Electric vehicle battery operation characteristic modeling method considering low temperature influence

Abstract

The invention relates to the field of battery modeling, and discloses an electric vehicle battery operating characteristic modeling method considering low-temperature influence, which comprises the following steps: calculating operating temperatures of an electric vehicle battery in idle, discharging, charging and preheating states; establishing a relationship between the operation temperature of the battery of the electric vehicle and the energy availability; electric vehicle battery operation constraints in the preheating process are considered immediately; carrying out linearization processing on the electric vehicle battery operation characteristic model; the method has the beneficial effects that the energy of the available electric vehicle battery in the charging period and after charging can be more accurately estimated through the electric vehicle battery operation characteristic model considering the low temperature influence in the extremely cold weather, meanwhile, the uncertainty of the departure time of the electric vehicle is more effectively managed, and the operation flexibility of the electric vehicle is ensured.

Description

Technical Field

[0001] The present invention relates to the field of battery modeling, and particularly to a method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature. Background Art

[0002] In extremely cold weather, low temperature will significantly reduce the charge and discharge performance of electric vehicle batteries. Different from fixed batteries, electric vehicle batteries are usually parked outdoors, and the operating temperature of their batteries is often close to the cold ambient temperature, which will affect the operating performance of electric vehicle batteries. Therefore, the influence of low temperature on the charge and discharge performance of electric vehicle batteries must be considered. Taking lithium-ion batteries as an example, lithium-ion batteries are one of the most common battery types used in electric vehicle batteries. In terms of charging performance, both the charging time and the consumed energy increase significantly in cold weather. Charging below zero degrees Celsius will increase the loss, aging, and safety risks of electric vehicle batteries.

[0003] Therefore, an additional preheating process will be performed before charging to raise the temperature of the electric vehicle battery above 0°C. According to the existing literature, it takes 20 minutes to raise the operating temperature of an electric vehicle battery from -20°C to 0°C, and 48 minutes to raise it from -19.3°C to -2.4°C.

[0004] Regarding the discharge performance, although lithium-ion batteries can be discharged in the temperature range of -30°C to 40°C, the energy that can be released at cold temperatures will be significantly reduced. For example, compared with 20°C, when a lithium-ion battery is discharged at -20°C, there will be a reduction of about 40% in energy. This decrease is because the lithium-ion battery reaches its cut-off voltage prematurely, preventing the complete release of the stored energy. This conclusion also applies to other electrochemical batteries, such as lead-acid batteries and supercapacitor batteries. Ignoring the influence of low temperature on the operating characteristics of electric vehicle batteries will overestimate the available energy of electric vehicle batteries, resulting in an infeasible energy management optimization scheme in extremely cold weather. Therefore, it is very important to establish an improved model of the operating characteristics of electric vehicle batteries considering the influence of low temperature for ensuring accurate energy assessment.

[0005] To reflect the impact of low temperatures, some studies have developed temperature-related battery operation models. For example, some studies have proposed a linear function based on data fitting to reflect the impact of low temperatures on the discharge / charge power of electric vehicle batteries. In contrast, some studies have explored the impact of low temperatures on battery energy availability, considering energy limitations and energy change constraints in the operation model. For example, some studies have proposed a temperature-aware energy limitation constraint by modeling the rated energy as a polynomial function of the ambient temperature. However, their studies only used the daily average ambient temperature and ignored the temperature fluctuations during the day. Some studies have developed a temperature-aware energy change model, but the modeling of the battery state of charge in it ignores the adaptability to the idle state of the battery. When there is an idle state, this limitation may lead to inaccurate estimates of energy availability. Some studies have proposed a new formula for the change in the battery state of charge, which takes into account the static battery state of charge (not affected by temperature) and the working battery state of charge (affected by temperature). However, the electrothermal coupling model used in it involves non-linear relationships, increasing the computational complexity. In addition, the models in some studies ignore the preheating process of electric vehicle batteries required to ensure safe charging. The optimal preheating temperature of electric vehicle batteries has been considered in some studies, but the preheating process is not included in the electric vehicle battery operation characteristic model.

[0006] In summary, the existing operating states of electric vehicle batteries do not simultaneously consider situations such as energy limitations, energy changes, and the preheating process. Imprecise modeling may lead to inaccurate evaluations of the performance of electric vehicle batteries at cold temperatures. Summary of the Invention

[0007] The object of the present invention is to propose a method for modeling the operating characteristics of electric vehicle batteries considering the impact of low temperatures, and to solve the technical problem that the existing technology fails to consider situations such as energy limitations, energy changes, and the preheating process in the modeling of the battery operating state, resulting in inaccurate evaluations of battery performance in extremely cold weather.

