In recent years, electric vehicles have developed rapidly, and as a key component of electric vehicles, power batteries have also undergone rapid technological development. The performance and service life of power batteries directly determine the performance and cost of electric transmission vehicles. At present, the main power batteries used in electric drive vehicles are lead-acid batteries, Nickel–cadmium battery, Nickel–metal hydride battery, lithium ion batteries and supercapacitor batteries. Lithium ion power batteries have gradually replaced lead-acid batteries, Nickel–cadmium battery and Nickel–metal hydride battery as the main power batteries for electric drive vehicles because of their advantages such as high specific power, large energy density, long life, low self discharge rate, long storage time and no pollution.
The power battery used in electric vehicles is composed of multiple battery cells arranged in series and parallel to form a battery pack. The battery cells are tightly arranged together, and during charging and discharging, each cell generates a large amount of heat. The heat generated by the battery cells will affect each other. If the heat dissipation is uneven, it will cause a rapid increase in local temperature of the battery pack, deteriorate the consistency of the battery, and greatly shorten its service life, In serious cases, Thermal runaway of some single batteries will be caused, resulting in relatively serious accidents.
At the same time, when the power battery is in a low temperature environment, the charge and discharge performance of the lithium-ion battery will significantly decline. The manufacturers and professionals of the lithium-ion battery have carried out a lot of research work on this. At present, it is generally believed that the low-temperature performance of the lithium-ion battery is due to SEI film, Surface charge transfer impedance, lithium ion diffusion in the electrode and other reasons, but the main factors affecting the low-temperature performance of the battery have not been determined, During the charging and discharging process of batteries at low temperatures, heat is also generated. Whether this heat can help restore battery performance has not yet been studied in relevant literature. Therefore, it is necessary to conduct relevant research on the thermal characteristics of batteries.
1 Battery Charging and Discharging at Room Temperature
This section investigates the heat generation characteristics of a 35Ah lithium manganese oxide battery during charging and discharging under natural heat dissipation conditions. The battery cells are suspended in a space without forced heat dissipation, under natural heat dissipation conditions. During the battery charging and discharging process, a 16 channel temperature measurement system is used to measure the battery temperature.
1.1 Heat generation characteristics of battery discharge
Under natural heat dissipation environment, discharge the batteries at rates of 0.3C, 0.5C, 1C, 2C, 3C, and 4C, respectively. Firstly, suspend the battery in an environment without forced heat dissipation at room temperature. Before discharging, charge the battery at a constant current to constant voltage rate of 1C/3, and let it stand for 2 hours after fully charging; Then perform constant current discharge at a certain rate, with a cut-off voltage of 3V. Due to the fact that the experiment was conducted in a natural heat dissipation environment, the room temperature varies slightly at different time periods. For ease of comparison, the starting temperature of the battery was uniformly set at 20 ℃ during the drawing process.
From the experimental results, it can be seen that during the discharge process, the temperature of the positive ear of the battery is slightly higher than that of the negative ear, and this trend is more obvious during high rate discharge. As the discharge rate of the battery increases, the temperature of the positive and negative ears of the battery rapidly increases. When discharged at 0.3C, the temperature of the positive ear of the battery increases from 20 ℃ to 21.9 ℃, only increasing by 95%;
When discharging at 1C, the temperature of the positive electrode ear of the battery increased from 20 ℃ to 24.3 ℃, an increase of 21.5%; When discharging at 2C, the temperature of the positive electrode ear of the battery increased by 48% from 20 ℃ to 29.6 ℃; When discharging at 4C, the temperature of the positive electrode of the battery increased from 20 ℃ to 36.96 ℃, an increase of 84.8%. Therefore, in the high temperature environment, when the battery is discharged at a large rate, corresponding heat dissipation measures must be taken, otherwise the battery will be overheated, resulting in performance degradation, shortened life, and even a dangerous state of Thermal runaway.
Under different discharge rates, the temperature rise trend of the battery body is the same as that of the positive and negative electrode ears: the temperature rise is faster in the early stage of discharge, slows down in the middle stage, and rises rapidly again in the later stage of discharge.
1.2 Heat generation characteristics of battery charging
Similar to the discharge temperature rise experiment, during the charging temperature rise experiment, the battery is suspended in an environment without forced heat dissipation. Firstly, the battery is discharged at a constant current rate of 1C/3, with a cut-off voltage of 3V. After the discharge is completed, it is allowed to stand for 2 hours.
