The rapid development of new energy vehicles also faces a series of severe challenges. Charging time, driving range, and safety are all affected by power batteries. Lithium-ion power batteries exhibit different characteristics at different ambient temperatures. In high-temperature environments, power batteries undergo intense chemical reactions during high-rate charging and discharging, generating a large amount of heat. If the heat generated by the power battery cannot be dissipated in time, it will accumulate inside the battery, causing the temperature to rise, and in severe cases, may lead to explosion. In low-temperature environments, the significant attenuation of available energy and power severely affects the driving range and low-temperature power performance of power batteries. In low-temperature environments, the deterioration of power battery power characteristics and the decrease in charging and discharging efficiency are among the factors restricting the development of electric vehicles. Therefore, research on thermal management of electric vehicles in high and low-temperature environments is one of the key development areas for new energy vehicles.
Thermal management of power batteries can be divided into cooling management in high-temperature environments and heating management in low-temperature environments. High-temperature cooling technologies can be categorized into air cooling, liquid cooling, heat pipe cooling, and phase change cooling.
Low-temperature heating technologies are mainly divided into internal heating methods and external heating methods. Internal heating methods utilize the Joule heat generated by the power battery itself, but their impact on the battery's lifespan and safety is not yet clear, and they are still in the early stages of research in the field of electric vehicles, so they are less commonly used. External heating methods use high-temperature gas, high-temperature liquid, electric heating plates, phase change materials, and the Peltier effect to heat the power battery through an external heat source. This method relies mainly on an external heat source to heat the power battery and is safer compared to internal heating methods.
Liquid cooling is widely used in the field of electric vehicles due to its high heat transfer efficiency and cooling speed. By changing the temperature of the coolant on the basis of liquid cooling, the power battery can be heated in low-temperature environments without the need for additional equipment costs. At present, research on low-temperature thermal management of power batteries mainly focuses on charging thermal management, with relatively little research on thermal management strategies during the driving process of electric vehicles. Based on the 1D simulation model of AMESim, this paper analyzes the impact of different power sources and different initial SOC on the discharge power of the power battery when starting the low-temperature driving heating function, aiming to improve the low-temperature power performance of the power battery through the low-temperature thermal management system and provide some guidance for the thermal management scheme during low-temperature driving of electric vehicles.
1 Introduction to Low-Temperature Thermal Management Principles and Scheme Design
1.1 Introduction to Low-Temperature Thermal Management Principles
This paper takes the low-temperature thermal management system principle of a certain electric vehicle as an example to design a low-temperature heating scheme, as shown in Figure 1, which mainly includes PTC, PTC circuit water pump, Chiller (heat exchanger), battery circuit water pump, liquid cooling system, power battery, and pipelines. The battery thermal management system performs low-temperature heating management on the power battery based on its minimum temperature. When the power battery needs heating, the PTC heats the coolant in the PTC circuit, and then the coolant in the battery circuit is heated through the Chiller. The battery circuit heats the power battery through the liquid cooling system, thereby achieving temperature control of the power battery.
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1.2 Low-Temperature Thermal Management Scheme Design
The preheating function of electric vehicles is a low-temperature thermal management strategy. Users can set whether to enable this function in advance according to travel needs. After the user enables this function, the electric vehicle thermal management system will determine whether to start heating the power battery based on the battery temperature. This function can improve the low-temperature characteristics of the power battery to a certain extent. This paper designs a preheating scheme based on the preheating function of electric vehicles and studies the improvement effect of the preheating function on the low-temperature power performance of the power battery. Generally, when the ambient temperature is below -10°C, the capacity and working voltage of lithium-ion power batteries decrease significantly. Moreover, according to big data of electric vehicle users, the lowest ambient temperature used by users is also around -10°C. Therefore, based on the actual user environment, this paper selects an ambient temperature of -10°C as the lowest temperature for the study. In addition, research shows that the optimal temperature range for lithium-ion power batteries is 10-35°C. According to the discharge power of the power battery (Table 1), when the battery temperature is 10°C, the initial SOC between 20% and 100% can meet the target discharge power (80kW) requirement. Therefore, 10°C is selected as the target temperature for preheating. The specific preheating scheme is to preheat the power battery to 10°C at an ambient temperature of -10°C before discharging. According to the power source of the PTC, there are two schemes: Scheme A, the PTC power comes from the charging pile; Scheme B, the PTC power comes from the power battery.
