulated for a long time, some batteries will be overcharged or overdischarged, which will affect the life and performance of the battery and cause safety hazards. The temperature difference of the battery module in the battery box has a great relationship with the battery pack arrangement. Under normal circumstances, the battery in the middle position is easy to accumulate heat, and the heat dissipation condition of the edge battery is better. Therefore, when performing the battery pack structure arrangement and heat dissipation design, it is necessary to ensure the uniformity of heat dissipation of the battery pack. Taking air cooling as an example, the ventilation methods are generally serial and parallel, as shown in the following figure.
In the serial ventilation mode, cold air is blown from the left side and blown out from the right side. The air is continuously heated during the flow, so the cooling effect on the right side is worse than the left side, and the temperature of the battery pack in the battery box rises from left to right.
Parallel ventilation allows for a more even distribution of air flow between the battery modules. Parallel ventilation requires good design of the inlet and exhaust passages, and the battery arrangement position. The wedge-shaped intake and exhaust passages make the pressure difference between the different modules basically the same, ensuring the air flow through different battery modules. Consistency, thus ensuring the consistency of the temperature field distribution of the battery pack.
6, fan and temperature point selection
When designing a battery thermal management system, you want to choose the type of fan and power, the number of temperature sensors and the location of the temperature measurement point are just right.
Taking the air cooling method as an example, when designing the heat dissipation system, the flow resistance should be minimized, the fan noise and power consumption should be reduced, and the efficiency of the whole system should be improved while ensuring a certain heat dissipation effect. The power consumption of the fan can be estimated by estimating the pressure drop and flow using experimental, theoretical calculations and fluid dynamics CFD simulations (FloEFD software in this case). When the flow resistance is small, an axial flow fan can be considered; when the flow resistance is large, a centrifugal fan is suitable. Of course, we must also consider the size and cost of the space occupied by the fan. Finding the optimal fan control strategy is also one of the functions of the thermal management system.
Ipsilateral airflow streamline diagram
Irregular airflow diagram
The temperature distribution of the battery pack in the battery compartment is generally non-uniform, so it is necessary to know the thermal field distribution of the battery pack under different conditions to determine the dangerous temperature point. The number of temperature sensors is large, and there is a comprehensive advantage of temperature measurement, but it will increase the system cost. Considering that the temperature sensor may fail, the number of temperature sensors in the entire system should not be too small, at least two. According to different practical engineering backgrounds, theoretically using finite element analysis, infrared thermal imaging or real-time multi-point temperature monitoring in the test can analyze and measure the thermal field distribution of battery packs, battery modules and battery cells, and determine the temperature measurement. Find the appropriate temperature measurement points for different areas by the number of points. The general design should ensure that the temperature sensor is not blown by the cooling air to improve the accuracy and stability of the temperature measurement. When designing the battery, consider the space for the temperature sensor, for example, you can design the appropriate hole in the appropriate position.
05 thermal management system performance evaluation
Simulation is one of the most effective evaluation methods for battery thermal management systems. According to the existing experience of air-cooling and water cooling projects, the simulation can complete the following work:
1) Calculation of pressure drop of cooling plate of water cooling system and calculation of consistency of cooling water flow;
2) Calculation of the thermal performance of the battery pack;
3) Optimization calculation of air cooling system.
1, heat dissipation type battery pack thermal management case
The following is a complete vehicle thermal management for a hybrid vehicle, including battery pack thermal management model, passenger compartment model, engine cooling, HVAC, oil cooling system and Motor cooling system FloMASTER software (software original name Flowmaster) simulation model, The battery cooling system has carried out a series of design simulation work.
For the battery pack, the battery model and the cooling model were established. The heat capacity, thermal resistance and thermal bridge of the battery were considered. The cooling and heating processes were studied and the cooling temperature requirements were met (the battery core did not exceed 40 °C). The water flow rate and the heating power of 30 ° C in the specified 30 minutes, as well as the temperature uniformity and hysteresis performance of each cell during heating.
2, direct air cooling battery pack
This case is the thermal management simulation of the Mitsubishi Oland model, and the cold air condition and cell temperature of the evaporator outlet under different meteorological conditions and the whole test cycle conditions are obtained.
3, air / water mixed cooling battery pack
The following models are air/water hybrid cooling battery thermal management and vehicle thermal management model, and the system is subjected to thermal management simulation of different seasons and different vehicle conditions, and combined with control strategy, heating and battery heating of different gear positions are studied. Working conditions and pure heating conditions provide an important basis for system design and control strategy optimization.
Finally, Xiaobian wants to say that the temperature of the battery directly affects the safety of the battery. Therefore, the design of the thermal management system of the battery is one of the most critical tasks in the design of the battery system. The design and verification of the thermal management of the battery system must be carried out in strict accordance with the thermal management design process of the battery, the thermal management system and component types of the battery, the component selection of the thermal management system, and the performance evaluation of the thermal management system. To ensure the performance and safety of the battery.
Solar Generator,Wind Power Generator,Home Generators,Solar Generator
Shaoxing AnFu Energy Equipment Co.Ltd , https://www.sxanfu.com