Then, constant current constant voltage charging is carried out at a rate of 0.3C, 0.5C, 1C, 2C, 3C, and 4C, respectively. The temperature difference between the positive and negative ears of the battery during charging is smaller than when discharging at the same rate. During the constant current charging process, the temperature of the positive and negative ears of the battery increases rapidly; During the constant voltage charging stage, the temperature of the battery ears begins to decrease, mainly due to the continuous decrease in charging current and the decrease in battery heat generation rate. Therefore, in the process of constant current and constant voltage charging of batteries, the constant current charging process is an important stage of internal heat accumulation in batteries.
The temperature of the front and back sides is almost equal, and the temperature rise trend of the battery body is the same as that of the positive and negative ears. Through the above experiments on battery charging and discharging heat generation in a room temperature environment without forced heat dissipation, it can be seen that during high rate charging and discharging, the temperature of the battery rapidly increases, which can easily cause battery damage and even dangerous working conditions. Therefore, electric vehicles must be equipped with a heat dissipation system to dissipate heat from the battery, in order to control the battery temperature within a reasonable range.
2 Battery Discharge at Low Temperature
Due to the fact that the low-temperature charging and discharging test is conducted in temperature test chamber, it is not possible to analyze the heat generation of the battery by directly measuring the surface temperature of the battery, and can only be analyzed based on the charging and discharging curve of the battery.
The battery is almost unable to charge with high current at low temperatures. In the experiment, constant currents of 35A and 70A were used to charge the battery. Below 0 ℃, the backend voltage of the battery immediately rises to 4.2V, and then enters the constant voltage charging stage. At this time, the charging current of the battery is relatively small, so there is no obvious heating phenomenon. Batteries can undergo short-term high current discharge at low temperatures, so in-depth research can be conducted on the heat generation during discharge of low-temperature batteries.
To study the heat generation of batteries at low temperatures, the batteries were placed in a low-temperature environment and subjected to constant current discharge at the same rate. The battery is first charged at a constant current to constant voltage rate of 1C/3C at room temperature, fully charged, and then left in a temperature chamber for 5 hours.
Then, it is discharged at a constant current rate with a cut-off voltage of 3V. Within the temperature range of 0 ℃~-40 ℃, discharge at constant currents of 10A, 35A, 70A, and 140A, respectively. In order to compare with room temperature discharge, the discharge situation of the battery at 20 ℃ is shown in the figure, and the same method will be used for subsequent treatment.
The experimental results indicate that:
(1) When discharging at low current, the heat generation of the battery is not significant, and there is no significant fluctuation in the discharge curve of the battery at different low temperatures.
(2) When discharging at low temperature and high current, there is significant heat generation in the battery, as the discharge curve of the battery exhibits a nonlinear state with obvious valley and peak shapes, and the discharge voltage fluctuates greatly.
Taking 70A constant current discharge as an example, when discharging at 20 ℃ and 0 ℃, the discharge curve is relatively normal without any valley peaks. When the ambient temperature is -10 ℃, the discharge curve shows obvious valleys. When the ambient temperature is -20 ℃, the discharge curve shows obvious valley peaks. The voltage at both ends of the battery drops from 4.15V before discharge to 3.07V, with a voltage drop of 108V. Subsequently, the voltage begins to rise, reaching a maximum of 3.35V, and then begins to decrease. This indicates that during high current discharge at low temperatures, during the initial stage of discharge, due to the low temperature of the battery, the active substances in the battery cannot be fully utilized, the electrode polarization is severe, and the internal resistance of the battery is high.
Therefore, the discharge voltage of the battery rapidly decreases during the initial stage of discharge. As the discharge progresses, due to the high internal resistance of the battery, a large amount of heat is generated inside the battery, causing the temperature of the battery to rapidly rise, activating the active material part of the battery. Therefore, the discharge voltage of the battery begins to rise. As the temperature of the battery increases, the internal resistance of the battery begins to decrease, and the heat generated decreases. As the ambient temperature remains at -20 ℃, the temperature of the battery decreases, and the discharge voltage of the battery also decreases.
(3) In low-temperature environments, as the discharge current increases, the performance of the battery improves more significantly after being heated. Therefore, it can be seen that at low temperatures, if the power battery is pre heated, relying on the heat generated during the battery’s operation can fully maintain the battery’s performance.
As the charging and discharging current increases, the battery temperature rapidly increases. Therefore, the discharge rate of the power battery needs to be controlled to a certain extent, and it is not advisable to conduct high rate discharge for a long time. In high environmental temperatures or high rate discharge, corresponding heat dissipation needs to be used. Therefore, electric vehicles need to install a battery heat dissipation system to control the battery temperature within a reasonable range.
In low-temperature environments, the heat generated during battery discharge can be utilized to improve the low-temperature performance of the battery. When designing a battery heating system, this feature can be utilized by simply considering pre heating the battery.