Shortening the charging time is of great significance to accelerate the development of electric vehicles. Ultra-fast charging technology (XFC) can shorten the charging time of the battery to less than 10 minutes. However, the lithium battery will cause rapid heat generation under ultra-fast charging conditions, which may cause the battery to overheat, so as to shorten the battery life and bring safety hazards. Therefore, it is necessary to understand the thermal behavior of batteries under fast charging conditions. Here, the University of Alabama and the Oak Ridge National Laboratory of the United States, by implanting micro-thermocouples inside the battery, measured in situ the ultra-low temperature of the 2Ah LiNi0.6Co0.2Mn0.2O2/graphite soft-pack lithium battery at a rate of 7C. The internal temperature during fast charging examines the difference between the internal temperature and the surface temperature, and estimates the heat generation rate during charging. The effect of the cooling method is also discussed. Finally, the interesting phenomenon of battery voltage drop during the ultra-fast charging phase is discussed.
The figure below is a schematic diagram of the thermocouple implanted in the experiment. The diameter of the metal wire used by the two miniature thermocouples is 80um. One of the thermocouples is implanted in the center of the cell, and the other thermocouple is placed at the interface between the cell and the soft casing. In addition, a thermocouple is pasted on the outer surface of the soft pack lithium battery to monitor the temperature of the battery surface. Before each charge, the battery was discharged to 2.8V at 1.0A (0.5C), and then stood for 30min. Charge in CCCV mode at room temperature (23±1°C), and the cut-off current of constant voltage charging is 0.1A (corresponding to 0.05C). The battery is placed horizontally in the incubator with the thermocouple facing up. Cooling is done in two ways: forced convection and natural convection. The temperature of the thermostat was set to 23°C for forced convection, and then the air was circulated by an internal fan to cool the lithium battery by forced convection. The incubator does not work during natural convection, and the lithium battery is naturally cooled by the air in the incubator.
The figure below shows the comparison results of the experimental battery implanted with the sensor and the control battery without the implanted sensor. Where a is the charging voltage and b is the surface temperature. The voltage and surface temperature of the experimental group and the control group are very close. When charging at 5C, the voltage of the battery in the experimental group was slightly higher than that in the control group, which may be attributed to the slight increase in impedance after the sensor was implanted.Also read:https://www.aimeno.com/lithium-battery-pack/197.html
The figure below shows the current, voltage, SOC and temperature changes of lithium batteries when they are charged at different rates when forced convection cooling is used. Figure a shows that when the battery voltage starts to stabilize at 4.2V, the current drops rapidly. It only takes 280s for 7C to reach the cut-off voltage of 4.2V, while the time required for 5C, 3C and 1C is 450s, 860s and more than 3000s respectively. Figure b shows that it takes only 7.5 minutes to charge to 80% SOC at a rate of 7C, and it takes 10 minutes to charge to 80% SOC at a rate of 5C. However, it takes more than 16min and 50min for 3C and 1C rate charging to reach 80% SOC. Although 5C or 7C rate charging can greatly shorten the charging time, the temperature increase and speed of the battery are much higher than 1C rate charging.
After charging at 7C for 300s, the temperature at the center of the battery increased by 22.5°C, while charging at 1C for 300s, the temperature at the center of the battery increased by less than 0.5°C, and the temperature at the center of the battery increased by less than 1.5°C during the entire charging process. Charged at a faster rate, the thickness-wise temperature gradient from the center of the cell to the surface and ambient is also greater. The temperature gradient is less than 0.2°C when charging at 1C rate, and reaches 3.4°C when charging at 7C rate. Embedded miniature thermocouples add additional thermal resistance, creating artificially larger temperature gradients. Therefore, thinner temperature sensors or non-intrusive internal temperature measurement techniques should be used to eliminate this effect.
The above discussion found that the temperature rise was higher and faster when charging at 7C and 5C, which was attributed to the higher heat generation rate. By ignoring the mixed heat effect, the heat generation of the battery can only be expressed as the sum of irreversible heat and entropy heat, as shown in Equation 1:
Among them, I is the current, which is a positive value when charging; V is the battery voltage, U is the open circuit voltage, T is the absolute temperature (K),
is the entropy change coefficient, I(V-U) is the irreversible heat production rate,
is the reversible heat production rate.