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2 1D Simulation Model Based on AMESim
This paper mainly studies the voltage and temperature changes of the power battery. In the low-temperature thermal management system of the power battery, the minimum battery temperature is used as the judgment criterion. Therefore, the simulation model is calibrated based on the minimum temperature of the power battery. Before building the model, the electrical model and thermal model of the power battery are calibrated. After inputting the test boundary and test current into the model, the comparison results between the simulated voltage and the test voltage are shown in Figure 2, with a voltage error within 4%; the comparison results between the simulated temperature and the test temperature are shown in Figure 3, with a temperature error within 2°C. The calibration results show that the simulation model has sufficient accuracy for subsequent simulation verification of the preheating scheme. During the simulation, the coolant flow rates of the PTC circuit and the power battery circuit are both controlled at 10 L/min, and the PTC power (rated power 5kW) is controlled to ensure that the coolant inlet temperature of the power battery circuit liquid cooling system is 30°C.
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3 Effect of Preheating on Power Battery Power Performance
By using the preheating function to heat the initial temperature of the power battery to 10°C, no heating measures are taken during the discharge process, and only the battery's own insulation measures are used to maintain the battery temperature, thereby ensuring the power performance of the power battery. This method can meet the power performance requirements of the power battery to a certain extent, but it is greatly affected by the insulation performance of the power battery.
3.1 Effect of Power Source on Target Power Duration
The power for preheating the power battery mainly comes from the charging pile or the battery itself. Different power sources have different effects on the target power duration during preheating. When the power for preheating comes from the charging pile, it has no effect on the battery's own power. However, when using the battery's own power for preheating, it affects the battery's own power, thereby affecting the target power duration. The effect of different power sources on the increment of target power duration (compared to no preheating) for different initial SOC of the power battery is shown in Table 2.
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From Table 2, it can be seen that when the initial SOC of the power battery is 100%, 90%, and 80%, using the battery's own power for preheating reduces the target power duration, while using the charging pile power for preheating increases the target power duration. When the initial SOC is below 70%, both using the charging pile power and the battery's own power for preheating can increase the target power duration.
The discharge power of the power battery is directly related to the battery temperature and SOC. When the power battery is at different initial SOC, the discharge power and temperature changes during the discharge process after preheating are shown in Figures 4-12. It can be seen from the figures that when the power battery is preheated using the charging pile power, the target power duration is improved compared to the control group. However, at high initial SOC, as the discharge duration increases, the battery temperature shows a decreasing trend. The higher the initial SOC, the closer the temperature at the end of discharge is to the control group. Therefore, the higher the initial SOC, the smaller the increment in target power duration. When the power battery is preheated using its own power and the initial SOC is high, the discharge power at the initial stage of discharge is higher than that of the control group. However, as the discharge time increases, the discharge power becomes lower than that of the control group. Moreover, when the initial SOC is high, the battery's own discharge power already meets the target power requirement, so preheating would waste power.
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3.2 Effect of Ambient Temperature on Target Power Duration
Ambient temperature is a direct factor affecting the power performance of the power battery. The power performance of the power battery varies at different ambient temperatures. Therefore, it is crucial to study the effect of preheating on the power battery at different ambient temperatures. Based on the analysis in the previous section, the effect of preheating on the power battery at different ambient temperatures is analyzed from the following two aspects.
3.2.1 Preheating Power from Charging Pile
When the preheating power of the power battery comes from the charging pile, the increment of target power duration at different ambient temperatures (default initial state: battery temperature equals ambient temperature) is shown in Table 3. It can be seen from the table that at different ambient temperatures, the increment of target power duration varies, and the increment first increases and then decreases as the initial SOC decreases. The lower the ambient temperature, the greater the increment of target power duration.
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3.2.2 Preheating Power from Power Battery
When the preheating power of the power battery comes from the battery itself, the increment of target power duration at different ambient temperatures is shown in Table 4. It can be seen from the table that at different ambient temperatures, when the preheating power comes from the power battery, the increment of target power duration is not only related to the initial SOC of the power battery but also affected by the ambient temperature. The lower the ambient temperature, the higher the initial SOC at which preheating can increase the increment of target power duration.