[0008] Specifically, a method for modeling the operating characteristics of electric vehicle batteries considering the impact of low temperatures provided by the present invention includes the following steps: S1. Calculate the operating temperatures of the electric vehicle battery in the idle, discharge, charge, and preheating states; S2: Establish the relationship between the operating temperature of the electric vehicle battery and energy availability; S3: Establish the operating constraints of the electric vehicle battery considering the preheating process; S4: Linearize the electric vehicle battery operation characteristic model.

[0009] A storage medium stores instructions and data for implementing a method for modeling the operating characteristics of electric vehicle batteries considering the impact of low temperatures.

[0010] An electric vehicle battery operating characteristic modeling device considering the influence of low temperature, comprising: a processor and the storage medium; the processor loads and executes the instructions and data in the storage medium to implement a method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature.

[0011] The beneficial effects provided by the present invention are: it can more accurately estimate the energy of the available electric vehicle battery during and after charging through the electric vehicle battery operating characteristic model considering the influence of low temperature in extremely cold weather, while more effectively managing the uncertainty of the departure time of the electric vehicle and ensuring the flexibility of the electric vehicle operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic diagram of the process of the method of the present invention; Figure 2 It represents the energy state conversion of the electric vehicle battery in the idle mode; Figure 3 It represents the energy state conversion of the electric vehicle battery in the V2R mode; Figure 4 It represents the energy state conversion of the electric vehicle battery in the R2V mode; Figure 5 It is a schematic diagram of the operation of the hardware device in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0014] Before formally elaborating on the present invention, first, a general description of the solution of the present invention is given for easy understanding.

[0015] Please refer to Figure 1 , a method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature provided by the present invention, includes: S1. Calculate the operating temperature of the electric vehicle battery in the idle, discharging, charging, and preheating states; The specific process of step S1 is as follows: The operating temperature of the electric vehicle battery is greatly affected by its four working states. The four working states include the idle state ( ), the discharging state ( ), the charging state ( ), and the preheating state ( ). And the energy availability of the electric vehicle battery depends on the operating temperature. Therefore, evaluating the operating temperature of the electric vehicle battery is the first step in establishing the operating characteristic model of the electric vehicle battery.

[0016] The operating temperature formula of the electric vehicle battery is as follows: (1) (2) (3) (4) (5) (6) Among them, Equation (1) is the modeling formula for the operating temperature of the battery when the electric vehicle is in the idle state and is equal to the ambient temperature , , where is a binary variable indicating whether it is in the idle state; Equation (2) is the modeling formula for the operating temperature of the battery when the electric vehicle is in the discharging state . When discharging, when using a bi-directional charger, the operating temperature of the electric vehicle battery will rise slightly, but can be ignored. Therefore, is also equal to , where is a binary variable indicating whether it is in the discharging state; Equation (3) is the modeling formula for the operating temperature of the battery when the electric vehicle is in the charging state . When charging, if is higher than the charging threshold temperature , can be equal to . Otherwise, a heater powered by the power grid is required to preheat the electric vehicle battery to raise to the threshold , where is a binary variable indicating whether it is in the charging state; Equation (4) is the modeling formula for the operating temperature of the battery when the electric vehicle is in the preheating state . where is a binary variable indicating whether it is in the preheating state; Equation (5) is the modeling formula for the change in the operating temperature of the electric vehicle battery in the preheating state . is the power consumed by the heater at time t , is the heater efficiency, is the number of battery cells, is the mass of each battery cell, is the specific heat capacity of the battery, is the duration of each time interval; Equation (6) is the modeling formula for the temperature of an electric vehicle battery expressed by a state-related function.

[0017] S2: Establish the relationship between the operating temperature of the electric vehicle battery and energy availability; As an example, four key energy state parameters are defined in the present invention to describe the influence of temperature on the energy availability of the electric vehicle battery, including the rated energy , stored energy , unavailable energy and available energy .

[0018] Specifically, the rated energy: refers to the maximum energy that the battery can store and deliver at a specific operating temperature .

[0019] Stored energy: refers to t the total energy stored in the battery at time, which is only affected by charging and discharging activities.

[0020] Unavailable energy: At low temperatures, a part of the energy will freeze and cannot be released at the operating temperature , and this energy is only affected by . Although this energy is still stored in the battery, it will prevent the battery from further discharging due to reaching the cut-off voltage prematurely during the discharge process.

[0021] Available energy: refers to the stored energy that can be released at the operating temperature , which is affected by temperature and charging / discharging activities.

