A method for calculating open circuit voltage during battery charging

Abstract

The present invention discloses a method for calculating the open circuit voltage during the charging process of a battery, obtaining the open circuit voltage Vocv of the battery before charging, the charging current Icc and the battery voltage Vccv during the constant current charging stage, and establishing a linear equation between the charging current and the charging voltage. Then, the charging current Icc is switched between the constant current value Iccl and the constant current value Icch at a frequency of 20 Hz, and the battery voltage difference △Vchl (t1) under two different currents is calculated, and the change amount of the slope k in the linear equation is used to obtain a curve equation with a time parameter, which can more accurately calculate the actual open circuit voltage of the battery. The method for calculating the open circuit voltage during the charging process of the battery of the present invention has the advantages of being able to more accurately calculate the actual open circuit voltage of the battery, truly and effectively reflecting the battery's electric quantity, and enabling users or program developers to more easily see the battery charging progress.

Description

Technical Field

[0001] The invention relates to a battery charging technology, in particular to a method for calculating an open circuit voltage during a battery charging process. Background Art

[0002] The accuracy of calculating the remaining power of a battery is very important for battery management. The remaining power of a battery can be measured using the Coulomb integral method and the open circuit voltage method.

[0003] The Coulomb integration method is a method for calculating the battery charge state by current integration. It is mainly done by connecting a resistor in the charge and discharge circuit, detecting the current by resistor voltage division, and integrating the current value with time. However, in the Coulomb integration method, since the hardware circuit of the charge and discharge circuit has a constant system error in the measurement of the charge and discharge current, and the integration process will cause the system error to accumulate continuously, resulting in a low accuracy of the coulomb meter in estimating the battery charge after a long period of operation, and the result is low reliability.

[0004] The terminal voltage of a battery in an open circuit state is called the open circuit voltage. The open circuit voltage of a battery is equal to the difference between the electrode potential of the positive electrode and the electrode potential of the negative electrode of the battery when the battery is disconnected (i.e. when no current passes through the two poles). The open circuit voltage method is based on the principle that there is an accurate correspondence between the open circuit voltage and the depth of discharge. The accurate depth of discharge can be obtained by measuring the accurate open circuit voltage. However, the disadvantage of the open circuit voltage method is that the open circuit voltage is usually difficult to obtain. The terminal voltage can only be identified as the open circuit voltage when the battery is in an open circuit state for a long time. The scope of application of the discharge depth correction by this method is extremely small and the real-time performance is poor.

[0005] During the battery charging process, the battery voltage often rises due to the internal resistance voltage drop and polarization effect. Especially when the charging current is large or the battery capacity is small, the battery voltage will be greatly increased. At this time, if the battery voltage is directly sampled, the sampled voltage value obtained will be very different from the actual open circuit voltage (OCV). At this time, the sampled voltage is difficult to truly reflect the actual charging progress of the battery.

[0006] Therefore, during the battery charging process, the actual open circuit voltage (OCV) of the battery is calculated in real time, which can easily and accurately determine the actual charging progress of the battery. At present, the methods that can clearly feedback the battery charging progress during the charging process include the ammeter method, the coulomb meter method, the model predictive control method (MPC) or more complex large model algorithms, but these measurement methods are relatively complicated, and most of them require a complete charge and discharge cycle of the battery to take effect. Therefore, it is urgent to design a method that can calculate the open circuit voltage of the battery in real time during the charging process. Summary of the invention

[0007] The present invention is to avoid the deficiencies existing in the above-mentioned prior art and provide a method for calculating the open circuit voltage of a battery during charging, so as to obtain the actual open circuit voltage of the battery more accurately during the charging process.

