A constant temperature and constant voltage lithium ion battery charging method with current constraint

Abstract

The application discloses a constant temperature and constant voltage type lithium ion battery charging method containing current constraint. The application is based on a nonlinear penalty term design technology, a proportional-integral-derivative control method and a constant temperature and constant voltage type charging mode. Firstly, according to an equivalent circuit model of a lithium ion battery, a battery voltage dynamic model and a thermal model are established; secondly, based on the battery thermal model and the proportional-integral-derivative control method, a reference current controller is designed; then, combined with the nonlinear penalty term design technology, a current constraint controller is proposed on the basis of the reference controller; finally, an overall control flow is given, and fast and safe charging is realized. The application is simple and efficient, not only has the advantages of the constant temperature and constant voltage charging method, such as good applicability, fast charging and convenient temperature rise control, but also can improve the current constraint capability of the charging system and enhance the reliability of the charging system.

Description

Technical Field

[0001] This invention relates to a constant temperature and constant voltage lithium-ion battery charging method with current constraint, belonging to the technical field of lithium-ion battery charging control. Background Technology

[0002] Among existing batteries, lithium-ion batteries have attracted widespread attention due to their advantages such as high energy density, long cycle life, low self-discharge, and no memory effect, and have been successfully promoted in fields such as electric vehicles, portable electronic products, and grid energy storage. In the application of lithium-ion batteries, charging technology inevitably needs careful consideration. During battery charging, the charging current must be properly selected; too small a charging current will result in slow charging and reduced charging efficiency, while too large a charging current will affect battery life and performance. Furthermore, battery temperature also needs close monitoring during lithium-ion battery charging; excessively high temperatures will accelerate battery capacity decay and shorten cycle life. To address these issues, researchers have developed various charging methods, including constant current / constant voltage charging, multi-stage constant current charging, pulse current charging, and sinusoidal ripple current charging, each with different focuses regarding charging time, charging efficiency, battery temperature rise, and cycle life.

[0003] Currently, the most widely used method is the constant current constant voltage charging method (reference: F. Hoffart, Proper care extends Li-ion battery life[J]. Power Electronics Technology, 2008, 34: 24-28). Its design principle is: in the initial stage of charging, a constant current is used for charging; when the battery voltage reaches the maximum allowable voltage, it switches to constant voltage charging until charging is complete. The constant current constant voltage charging method can meet the charging requirements of lithium-ion batteries to a certain extent, but it has some drawbacks: First, it is an open-loop charging method, and the predefined parameters are not applicable to every battery. Furthermore, battery aging and changes in ambient temperature can also cause the parameters to become inapplicable. Moreover, to ensure the safety and reliability of lithium-ion batteries, the selection of the charging current in the constant current stage is often conservative, resulting in a slower charging speed. The constant-temperature constant-voltage charging method proposed in the literature (L. Patnaik, AVJ SPraneeth, SSW Williamson, A closed-loop constant-temperature constant-voltage charging technique to reduce charge time of lithium-ion batteries[J]. IEEE Transactions on Industrial Electronics, 2019, 66(2): 1059-1067.) can better solve the above problems: it is a closed-loop charging method that can adjust the charging current in real time according to the battery condition and ambient temperature. Under a certain temperature rise, constant-temperature constant-voltage charging can achieve a faster charging speed than constant-current constant-voltage charging. However, the disadvantage of constant-temperature constant-voltage charging is that in the initial stage of charging, the charging current may be too large due to the large temperature difference, and the overcurrent directly affects the cycle life and performance of lithium-ion batteries. Summary of the Invention

[0004] The technical problem solved by this invention is: to address the overcurrent problem that occurs in the initial stage of charging due to the large temperature difference in the constant temperature and constant voltage charging method, a constant temperature and constant voltage lithium-ion battery charging method with current constraint is proposed.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention proposes a constant-temperature, constant-voltage charging method for lithium-ion batteries with current constraint, comprising the following steps:

[0007] Step 1: Based on the equivalent circuit model of a lithium-ion battery, establish a dynamic voltage model and a thermal model for the battery.

[0008] Step 2: Based on the battery circuit model and thermal model described in Step 1, a reference current controller is designed to achieve closed-loop constant temperature charging using the proportional-integral-derivative control method.

[0009] Step 3: To address the insufficient current constraint capability of the reference current controller described in Step 2, a current constraint controller is designed based on the reference current controller, incorporating nonlinear penalty term design techniques, to achieve safe and efficient constant temperature charging.

[0010] Step 4: Based on the real-time detected battery voltage and charging current values, build a charging control flowchart to achieve overall constant temperature and constant voltage charging.