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4 Effect of Preheating + Driving Heating on Power Battery Power Performance
The preheating function solves the problem of insufficient low-temperature power performance of the power battery to a certain extent. However, the preheating strategy needs to consider the source of preheating power, the initial SOC of the power battery, and the ambient temperature. Moreover, the preheating strategy is greatly affected by the user. If the user uses the vehicle at high SOC and stops using it without affecting the power performance, the power consumed by preheating will be wasted. In addition, the target temperature of preheating also affects the target power duration. Therefore, based on the actual needs of the project and the actual usage of the vehicle by users, this paper proposes a low-temperature thermal management scheme of preheating + driving heating (Scheme C) to improve the low-temperature power performance of the power battery.
4.1 Design of Preheating + Driving Heating Scheme
The discharge power of the power battery is affected by the battery temperature and SOC. Different battery temperatures and SOC correspond to different discharge powers. The preheating + driving heating scheme aims to maintain the target power of 80kW. During preheating, heating is started when the discharge power is less than 80kW, and stopped when the discharge power is greater than 85kW. During discharge, heating is started when the discharge power is less than 85kW, and stopped when the discharge power is greater than 90kW.
From Table 1, it can be seen that when the battery temperature is -10°C and the initial SOC is 40% or above, preheating is not required. Heating is only started during discharge when the discharge power does not meet the target value set by the heating strategy. Similarly, when the battery temperature is 0°C and the initial SOC is 30% or above, preheating is not required. This simplifies the preheating strategy to a certain extent and avoids the waste of preheating power due to short driving distances.
4.2 Target Power Duration of Preheating + Driving Heating Scheme
4.2.1 Target Power Duration When Initial SOC ≥ 40%
When the initial SOC of the power battery is 40% or above, since preheating is not required, the discharge condition does not need to consider the source of preheating power. The target power duration at this time is shown in Table 5. It can be seen from the table that the new scheme improves the target power duration of the power battery. Moreover, compared with the preheating scheme, the new scheme has a greater improvement in target power duration.
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During the discharge process of the power battery, when the discharge power meets the target power requirement, driving heating is started. The change in discharge power during the discharge process is shown in Figure 13. It can be seen from the figure that when the discharge power triggers the heating start power set by the preheating + driving heating strategy, the preheating + driving heating function is started. At this time, the discharge power of the power battery increases. The time from triggering the preheating + driving heating function to stopping the function is about 12 minutes, and the power battery power can meet the target power requirement during this process.
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4.2.2 Target Power Duration When Initial SOC is 30% and 20%
When the initial SOC of the power battery is 30% and 20%, the initial discharge power cannot meet the target power requirement, so preheating is necessary. At this time, the target power duration is analyzed according to the source of preheating power, as shown in Table 6.
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From Table 6, it can be seen that when using the charging pile power for preheating, the increment of target power duration is still greater than when using the battery's own power for preheating. Compared with no low-temperature heating, the preheating + driving heating scheme improves the target power duration of the power battery.
When the initial SOC of the power battery is 30% and 20%, the change in discharge power during the discharge process according to different sources of preheating power is shown in Figure 14. It can be seen from the figure that when the initial SOC of the power battery is 30%, starting the preheating + driving heating function during discharge can increase the discharge power. When the initial SOC of the power battery is 20%, starting the preheating + driving heating function can also increase the discharge power.
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5 Conclusion
This paper addresses the low-temperature power performance issue of power batteries. Based on the liquid cooling thermal management system commonly used in electric vehicles, a low-temperature thermal management scheme of preheating + driving heating is proposed for low-temperature heating of power batteries. A 1D simulation model of AMESim is established to verify the effects of the two schemes, preheating and preheating + driving heating, on the low-temperature power performance of power batteries. The power performance of power batteries in low-temperature environments is improved through appropriate low-temperature thermal management schemes. The specific conclusions are as follows.
(1) When using the preheating scheme for low-temperature heating of the power battery, the source of preheating power, the initial SOC of the power battery, and the ambient temperature all affect the increment of target power duration. Moreover, when the initial SOC of the power battery is high, the preheating scheme causes power waste.
(2) When using the preheating + driving heating scheme for low-temperature heating of the power battery, the target power duration of the power battery can be improved. In addition, the time from triggering the low-temperature thermal management strategy to stopping it is about 12 minutes, meeting the actual driving needs of users.
(3) Compared with the preheating function, the control logic of the driving heating + preheating function is simple and clear. When the initial power of the power battery meets the target power requirement, preheating is not performed. This scheme can better meet the actual driving needs of users and avoid power waste.
