Lithium batteries are being used more and more in the field of electric vehicles due to their high specific energy, long cycle life, and lightweight. However, the low-temperature performance of lithium-ion batteries is not ideal. For example, when a lithium iron phosphate battery is charged at -20°C, the actual capacity is only 70% of the capacity under the same charging condition at 25°C. Even for a lithium manganate battery with a good low-temperature effect, the charging capacity at a low temperature of -20°C is only about 80% of that at room temperature. The low-temperature environment reduces the effective use of lithium batteries and limits the promotion and application of lithium-ion power batteries in cold northern regions.
Preheating the lithium battery is one of the effective methods to solve the problem of low-temperature characteristics of the lithium battery. Using the lithium battery's own power to provide power for heating equipment is a common battery heating method, but this method will consume a considerable part of the battery energy during the heating process, which reduces the effective discharge capacity of the lithium battery to a certain extent.
Before charging or using the lithium battery in a low-temperature environment, the battery must be preheated. The way that the battery management system (BMS) of the electric vehicle heats the lithium battery can be roughly divided into two categories: external heating and internal heating. External heating methods include air heating, liquid heating, phase change material heating, and heat resistance heater or heat pump heating. These heating methods are generally located in the battery pack or set in the container of the thermal cycle medium. The internal heating method heats the battery by exciting the electrochemical substances inside the battery through alternating current so that the battery itself generates heat.
External heating
Regarding the method of heating with air, some researchers have conducted experiments using batteries and a set of atmospheric simulation systems. The experimental results show that batteries with heated surrounding air can release more capacity than batteries exposed in a low-temperature environment.
Compared with air heating, liquid heating has better thermal conductivity and higher heat conversion efficiency. But liquid heating requires a more complicated heating system. There have been many practical cases of the application of liquid heating in electric vehicles and hybrid vehicles. For example: In a Chevrolet Volt car, the heat exchange fluid surrounding the battery pack is heated by a 360V heater.
Phase change material heating batteries have also been used. When the battery temperature drops to the phase change temperature point of the phase change material, the heat stored in the phase change material will be released to keep the ambient temperature constant, which is to transfer heat to the battery pack. The main advantage of phase change materials is that they can be used in environments with relatively rapid temperature changes.
Internal heating
Compared with external heating, AC excitation heating is another commonly used heating method, which is simpler in structure design, which is to heat the battery through alternating current. It does not need to design the heat transfer structure, just load a certain frequency of AC excitation on the positive and negative electrodes of the battery, and the excitation acts on the electrochemical substances in the battery, which is equivalent to the effect of cyclically reciprocating small-amplitude charging and discharging.
Compared with direct current heating current, alternating current or positive and negative square wave current can heat the battery during both the discharge and charge cycles, causing the battery temperature to rise, while the battery state of charge (SOC) is basically unchanged. Due to these characteristics, the AC internal preheating method has become a field of more research. In 2004, a foreign researcher first proposed to use alternating current to directly heat the lithium-ion battery, and only use the internal resistance effect of the battery to generate heat. They did some tests on different batteries in different SOC states and at different temperatures (-20℃~40℃). The test results show that under a certain rate of current, all batteries will quickly generate heat.
A team in the United States conducted a study on the influence of heating frequency on heating effect. They conducted simulations at different frequencies from 0.01Hz to 2KHz and compared the results with external heating methods. They believed that internal heating has obvious advantages.
Compared with external heating methods, internal heating avoids long-path heat conduction and the formation of local hot spots near the heating device. Therefore, internal heating can heat the battery more uniformly with higher efficiency to achieve a better heating effect and easier to achieve.
Most of the program research focuses on heating speed and efficiency, and the heating strategy has few clear considerations for preventing the occurrence of side reactions such as lithium deposition. To prevent the generation of lithium deposition during the preheating process, BMS needs to be able to estimate and control the conditions of lithium deposition in real-time. A model-based control battery heating technology at low temperatures is needed to achieve the above functions. With the development of new energy vehicles, the use of power lithium batteries is also increasing day by day. The use of lithium batteries at low temperatures urgently needs to solve the problem of battery preheating, which is a field very close to practical applications.
In addition, AC heating, which mobilizes electrochemical substances to produce movement, has an impact on the service life of the battery. I have not yet seen what conclusions have been obtained, which is also a problem worthy of continuous attention.
Lithium battery charging and preheating device: battery box, power management system and charger
The lithium battery or lithium battery module is located in the battery box, and is characterized in that: a resistance heating wire and a temperature detector are arranged in the battery box; the power management system includes: a heating circuit switch, a charge and discharge protection switch, and a temperature measurement circuit , Heating system control circuit, power management system microprocessor, charge and discharge switch control circuit and power management system communication port, the input end of the temperature measurement circuit is connected to a temperature detector, and the output end of the temperature measurement circuit is connected to the power management system microprocessor, One output terminal of the power management system microprocessor is connected to the control terminal of the heating circuit switch through the heating system control circuit, and the other output terminal of the power management system microprocessor is connected to the control terminal of the charge and discharge protection switch through the charge and discharge switch control circuit
The heating circuit switch is connected between the resistance heating wire-terminal and the negative output terminal of the charger, and the charging and discharging protection switch is connected between the negative electrode of the lithium battery or lithium battery module and the negative output terminal of the charger, and the resistance heating The other end and the positive electrode of the lithium battery or lithium battery module is connected to the positive output terminal of the charger;
The communication port of the power management system is connected to the microprocessor of the power management system, and is externally connected to the communication port of the charger; the charger includes a voltage regulation and switching circuit, a constant current source, conversion, and detection circuit, a charger communication port, and Charger microprocessor, the input end of the voltage regulation and switches circuit is connected to the mains, the output end of the voltage regulation and switch circuit is directly connected to the conversion and detection circuit on the one hand and is connected to the conversion and detection circuit through a constant current source, on the other hand, The output of the conversion and detection circuit is connected to the positive and negative output terminals of the charger, and connected to the input terminal of the charger microprocessor. The output terminal of the charger microprocessor is connected to the input terminal of the voltage regulation and switching circuit. The processor is connected to the charger communication port.