[0011] As described above, the isothermal and constant-voltage lithium-ion battery charging method with current constraint further includes the following: the dynamic voltage model of the lithium-ion battery in step one is as follows:

[0012]

[0013] Among them, V t V is the battery terminal voltage. oc I is the battery open-circuit voltage. L The current is the battery load current, which is positive during charging. R0 is the internal resistance in ohms, R1 is the polarization internal resistance, C is the polarization capacitor, and V... p This is the voltage across the capacitor.

[0014] In the battery model, all parameters are functions of the battery's state of charge (SOC), which is defined as:

[0015]

[0016] Among them, SOC t This represents the SOC value of the battery at time t, where SOC0 represents the initial SOC of the battery. Q η is the nominal capacity of the battery, and η is the coulombic efficiency.

[0017] According to the equivalent circuit model, the charging energy loss P of the battery loss It can be calculated using the following formula:

[0018]

[0019] The heat generation of a battery is mainly considered in terms of its ohmic internal resistance, assuming a uniform temperature distribution inside the battery. Therefore, the thermal model of the battery can be expressed as:

[0020]

[0021] Among them, C iFor the internal heat capacity of the battery, C s For the surface heat capacity of the battery, T i T represents the internal temperature of the battery. s T represents the surface temperature of the battery. a R represents the ambient temperature. θis R is the thermal conductivity between the inside and surface of the battery. θsa It is the thermal conductivity between the battery surface and the environment.

[0022] As described above, in a current-constrained constant-temperature and constant-voltage lithium-ion battery charging method, the reference controller in step two is further defined as follows:

[0023]

[0024] For ease of implementation, the above-mentioned reference controller is designed in the discrete-time domain. The (n) after each variable represents the value at the current time, and (n-1) represents the variable value at the previous time. L (n) is the charging current calculated by the controller mentioned above, I p (n) is the proportional term in proportional-integral-derivative control, k p It is a positive proportional term controlling the gain, I i (n) is the integral term, k i It is a positive integral term gain, I d (n) is the differential term, k d It is a positive differential gain. e1(n) = T * (n)-T(n) is the temperature deviation, T * (n) is the preset battery reference temperature, and T(n) is the real-time battery temperature. e2(n) = e1(n) - e1(n-1).

[0025] As described above, in a constant-temperature and constant-voltage lithium-ion battery charging method with current constraint, the current constraint controller in step three is further defined as follows:

[0026]

[0027] Where l is the positive constraint gain, and M is the current constraint value. The difference between the current constraint controller and the reference controller in step two lies in the nonlinear term l / (M). 2 -I L (n-1) 2 When the charging current I at the previous moment... L (n-1) When the temperature difference is large and the current constraint boundary M is approached, the nonlinear term l / (M) 2 -I L (n-1) 2The current will become very large, and the controller will generate a corresponding control action to pull the current back away from the constraint boundary.

[0028] As described above, the constant temperature and constant voltage lithium-ion battery charging method with current constraint further includes the following charging control process in step four:

[0029] 1) Obtain battery parameter information, including nominal capacity, nominal voltage, maximum allowable charging voltage, and charging cut-off current;

[0030] 2) Initialize the control parameters, including the control gain k described in step two. p k i and k d The initial value of the integral term I i (0), initial temperature deviation e1(0), and current constraint value M and constraint term control gain l as described in step three;

[0031] 3) Real-time measurement and storage of charging current, charging voltage, and temperature;

[0032] 4) Determine if the battery voltage has reached the maximum allowable voltage. If so, the charger enters the constant voltage charging phase and sets the charging voltage V. ch The charger is charged according to V. ch Charge the battery.

[0033] 5) If it is determined that the battery voltage has not reached the maximum allowable voltage, the charger enters the constant temperature charging stage, calculates the temperature deviation e1(n), and calculates the charging current I through the current constraint controller described in step three. L (n) The charger charges the lithium-ion battery according to the charging current.

[0034] 6) Monitor the charging current in real time. If the current is less than the cutoff current, the charging task is completed and charging stops; otherwise, charging continues. Compared with existing technologies, the above technical solution of this invention has the following technical advantages:

[0035] 1) A closed-loop charging current controller is designed based on the constant temperature and constant voltage charging method and proportional-integral-derivative control technology, which enables the charger to adjust the charging current in real time according to the battery condition and ambient temperature, thereby improving the charging speed under a certain temperature rise.

[0036] 2) A current constraint controller based on nonlinear penalty term technology is designed to effectively suppress the overcurrent problem caused by large temperature difference in the initial stage of charging, improve the current constraint capability of the charging system, and enhance the safety and reliability of the charging system.