[0022] The influence of temperature on the energy availability of the electric vehicle battery is as follows: (7) (8) (9) (10) Among them, Equation (7) fits the unavailable energy as a non-linear function of the operating temperature and the rated energy of the battery at room temperature ; Equation (8) represents the relationship between the rated energy at the operating temperature and the rated energy of the battery at room temperature and the unavailable energy ; Equation (9) represents the available energy Relationship with stored energy and unavailable energy ; Equation (10) represents t the state of charge of the battery at a given moment equals the ratio of the available energy to the rated energy .

[0023] S3: Establish the operating constraints of an electric vehicle battery considering the preheating process; It should be noted that the operating constraints of the electric vehicle battery during the preheating process include energy change constraints, energy limit constraints, discharge limit constraints, as well as preheating constraints and charging limit constraints.

[0024] As an example, the energy change constraints are as follows (11) (12) Figure 2 represents the energy state transition of the electric vehicle battery in the idle mode.

[0025] Among them, constraint (11) represents the energy change constraint of the electric vehicle in the idle mode. In this mode, the battery only experiences a temperature decrease but no charging or discharging activities. When the electric vehicle battery undergoes a temperature change from to , the energy state changes from point B to point C, and the unavailable energy changes. When the temperature decreases, is negative, representing that the difference in unavailable energy is negative, that is, the unavailable energy increases. Among them, constraint (12) represents the modeling method of the available energy after the temperature change . Although the total stored energy in the electric vehicle battery remains unchanged, due to the increase in unavailable energy at lower temperatures, the available energy decreases from at point B to at point C.

[0026] (13) Figure 3 represents the energy state transition of the electric vehicle battery in the V2R mode (the power flow of the electric vehicle battery to the residential power grid).

[0027] Among them, constraint (13) represents the modeling method of the available energy after the electric vehicle battery undergoes a discharge process and a temperature change . The available energy changes from at point D to . Among them, the change in available energy can be divided into two independent parts: (I) is the change in available energy caused by temperature change, (II) is the change in available energy caused by discharging. For the first part, the change during the transition from point D to point Q is defined in constraint (11). For the second part, the electric vehicle battery releases electrical energy, resulting in a decrease in available energy from point D to point P, which is negative. Among them represents t the discharging power at time represents the discharging efficiency.

[0028] (14) Figure 4 represents the energy state conversion of the electric vehicle battery in the R2V mode (the power of the residential power grid flows to the electric vehicle battery).

[0029] Among them, constraint (14) represents the modeling method of the available energy after the electric vehicle battery experiences the preheating and charging processes in the R2V mode. If the temperature of the electric vehicle battery is lower than the threshold temperature , the preheating process will first increase the temperature from at point F to at point G, which will increase the available energy, that is is positive. Then, the electric vehicle battery is charged, resulting in a further increase in available energy from point G to point H, which is positive. Among them represents t the charging power at time represents the charging efficiency.

[0030] (15) Among them, constraint (15) represents the change in available energy in the electric vehicle battery considering three operating modes and temperature change simultaneously. Among them represents the change in available energy caused by temperature change, represents the change in available energy caused by charging and discharging activities.

[0031] The energy limit constraints are as follows: (16) (17) (18) Among them, constraint (16) represents the upper and lower limit constraints of available energy, and constraint (17) represents the upper limit of available energy and In the relationship, constraint (18) represents the lower limit of available energy and the relationship. Among them represents the upper limit of available energy, represents the upper limit of the battery charge state, where represents the lower limit of available energy, represents the lower limit of the battery charge state.

[0032] The discharge limit constraints are as follows: (19) (20) Among them, constraint (19) represents the discharge power shall not exceed the maximum power of the bi-directional charger , constraint (20) represents preventing discharge when the temperature drops to or below. Among them is a binary state variable characterizing whether discharge occurs.

[0033] The preheating and charging limit constraints are as follows: (21) (22) (23) (24) (25) (26) (27) (28) Among them, constraint (21) means that in the R2V mode, the power flowing into the electric vehicle battery should be less than , is a binary state variable characterizing whether the electric vehicle battery is in the R2V mode; Constraint (22) means that the electric vehicle battery cannot be in the idle mode, R2V mode and V2R mode at the same time. Since the electric vehicle battery only discharges in the V2R mode, so is a binary state variable characterizing whether the electric vehicle battery is in the V2R mode; Constraint (23) is used to ensure that preheating and charging cannot occur simultaneously; Constraint (24) means that in the R2V mode, when the operating temperature of the electric vehicle battery is lower than the defined and at the same time greater than , preheating will occur; Constraint (25) indicates that charging will only occur when the operating temperature of the electric vehicle battery is higher than the defined and less than ; Constraint (26) indicates that the preheating power should not exceed the rated power of the heater ; Constraint (27) indicates that the charging power cannot exceed ; Constraint (28) indicates that both the preheating power and the charging power should satisfy the power relationship of the bidirectional charger.