[0008] A method for calculating the open circuit voltage during battery charging of the present invention comprises the following steps:

[0009] Step 1: Before charging, collect and save the open circuit voltage Vocv of the battery;

[0010] Step 2: After charging starts, the constant current charging stage of the charging current Icc is entered, and the battery voltage Vccv in the constant current charging stage is collected;

[0011] Step 3: Calculate the difference between the battery voltage Vccv during the constant current charging phase and the open circuit voltage Vocv of the battery collected before charging ;

[0012] Step 4: Establish the first-order RC equivalent model of the battery;

[0013] Step 5: In the first-order RC equivalent model, assume that the polarization internal resistance of the battery is R cr During the charging process, the linear relationship between the battery charging current Icc and the battery voltage Vccv is expressed by a straight line equation. The slope k of the straight line equation is calculated by the open circuit voltage Vocv, the charging current Icc and the battery voltage Vccv in the constant current charging stage, and a straight line relationship diagram between the charging current Icc and the battery voltage Vccv is drawn in the coordinate system.

[0014] Step 6: Switch the charging current Icc between the first constant current value Iccl and the second constant current value Icch at a frequency of 20 Hz;

[0015] Step 7: At 50 ms after switching to the first constant current value Iccl, the battery voltage Vccl is detected; at 50 ms after switching to the second constant current value Icch, the battery voltage Vcch is detected, and the difference between the battery voltage Vcch and the battery voltage Vccl is calculated. ;

[0016] Step 8: Obtain parameters , and set the parameter Combined with the linear equation between the charging current Icc and the battery voltage Vccv, a curve equation with the time variable added is obtained, and the open circuit voltage Vocv of the battery is obtained through the curve equation.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] The present invention discloses a method for calculating the open circuit voltage of a battery during charging, obtaining the open circuit voltage Vocv of the battery before charging, the charging current Icc and the battery voltage Vccv in the constant current charging stage, and establishing a linear equation between the charging current and the charging voltage. Then, the charging current Icc is switched between the constant current value Iccl and the constant current value Icch at a frequency of 20Hz, and the battery voltage difference under the two different currents is calculated. To calculate ,Will As the change in the slope k in the linear equation, a curve equation with a time parameter is obtained, which can more accurately calculate the actual open circuit voltage of the battery.

[0019] The method for calculating the open circuit voltage during the battery charging process of the present invention has the advantages of being able to more accurately calculate the actual open circuit voltage of the battery, thereby more truly and effectively reflecting the battery power, and allowing users or program developers to more easily see the battery charging progress. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention is a flow chart of a method for calculating an open circuit voltage during a battery charging process.

[0021] Figure 2 It is a schematic diagram of a first-order RC equivalent model of a battery of the present invention, showing an open circuit voltage during the battery charging process.

[0022] Figure 3 The figure is a relationship diagram between the charging current and the charging voltage of the open circuit voltage during the charging process of a storage battery of the present invention.

[0023] The present invention will be further described below through specific implementation modes in conjunction with the accompanying drawings. DETAILED DESCRIPTION

[0024] See also Figure 1 to Figure 3 A method for calculating the open circuit voltage during battery charging of the present invention comprises the following steps:

[0025] Step 1: Before charging, collect and save the open circuit voltage Vocv of the battery;

[0026] Step 2: After charging starts, the constant current charging stage of the charging current Icc is entered, and the battery voltage Vccv in the constant current charging stage is collected;

[0027] Step 3: Calculate the difference between the battery voltage Vccv during the constant current charging phase and the open circuit voltage Vocv of the battery collected before charging ;

[0028] The difference The calculation formula is shown in the following formula (1);

[0029] (1);

[0030] Step 4: Establish the first-order RC equivalent model of the battery;

[0031] like Figure 2 As shown, the storage battery takes lithium battery as an example. The internal equivalent circuit of lithium battery is Figure 2 It can be expressed as a first-order RC equivalent model. Among them, R fr Represents the ohmic internal resistance of the lithium battery (i.e. the fixed internal resistance of the lithium battery), R cr Represents the polarization internal resistance of the lithium battery (i.e. the changing internal resistance of the lithium battery), and C0 represents the polarization capacitance of the lithium battery. Ignoring the polarization capacitance of the lithium battery, the battery equivalent circuit is directly understood as the battery internal resistance. R ir The ohmic internal resistance R fr and polarization internal resistance R cr The sum of is shown in the following formula (2).