[0037] 3) The current-constrained lithium-ion battery charging control design concept proposed in this invention has good universality and can be applied to the charging control design of other types of batteries. It can be used in complex and varied scenarios and has good flexibility. Attached Figure Description

[0038] Figure 1 This is a diagram of the equivalent circuit model of a lithium-ion battery.

[0039] Figure 2 A thermal model diagram of a lithium-ion battery;

[0040] Figure 3 The curve showing the change of charging current of a lithium-ion battery with charging time;

[0041] Figure 4 This is a control block diagram of the method of the present invention;

[0042] Figure 5 This is a control flowchart of the method of the present invention. Detailed Implementation

[0043] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings:

[0044] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0045] The following describes the detailed design steps of the embodiments of the present invention with reference to the accompanying drawings.

[0046] Step 1: Establish dynamic and thermal models for battery voltage.

[0047] Figure 1 This is an equivalent circuit model diagram of a lithium-ion battery, where V... oc Indicates the battery open-circuit voltage, V t I is the battery terminal voltage. L Let R0 be the battery load current, which is positive during charging. R1 is the internal resistance in ohms, C is the polarization resistance, and C is the polarization capacitor. Based on the above circuit model, the dynamic model of the battery voltage can be obtained as follows:

[0048]

[0049] Among them, V p This is the voltage across the capacitor.

[0050] In the battery model, all parameters are functions of the battery's state of charge (SOC), which is defined as:

[0051]

[0052] Among them, SOC t This represents the SOC value of the battery at time t, where SOC0 represents the initial SOC of the battery. Q η is the nominal capacity of the battery, and η is the coulombic efficiency.

[0053] According to the equivalent circuit model, the charging energy loss P of the battery loss It can be calculated using the following formula:

[0054]

[0055] Figure 2 This is a thermal model diagram of a lithium-ion battery, where C i For the internal heat capacity of the battery, C s For the surface heat capacity of the battery, T i T represents the internal temperature of the battery. s T represents the surface temperature of the battery. a R represents the ambient temperature. θis R is the thermal conductivity between the inside and surface of the battery. θsa Let be the thermal conductivity between the battery surface and the environment. The mathematical expression for the battery thermal model is as follows:

[0056]

[0057] Step 2: For the battery circuit model, based on the proportional-integral-derivative control method, design a reference current controller. The reference current controller is designed as follows:

[0058]

[0059] To facilitate digital controller programming, the aforementioned reference controller is designed in the discrete-time domain. The (n) after each variable represents the value at the current time, and (n-1) represents the variable value at the previous time. L (n) is the charging current calculated by the controller mentioned above, I p (n) is the proportional term in proportional-integral-derivative control, k p It is a positive proportional term controlling the gain, I i (n) is the integral term, k i It is a positive integral term gain, I d (n) is the differential term, k d It is a positive differential gain. e1(n) = T *(n)-T(n) is the temperature deviation, T * (n) is the preset battery reference temperature, and T(n) is the real-time battery temperature. e2(n) = e1(n) - e1(n-1).

[0060] Step 3: Design a current constraint controller

[0061] During constant temperature and constant voltage charging, the charging current may be excessive at the beginning of charging due to the large temperature difference, which directly affects the cycle life and performance of the lithium-ion battery. The change in charging current with charging time is generally as follows: Figure 3 As shown, the acceptable charging current of a lithium-ion battery decreases rapidly with increasing charging time. If the charging current exceeds this acceptable current, a decomposition and reduction reaction will occur inside the lithium-ion battery, releasing a large amount of gas and causing irreversible damage. Therefore, current constraint control is essential.

[0062] Starting from the baseline controller and combining it with nonlinear penalty term design techniques, the following current-constrained controller is proposed:

[0063]

[0064] Where l is the positive constraint gain, and M is the current constraint value. The difference between the current constraint controller and the reference controller in step two lies in the nonlinear term l / (M). 2 -I L (n-1) 2 When the charging current I at the previous moment... L (n-1) When the temperature difference is large and the current constraint boundary M is approached, the nonlinear term l / (M) 2 -I L (n-1) 2 The current will become very large, and the controller will generate corresponding control actions to pull the current back away from the constraint boundary. The closed-loop system control block diagram is as follows: Figure 4 As shown.

[0065] Step 4: Design the charging control flowchart

[0066] Figure 5 The specific charging control flowchart is shown in the figure. The specific charging process is as follows:

[0067] 1) Obtain battery parameter information, including nominal capacity, nominal voltage, maximum allowable charging voltage, and charging cut-off current;

[0068] 2) Initialize the control parameters, including the control gain k described in step two. p k i and k d The initial value of the integral term I i(0), initial temperature deviation e1(0), and current constraint value M and constraint term control gain l as described in step three;

[0069] 3) Real-time measurement and storage of charging current, charging voltage, and temperature;

[0070] 4) Determine if the battery voltage has reached the maximum allowable voltage. If so, the charger enters the constant voltage charging phase and sets the charging voltage V. ch The charger is charged according to V. ch Charge the battery.

