Method for determining state of charge of electric energy storage unit

Abstract

The invention relates to a method for determining a state of charge of an electric energy storage unit, comprising the following steps: substantially determining the state of charge of the electric energy storage unit in such a way that the state of charge is ascertained at least with predefined tolerances; -transmitting a first control command for charging and / or discharging the electrical energy storage unit as a function of the determined general state of charge; ascertaining a rate of change of the no-load voltage of the electrical energy storage unit during charging and / or discharging; if the determined change rate exceeds or falls below a predetermined first change rate, determining a first state-of-charge value as a function of the determined change rate of the no-load voltage during charging and / or discharging. The invention further relates to a corresponding device for determining a state of charge of an electrical energy storage unit, to a corresponding computer program, and to a machine-readable storage medium having a computer program.

Description

Technical Field

[0001] The invention relates to a method for determining the state of charge of an electrical energy storage unit. Background Art

[0002] The state of charge (also called SOC or State of Charge) of an electrical energy storage unit, such as a rechargeable battery cell, can be obtained by measuring the relaxed no-load voltage (also called stable voltage or open circuit voltage OCV) or by integrating the measured current, so-called coulomb counting. The tolerances mainly in the current measurement lead to the fact that coulomb counting only provides sufficiently accurate SOC values ​​during ongoing operation and only for a limited time. Therefore, regular corrections are required regarding the measurement of the no-load voltage. In addition, the initial SOC value needs to be known when counting coulombs.

[0003] Depending on the type of electrical energy storage unit, for example, in the case of NMC battery cells, the determination of the state of charge can be meaningfully achieved by measuring the no-load voltage over the entire state of charge range. For other electrical energy storage units, for example, in the case of lithium iron phosphate battery cells, the trend of the no-load voltage over a wide range of the state of charge is so flat that a meaningful SOC determination cannot be achieved due to measurement tolerances. Only for almost discharged or practically fully charged battery cells does the measurement of the no-load voltage provide sufficiently accurate SOC values. If the lithium iron phosphate battery cell is charged to two-thirds, a slight voltage increase occurs, which can be used for a meaningful SOC determination. However, before measuring the OCV voltage, it is necessary to wait for the relaxation time, because otherwise, the shorter the relaxation time, the more drastically the dynamics of the cell voltage will distort the measurement result. At room temperature, this relaxation time is at least 20 minutes. However, at low temperatures, it can be assumed that the relaxation time is significantly longer. Since the range in which the voltage increase occurs is relatively narrow and can shift with temperature and aging, multiple OCV measurements are required in practical use, which can last for several hours due to the relaxation times.

[0004] Alternatively, measurements in a small SOC range are also conceivable, but such measurements in a small SOC range impair the usability.

[0005] In order to determine the current cell capacity, i.e. the storage performance of the battery cell, two SOC measurements with a clear interval relative to one another are required, so that a single SOC measurement is not sufficient for an almost fully charged battery. Furthermore, when a plurality of cells are connected in series, the SOC determination is difficult for almost fully charged cells, since due to the different cell capacities and the different charges, one cell has already reached the charging limit and does not allow further charging, while the other cells are not yet sufficiently charged for the SOC determination. This can be solved by equalizing the charging of the individual cells, so-called balancing, but this is very complex. Summary of the invention

[0006] Advantages of the present invention.

[0007] A method for ascertaining a state of charge of an electrical energy storage cell is disclosed having the features of the independent claim.

[0008] In this case, the state of charge of the electrical energy storage cell is ascertained at least approximately so accurately that it is known at least within a predetermined tolerance.

[0009] A first control command for charging and / or discharging the electrical energy storage unit is transmitted as a function of the ascertained approximate state of charge. This can be implemented, for example, by an electrical control unit.

[0010] Here, the rate of change of the no-load voltage of the electrical energy storage unit during charging and / or discharging is determined. If the determined rate of change exceeds or falls below a predetermined first rate of change, a first state of charge value is determined based on the determined rate of change of the no-load voltage during charging and / or discharging.

[0011] The method is advantageous because it allows the charge state of the electrical energy storage unit to be accurately determined even when there is a flat no-load voltage characteristic curve with only a few slight voltage changes. The method is therefore suitable in particular for batteries or battery cells with lithium iron phosphate chemistry or other chemistries with a flat no-load voltage characteristic curve.