[0034] S4: Linearize the electric vehicle battery operating characteristics model.

[0035] It should be noted that the linearization method of the electric vehicle battery operating characteristics model is as follows: (29) (30) (31) (32)(32a) (32b) (32c) (32d) (32e) (32f) (33) (34) (35) Among them, based on the relationship in Equation (22), Equation (6) can be transformed into the form of Equation (29), but Equation (29) still has bilinear terms , . According to Equation (26), the bilinear term can be replaced by ; the bilinear term is the product of the binary variable and the continuous variable , and the big-M method can be used for linearization. Introduce the auxiliary variable , and replace through Constraint (30); for Equation (7) regarding and The non - linear function is linearized using the piece - wise function shown in Equation (31); Equation (32) introduces continuous variables { } and binary variables { } to further describe the selection of the piece - wise function; the bilinear terms in Equations (24) and (25) are linearized by the big - M method in Equation (33), and then Equations (24) and (25) can be converted into the forms of Equations (34) and (35) respectively.

[0036] In summary, the electric vehicle battery operation characteristic model consists of continuous variables { , }, binary variables { } and constraints (8), (10), (15) - (23), (26) - (35).

[0037] Please refer to Figure 5 , Figure 5 which is the schematic diagram of the working of the hardware device in the embodiment of the present invention. The hardware device specifically includes: a modeling device 401 for the operation characteristics of an electric vehicle battery considering low - temperature effects, a processor 402, and a storage medium 403.

[0038] A modeling device 401 for the operation characteristics of an electric vehicle battery considering low - temperature effects: The modeling device 401 for the operation characteristics of an electric vehicle battery considering low - temperature effects implements the method for modeling the operation characteristics of an electric vehicle battery considering low - temperature effects.

[0039] Processor 402: The processor 402 loads and executes the instructions and data in the storage medium 403 to implement the method for modeling the operation characteristics of an electric vehicle battery considering low - temperature effects.

[0040] Storage medium 403: The storage medium 403 stores instructions and data; the storage medium 403 is used to implement the method for modeling the operation characteristics of an electric vehicle battery considering low - temperature effects.

[0041] The beneficial effect of the present invention is that it can more accurately estimate the energy of the available electric vehicle battery during and after charging through the electric vehicle battery operation characteristic model considering low - temperature effects in extremely cold weather, and at the same time more effectively manage the uncertainty of the electric vehicle departure time, ensuring the flexibility of the electric vehicle operation.

[0042] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A modeling method for the operating characteristics of an electric vehicle battery considering the influence of low temperature, characterized in that: Including: S1. Calculate the operating temperature of the electric vehicle battery in the idle, discharging, charging, and preheating states; S2: Establish the relationship between the operating temperature of the electric vehicle battery and the energy availability; S3: Establish the operating constraints of the electric vehicle battery considering the preheating process; S4: Linearize the operating characteristic model of the electric vehicle battery.

2. The modeling method for the operating characteristics of an electric vehicle battery considering the influence of low temperature according to claim 1, characterized in that: The step S1 is as follows: The formula for the operating temperature of the electric vehicle battery is as follows: (1) (2) (3) (4) (5) (6) Among them, Equation (1) is the modeling formula for the battery operating temperature when the electric vehicle is in an idle state and is equal to the ambient temperature , where is a binary variable indicating whether it is in an idle state; Equation (2) is the modeling formula for the battery operating temperature when the electric vehicle is in a discharging state . In the discharging state, differs from by no more than a preset value, where is a binary variable indicating whether it is in a discharging state; Equation (3) is the modeling formula for the battery operating temperature when the electric vehicle is in a charging state . In the charging state, if is higher than the charging threshold temperature , then differs from by no more than a preset value. Otherwise, a heater powered by the grid is required to preheat the electric vehicle battery to raise to the threshold , where is a binary variable indicating whether it is in a charging state; Equation (4) is the modeling formula for the battery operating temperature when the electric vehicle is in a preheating state , where is the power consumed by the heater at time t , is the heater efficiency, is the number of battery cells, is the mass of each battery cell, is the specific heat capacity of the battery, is the duration of each time interval; Equation (6) is the modeling formula for representing the temperature of the electric vehicle battery by a state-dependent function.