[0032] (2);

[0033] Step 5: In the first-order RC equivalent model, assume that the polarization internal resistance of the battery is R cr During the charging process, the linear relationship between the battery charging current Icc and the battery voltage Vccv is expressed by a straight line equation. The slope k of the straight line equation is calculated by the open circuit voltage Vocv, the charging current Icc and the battery voltage Vccv in the constant current charging stage, and a straight line relationship diagram between the charging current Icc and the battery voltage Vccv is drawn in the coordinate system.

[0034] In the present invention, it is assumed that the polarization internal resistance of the lithium battery is R cr During the charging process, it is a fixed value. R cr There will be no change. Under this assumption, the difference between the battery voltage Vccv of the lithium battery and the open circuit voltage Vocv of the lithium battery will remain consistent at different currents, that is, there is a linear relationship between the charging current Icc and the battery voltage Vccv of the battery, so the relationship between the charging current Icc and the battery voltage Vccv of the lithium battery can be expressed by a linear equation.

[0035] like Figure 3, define the straight line equation as y=kx+d; then y is the battery voltage Vccv during constant current charging, x is the charging current Icc, and d is the open circuit voltage Vocv of the battery. The straight line equation in the first-order RC equivalent model can be expressed by the following formula (3).

[0036] (3);

[0037] By using the above formula (3), the open circuit voltage Vocv, charging current Icc and battery voltage Vccv in the constant current charging stage in step 1 and step 2 are substituted into formula (3) to calculate the slope k, so that the slope k can be plotted in the coordinate system as follows: Figure 3 A straight line is shown, which can express the operational relationship between the open circuit voltage Vocv, the charging current Icc and the battery voltage Vccv in the constant current charging stage during the entire constant current charging process.

[0038] Step 6: Switch the charging current Icc between the first constant current value Iccl and the second constant current value Icch at a frequency of 20 Hz;

[0039] In specific implementation, the first constant current value Iccl=Icc+0.1A, and the second constant current value Icch=Icc-0.1A; during the battery charging process, the polarization internal resistance must also be considered. R cr Therefore, the charging current Icc is switched back and forth between the two constant current values ​​of Iccl and Icch at a frequency of 20Hz. Because the deviation of the charging current Icc is small, it can be equivalent to charging with the charging current Icc for a period of time. Among them, Iccl=Icc-0.1A, Icch=Icc+0.1A. It should be noted that Icc-0.1A>0, and the positive and negative deviation values ​​of the charging current Icc are also adjusted according to the actual situation. The reason why the charging current Icc is switched at a frequency of 20HZ is that the voltage change of the battery will have a certain hysteresis due to the influence of the internal capacitance, so enough time needs to be reserved for the battery voltage to stabilize after switching the current.

[0040] Step 7: At 50 ms after switching to the first constant current value Iccl, the battery voltage Vccl is detected; at 50 ms after switching to the second constant current value Icch, the battery voltage Vcch is detected, and the difference between the battery voltage Vcch and the battery voltage Vccl is calculated. ;

[0041] The battery voltage Vccl is detected at 50ms after switching to Iccl; similarly, the battery voltage Vcch is detected at 50ms after switching to Icch. The battery voltage difference under two different currents is calculated and saved. ,Right now .

[0042] Step 8: Obtain parameters , and set the parameter Combined with the linear equation between the charging current Icc and the battery voltage Vccv, a curve equation with the time variable added is obtained, and the open circuit voltage Vocv of the battery is obtained through the curve equation.

[0043] Since charging has just started, the charging time has little effect on the internal resistance of the battery. Also, since the current change is small, it can be considered that and Is the same ohm internal resistance R fr Polarization internal resistance R cr The result is obtained when only the charging current Icc is affected, so the following equation (4) can be listed.

[0044] (4);

[0045] In formula (4), It is the battery voltage difference under 200mA current difference collected in real time at a certain time t during the charging process; represents the battery voltage difference under the first 200mA current difference collected after the battery starts charging; t1 refers to the completion time of the first Vccl and Vcch collection action after charging starts; t is the completion time of a certain Vccl and Vcch collection action during the charging process. And according to the equation (4), , which represents the formula (3) we obtained earlier The change in slope k. It is the slope of the current cycle calculated by using the two voltage coordinate points Vccl and Vcch in the tth cycle after collecting Vccl and Vcch. The two voltage coordinate points are connected into a straight line. There is a linear relationship between the battery charging current Icc and the battery voltage Vccv in each cycle T. The slope in each cycle T is a fixed value. The slope of different cycles is changing, but the slope in the same cycle remains unchanged.