The figure below shows the results of heat generation estimation during charging, including the SOC-OCV curve measured by the control group battery, the entropy change coefficient of the control group battery at different SOC, the irreversible heat generation, reversible heat generation, total heat generation and average heat generation of the experimental battery Heat production. Under the same SOC, the open circuit voltage of the battery changes little with temperature (the entropy change coefficient is very small), between -0.39mV/K and 0.09mV/K, so the open circuit voltage at room temperature is used to estimate the irreversible heat production, which brings The error can be ignored. When charging at 1C, the irreversible heat generation is less than 0.3W, while at 7C and 5C charging, the irreversible heat generation increases sharply to 9.1W and 4.6W. After reaching the maximum value, the rate of irreversible heat generation starts to decrease, which is attributed to the decrease of battery internal resistance with increasing temperature. When the battery voltage reaches the upper limit, the current begins to decrease rapidly, and the rate of irreversible heat generation also decreases accordingly.
At the beginning of charging, the reversible heat generation is negative and then gradually increases to positive. When charging at 5C or 7C, the amplitude of the reversible heat production rate is much smaller than that of the irreversible heat production rate, but at the beginning of charging, the reversible heat production rate has a significant impact on the total heat production rate. From the curves of the average irreversible, reversible and total heat production rates, it can be seen that the average total heat production rate of 7C charging is as much as 34 times that of 1C charging.
Then the authors investigated the effect of cooling methods on the temperature rise and temperature gradient of the battery. The figure below shows the electrochemical and thermal behavior of the battery when charging at 3C and 5C under natural convection, including current, voltage, SOC and temperature curves. Under natural convection, the maximum temperature rise of the battery surface during 5C charging reaches 20.6°C, which is higher than the maximum temperature rise during 5C charging under forced convection cooling (only 13°C), and even higher than the maximum temperature rise during 7C charging under forced convection cooling. Lift. This comparison shows that the temperature rise of the lithium battery during super fast charging can be controlled by a cooling method similar to forced convection. In addition, the temperature gradient for forced convection cooling is also greater than for natural convection because forced convection results in faster heat dissipation.
By carefully observing the test results, the author found that the battery voltage will drop significantly when charging at 7C under forced convection cooling and at 5C charging under natural convection cooling. For easier discussion, the figure below plots the battery charging voltage and open circuit voltage as a function of SOC in two cases. According to Equation 1, during constant current charging, when the open-circuit voltage U of the battery increases with the increase of SOC, the battery voltage V decreases, indicating that the rate of heat generation decreases. A lower rate of heat generation results in a slower temperature rise. The transient voltage drop phenomenon during fast charging is attributed to the reversal effect of open circuit voltage and internal resistance. A higher SOC during charging leads to an increase in the open circuit voltage U (shown in the figure a below), while a higher temperature leads to a decrease in the internal resistance of the battery (shown in the figure b below).Also read:https://www.aminobattery.com/lithium-battery/204.html
The battery is heated/cooled to a specific temperature by an incubator, and when the temperature of all batteries is stable, 0.5C (1A) is used for pulse charging/discharging. If the effect of the open circuit voltage is less than the internal resistance of the battery, the battery voltage will drop. Under the two cooling methods, the voltage of the battery starts to drop from ~22%SOC to ~40%SOC. The open-circuit voltage of the battery in this SOC interval increases slowly with the SOC. If the temperature of the battery increases rapidly at this time, the impact of the internal resistance of the battery will exceed the open-circuit voltage, resulting in a decrease in the battery voltage. From 22% SOC to 40% SOC, under forced convection cooling, the temperature at the center of the 7C rechargeable battery increased by 6.1°C, while the temperature rise of the 5C rechargeable battery was 6.8°C under natural convection cooling. The temperature rise of the 5C rechargeable battery during forced convection cooling is only 3.8°C, which is why the battery voltage temporarily drops. After exceeding 40% SOC, the open circuit voltage increases rapidly with the increase of SOC, and takes the lead, causing the charging voltage of the battery to increase again until it reaches the upper limit voltage.