With the promotion and popularization of electric vehicles, more and more power batteries used in electric vehicles, power lithium-ion battery is facing a big problem is the rapid decline in winter mileage, which is mainly related to the characteristics of lithium-ion battery. Lithium-ion batteries at low temperatures, the dynamic conditions are poor, so the capacity, magnification performance, especially the charging performance has greatly declined, which in the cold winter in the north is particularly serious, so in order to ensure that the electric car in the winter The use of performance, the battery pack will generally increase the heating system, but the current heating system is relatively low efficiency (usually takes tens of minutes to heat the battery), seriously affecting the convenience of the use of electric vehicles, so the lithium-ion battery at low temperatures Thermal management problem is that we in the battery design, battery pack design process need to focus on the issue.
Recently, the University of Pennsylvania, the University of Pennsylvania, etc. proposed a lithium-ion battery based on the internal heating of the battery pack thermal management strategy, in a short time to restore the lithium-ion battery to room temperature performance, for example, at -40 ℃ only need 112s Making the performance of lithium-ion battery completely restored, the technology also makes the electric vehicle mileage significantly increased, for example, at -40 ℃ to make electric vehicles mileage increased by 49%.
The core of the technology is self-heating battery, the battery design as shown above, the need for the internal placement of two Ni film, each piece of Ni chip impedance of 78mW, respectively, placed in the battery 1/4 and 3/4 thickness , Two pieces of Ni pieces are connected in parallel and connected together by a switch and positive and negative terminals for controlling the heating of the lithium ion battery. On the basis of the self-heating battery described above, Program, according to the electric car load conditions to control the heating switch on and off.
We know that the use of electric vehicles in the process, in addition to normal driving, but also the occurrence of braking, etc., electric vehicles in the braking process will generally be energy recovery, lithium-ion battery temperature is sufficient, this part of the energy will be stored directly In the lithium-ion battery, but when the lithium-ion battery temperature is too low, in order to prevent lithium-ion battery charging at low temperatures caused by negative electrode surface lithium, this part of the energy is usually wasted. Self-heating battery self-heating characteristics , The design of a low temperature braking energy recovery management program, the battery temperature is low, the recovery of energy will be used for heating the battery first, when the battery temperature to the appropriate temperature, and then the energy will be stored to the battery.
The following figure shows the change of current and power during the operation of the lithium ion battery. It can be noted from Figure d that the battery current is the same when the external discharge current exists, only when the battery is charged and the battery is in the standby state When the manager will use the internal current to heat the lithium-ion battery, the power curve also shows the same characteristics, which indicates that the management strategy does not affect the use of electric vehicles at the same time the strategy also makes the brake recovery of energy Full application, the first is used in the battery heating, and then the battery is charged. Figure c for the battery temperature and internal resistance diagram, from the figure can be noted that the heating inside the battery due to the characteristics of rapid heating, the battery temperature from -40 ℃ to 10 ℃, only 112s, while the battery resistance is also rapidly from 125mW down to 10 mW, which for the rapid recovery of low-temperature lithium-ion battery performance is closely related to such a fast Of the heating rate of lithium-ion batteries from 0, -10, -20 and -30 ℃ to 10 ℃ only need 13s, 33s, 46s and 56s, which for the use of electric vehicles in the winter with heavy Meaning.
The following figure shows the use of self-heating batteries and the use of ordinary battery control group at -40 ℃ in the course of the use of current, power, battery temperature and battery voltage and other data curve, the left side of the self-heating battery with the experimental group, the right is blank In the control group, the temperature rise rate of the self-heating battery was much faster than that of the control group, and the experimental group returned to 20 ℃ with 112s To achieve the recovery of all the braking energy, while the control group of batteries with 3000s, the temperature was restored to 0 ℃, resulting in can not recover the brake energy.
The following figure shows the ratio of the energy consumed by the various groups during the use of the experimental and control batteries. It can be seen that the energy of the experimental group can be greatly increased compared to the control group, which is important Part of the reason is due to the experimental group of the battery temperature is very fast, so the recovery of the braking energy is much more than the control group, thus significantly improve the car driving mileage.
The figure below shows the energy change curve that can be used for driving at different temperatures in the self-heating test group. As can be seen from the figure, at -30, -20, -10 and 0 ° C, the battery can be used for The driving energy is 78%, 80%, 85% and 90% of the total temperature.Atother self-heating battery design and management strategy for the future high specific energy battery is still very effective, the calculation shows that when the specific energy of lithium-ion battery To 300Wh / kg, at -40 ℃ under the battery for heating energy from 8.7% to 4.8%, heat loss can be reduced from 11.9% to 6.6%, can be used for driving energy from 74% 85%, by further improving the thermal insulation properties of the battery, but also reduce the battery can be used to drive the energy from 85% to 94%.
The use of self-heating battery to quickly heat the characteristics and with the corresponding management strategy can make the lithium-ion battery idle time quickly from low temperature to restore the room temperature, thus helping the lithium-ion battery to quickly restore the electrochemical performance, and does not affect the electric The normal driving of the car is of great significance for improving the ease of use of the electric vehicle in the alpine region, and the rapid recovery of the lithium ion battery temperature means that the lithium ion battery can be charged in a short time to achieve Full and effective recovery of the braking energy, which can also significantly increase the lithium-ion battery can be used to drive the energy of electric vehicles, which significantly improve the life of electric vehicles mileage. In general, Guangsheng Zhang designed battery management strategy, make full use of the The characteristics of the heating battery, for the promotion of electric vehicle use convenience and increase the electric car in the low temperature mileage is of great significance.