[0071] 5) If it is determined that the battery voltage has not reached the maximum allowable voltage, the charger enters the constant temperature charging stage, calculates the temperature deviation e1(n), and calculates the charging current I through the current constraint controller described in step three. L (n) The charger charges the lithium-ion battery according to the charging current.

[0072] 6) Monitor the charging current in real time. If the current is less than the cutoff current, the charging task is completed and charging is stopped; otherwise, continue charging.

[0073] The above description is only a partial embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for charging a constant-temperature and constant-voltage lithium-ion battery with current constraint, characterized in that the steps include... include: Step 1: Based on the equivalent circuit model of a lithium-ion battery, establish a dynamic voltage model and a thermal model for the battery. Step 2: Based on the battery circuit model and thermal model described in Step 1, a reference current controller is designed to achieve closed-loop constant temperature charging, using the proportional-integral-derivative control method. The reference current controller is in the following form: ； For ease of implementation, the aforementioned reference controller is designed in the discrete-time domain, with each variable... This represents the value at the current moment. Represents the value of the variable at the previous moment; The charging current is calculated by the reference controller. It is the proportional term in proportional-integral-derivative control. It is a positive proportional term controlling the gain. It is an integral term. It is a positive integral term gain. It is a differential term. It is a positive differential gain. It is the temperature deviation at the current moment. The preset battery reference temperature, Real-time battery temperature. It is the change in temperature deviation within a sampling period. It is the temperature deviation from the previous moment; Step 3: To address the insufficient current constraint capability of the reference current controller described in Step 2, a current constraint controller is designed based on the reference current controller, incorporating nonlinear penalty term design techniques, to achieve safe and efficient constant temperature charging. The current constraint controller is: ； in, It is a positive constraint term gain. The current constraint value differs from the baseline controller in step two in that it has a nonlinear term. When the charging current at the previous moment Approaching the current-constrained boundary due to a large temperature difference When, nonlinear term It will become very large, and the controller will generate corresponding control actions to pull the current back away from the constraint boundary; Step 4: Based on the real-time detected battery voltage and charging current values, build a charging control flowchart to achieve overall constant temperature and constant voltage charging.

2. The method for charging a constant-temperature and constant-voltage lithium-ion battery with current constraint according to claim 1, characterized in that: The dynamic model of lithium-ion battery voltage in step one is as follows: ； in, This refers to the battery terminal voltage. This is the battery open-circuit voltage. This is the battery load current; the current is positive during charging. For ohmic internal resistance, For polarization internal resistance, Polarized capacitor, The voltage across the capacitor; In the battery model, all parameters are functions of the battery's state of charge (SOC), which is defined as: ； in, This represents the SOC value of the battery at time t. This represents the battery's state of charge (SOC) at the initial moment. This refers to the battery's nominal capacity. For Coulomb efficiency, For integration variables; According to the equivalent circuit model, the charging energy loss of the battery The following formula is used to calculate: ； The heat generation of the battery is considered in terms of its ohmic internal resistance, and it is assumed that the internal temperature distribution of the battery is uniform; therefore, the thermal model of the battery is expressed as: ； in, For the internal heat capacity of the battery, For the surface heat capacity of the battery, This refers to the internal temperature of the battery. The surface temperature of the battery. For ambient temperature, The thermal conductivity between the battery's interior and surface. It is the thermal conductivity between the battery surface and the environment.

3. The method for charging a constant-temperature and constant-voltage lithium-ion battery with current constraint according to claim 1, characterized in that: The charging control process in step four is as follows: 1) Obtain battery parameter information, including nominal capacity, nominal voltage, maximum allowable charging voltage, and charging cut-off current; 2) Initialize the control parameters, including the control gain described in step two. , and Initial value of the integral term Initial value of temperature deviation and the current constraint value described in step three and constraint terms control gain ; 3) Real-time measurement and storage of charging current, charging voltage, and temperature; 4) Determine if the battery voltage has reached the maximum allowable voltage. If so, the charger enters the constant voltage charging phase and sets the charging voltage. The charger is in accordance with Charge the battery; 5) If the battery voltage is determined not to have reached the maximum allowable voltage, the charger will enter the constant temperature charging stage and the temperature deviation will be calculated. The charging current is calculated using the current constraint controller described in step three. The charger charges the lithium-ion battery according to the charging current; 6) Monitor the charging current in real time. If the current is less than the cutoff current, the charging task is completed and charging is stopped; otherwise, charging continues.

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