3. A modeling method for the operating characteristics of an electric vehicle battery considering the influence of low temperature as described in claim 2, characterized in that: The step S2 is specifically as follows: The influence of temperature on the energy availability of the electric vehicle battery is as follows: (7) (8) (9) (10) Among them, Equation (7) fits the unavailable energy as a non-linear function of the operating temperature and the rated energy of the battery at room temperature ; Equation (8) represents the rated energy at the operating temperature and the rated energy of the battery at room temperature as well as the unavailable energy ; Equation (9) represents the relationship between the available energy and the stored energy and the unavailable energy ; Equation (10) represents that t the state of charge of the battery at time is equal to the ratio of the available energy to the rated energy .

4. A method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature, as described in claim 3, characterized in that: In step S3, the operating constraints of the electric vehicle battery during the heating process include: energy change constraint, energy limit constraint, discharge limit constraint, preheating constraint, and charging limit constraint.

5. A method for modeling the operating characteristics of an electric vehicle battery considering low-temperature effects according to claim 4, characterized in that: The energy change constraint is as follows: (11) (12) (13) (14) (15) Constraint (11) represents the energy change constraint of the electric vehicle in the idle mode; Constraint (12) represents the available energy after a temperature change and its modeling formula; Constraint (13) represents the discharge process and temperature change experienced by the electric vehicle battery in the V2R mode The available energy after The modeling formula of; among them, the change of available energy can be divided into two independent parts, Is the change of available energy caused by temperature change, Is the change of available energy caused by discharge; Constraint (14) represents the modeling formula for the available energy of an electric vehicle battery after experiencing the preheating and charging processes in the R2V mode. of the modeling formula; Constraint (15) represents the change in the available energy in the electric vehicle battery considering three operating modes and temperature changes.

6. A method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature, as claimed in claim 5, characterized in that: The energy limit constraint is as follows: (16) (17) (18) Among them, constraint (16) represents the upper and lower bound constraints of the available energy, constraint (17) represents the relationship between the upper bound of the available energy and and constraint (18) represents the relationship between the lower bound of the available energy and . Among them, represents the upper bound of the available energy, represents the upper bound of the battery charge state. Among them, represents the lower bound of the available energy, represents the lower bound of the battery charge state.

7. A method for modeling the operating characteristics of an electric vehicle battery considering the influence of low temperature, as described in claim 6, characterized in that: The discharge limit constraint is as follows: (19) (20) Among them, the constraint (19) represents the discharge power which should not exceed the maximum power of the bi-directional charger , and the constraint (20) represents preventing discharge when the temperature drops to or below; where is a binary state variable characterizing whether discharge is carried out.

8. A modeling method for the operating characteristics of an electric vehicle battery considering the influence of low temperature, as described in claim 7, characterized in that: The preheating and charging limit constraints are as follows: (21) (22) (23) (24) (25) (26) (27) (28) Among them, the constraint (21) indicates that in the R2V mode, the power flowing into the electric vehicle battery should be less than , is a binary state variable characterizing whether the electric vehicle battery is in the R2V mode; Constraint (22) indicates that the electric vehicle battery cannot be in the idle mode, R2V mode, and V2R mode simultaneously. Since the electric vehicle battery only discharges in the V2R mode, the binary state variable characterizing whether the electric vehicle battery is in the V2R mode; Constraint (23) is used to ensure that preheating and charging cannot occur simultaneously; The constraint (24) indicates that in the R2V mode, preheating occurs only when the operating temperature of the electric vehicle battery is lower than the defined and at the same time greater than ; preheating will occur only when the operating temperature of the electric vehicle battery is lower than the defined Constraint (25) indicates that charging will only occur when the operating temperature of the electric vehicle battery is higher than the defined and simultaneously lower than ; Constraint (26) indicates that the preheating power should not exceed the rated power of the heater ; The constraint (27) indicates that the charging power cannot exceed ; Constraint (28) represents that both the preheating power and the charging power should satisfy the power relationship of the bi-directional charger.

9. A storage medium, characterized in that: The storage medium stores instructions and data for implementing a method for modeling the operating characteristics of an electric vehicle battery considering low-temperature effects according to any one of claims 1 to 8.

10. An electric vehicle battery operation characteristic modeling device considering the influence of low temperature, characterized in that: Including: A processor and a storage medium; the processor loads and executes the instructions and data in the storage medium for implementing a method for modeling the operating characteristics of an electric vehicle battery considering low-temperature effects according to any one of claims 1 to 8.

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