[0046] The formula (4) Combining with formula (3), we get a curve equation with time parameter t, which can be expressed by the following formula (5).

[0047] (5)

[0048] In this way, the battery internal resistance change parameter is added to the linear equation that originally assumes that the battery internal resistance will not change, and a curve equation with a time parameter t is obtained, so that the actual open circuit voltage Vocv of the battery can be calculated more accurately during the charging process.

[0049] like Figure 1 The present invention is a flow chart of a method for calculating the open circuit voltage during the battery charging process. In the present invention, the relationship between the charging current Icc and the battery charging voltage Vccv is first expressed by a linear equation, and then the time parameter is added to the linear equation to obtain a curve equation with a time parameter, which can more accurately calculate the actual open circuit voltage of the battery, thereby more truly and effectively reflecting the battery power, so that users or program developers can easily see the battery charging progress.

[0050] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0051] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for calculating the open circuit voltage during battery charging, characterized in that: The steps include: Step 1: Before charging, collect and save the open circuit voltage Vocv of the battery; Step 2: After charging starts, the constant current charging stage of the charging current Icc is entered, and the battery voltage Vccv in the constant current charging stage is collected; Step 3: Calculate the difference between the battery voltage Vccv during the constant current charging phase and the open circuit voltage Vocv of the battery collected before charging ; Step 4: Establish the first-order RC equivalent model of the battery; Step 5: In the first-order RC equivalent model, assume that the polarization internal resistance of the battery is R cr During the charging process, the linear relationship between the battery charging current Icc and the battery voltage Vccv is expressed by a straight line equation. The slope k of the straight line equation is calculated by the open circuit voltage Vocv, the charging current Icc and the battery voltage Vccv in the constant current charging stage, and a straight line relationship diagram between the charging current Icc and the battery voltage Vccv is plotted in the coordinate system. The straight line equation in the first-order RC equivalent model is expressed by the following formula (3); （3）； In the formula (3), Vocv is the open circuit voltage of the battery; Vccv is the battery voltage in the constant current charging stage; Icc is the charging current in the constant current charging stage; k is the slope of the straight line equation in the first-order RC equivalent model; Step 6: Switch the charging current Icc between the first constant current value Iccl and the second constant current value Icch at a frequency of 20 Hz; Step 7: At 50 ms after switching to the first constant current value Iccl, the battery voltage Vccl is detected; at 50 ms after switching to the second constant current value Icch, the battery voltage Vcch is detected, and the difference between the battery voltage Vcch and the battery voltage Vccl is calculated. ; Step 8: Obtain parameters , and set the parameter Combined with the linear equation between the charging current Icc and the battery voltage Vccv, a curve equation with time variables added is obtained, and the open circuit voltage Vocv of the battery is obtained through the curve equation; The curve equation is expressed by the following formula (5): （5）； In the formula (5), Vocv is the open circuit voltage of the battery; Vccv is the battery voltage in the constant current charging stage; Icc is the charging current in the constant current charging stage; k is the slope of the straight line equation in the first-order RC equivalent model; It is the slope in the current cycle calculated using the two voltage coordinate points Vccl and Vcch in the tth cycle after Vccl and Vcch are collected.

2. The method for calculating the open circuit voltage during battery charging according to claim 1, characterized in that: In step 3, the difference The calculation formula is shown in the following formula (1); （1）； In the formula (1), Vocv is the open circuit voltage of the battery; Vccv is the battery voltage during the constant current charging stage.

3. The method for calculating the open circuit voltage during battery charging according to claim 1, characterized in that: In step 6, the first constant current value Iccl=Icc+0.1A, and the second constant current value Icch=Icc-0.1A.

4. The method for calculating the open circuit voltage during battery charging according to claim 1, characterized in that: In step 7, the battery voltage difference ;in, is the difference between the battery voltage Vcch and the battery voltage Vccl.

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