[0012] Further advantageous embodiments of the invention are the subject matter of the dependent claims.

[0013] It is expedient to determine the first state of charge value as a function of the extreme point of the rate of change of the no-load voltage. This is advantageous because the state of charge can thereby be determined even more accurately because the extreme point can be determined relatively accurately.

[0014] It is advantageous if a predetermined current value is not exceeded during charging and / or discharging. This is advantageous because the voltage gradient decreases as the current level increases in value, which would be at the expense of determination accuracy. Thus, a sufficiently precise determination accuracy is ensured by the method.

[0015] It is expedient to determine the second state of charge value as a function of the determined rate of change of the no-load voltage when a predetermined second rate of change is exceeded. In addition, the current capacity of the electrical energy storage unit is determined as a function of the first state of charge value, the second state of charge value, and the amount of charge converted between the two state of charge values ​​during charging and / or discharging. This is advantageous because the current capacity of the electrical energy storage unit, i.e., the storable energy, can thereby be determined accurately.

[0016] The first predetermined rate of change and the second predetermined rate of change preferably correspond to a first predetermined state of charge and a second predetermined state of charge. This is advantageous because a simple relationship between the rate of change and the state of charge exists and the state of charge can thus be easily determined given a known rate of change.

[0017] The first rate of change is expediently in the range of 3 mV to 10 mV per state of charge step and the second rate of change is in the range of 25 mV to 75 mV per state of charge step. This allows the corresponding state of charge to be reliably detected.

[0018] The subject matter of the present invention is also a device for determining the charge state of an electrical energy storage cell, wherein the device comprises at least one component which is designed to carry out the steps of the method according to the present invention. This allows the advantages mentioned above to be achieved. In particular, the component can be designed as an electronic battery management controller.

[0019] The subject matter of the invention is also a computer program which includes commands which cause the device according to the invention to carry out all the steps of the method according to the invention. The advantages mentioned can thereby be achieved.

[0020] The subject matter of the invention is also a machine-readable storage medium on which the computer program according to the invention is stored. This allows the advantages mentioned to be achieved and ensures simple distributability of the computer program according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Advantageous embodiments of the invention are shown in the drawings and are explained in more detail in the following description.

[0022] in:

[0023] Figure 1A flow chart of a method according to the invention is shown according to one embodiment;

[0024] Figure 2 A schematic diagram showing the trend of a no-load voltage characteristic curve according to one embodiment;

[0025] Figure 3 Shows Figure 2 A schematic diagram of the no-load voltage characteristic curve shown in FIG. 1 with respect to the gradient of the charge state trend; and

[0026] Figure 4 A schematic diagram of a device for ascertaining a state of charge of an electrical energy storage cell according to one specific embodiment is shown.

[0027] The same reference numerals in all the figures denote the same device components or the same method steps. DETAILED DESCRIPTION

[0028] Figure 1 A flow chart of a method according to the invention for ascertaining a state of charge of an electrical energy storage cell is shown according to one specific embodiment.

[0029] In a first step S11 , the charge state of the energy storage cell is roughly determined, so that, for example, it can be distinguished whether the energy storage cell is charged to less than two thirds, thus having a charge state of less than approximately 66%. The charge state is thus known at least within a predetermined tolerance.

[0030] Subsequently, in a second step S12, a first control command for charging and / or discharging the electrical energy storage unit is transmitted according to the determined approximate charge state. This can be implemented, for example, at a power electronic component, with which the electrical energy storage unit is discharged, for example, via a resistor or during the driving of the electric motor.

[0031] In a third step S13, the rate of change of the no-load voltage of the electrical energy storage unit during charging and / or discharging is determined. This can be achieved, for example, by numerically differentiating the no-load voltage. For this purpose, for example, discrete values ​​of the no-load voltage are determined and associated with the corresponding charge state changes.

[0032] In a fourth step S14, if the rate of change determined in the third step S13 exceeds a predetermined first rate of change, a first state of charge value is determined, wherein the determination is carried out according to the determined rate of change of the no-load voltage during charging and / or discharging. This can be achieved, for example, by assigning a predetermined first rate of change to a predetermined first state of charge. The state of charge can thus be inferred from the rate of change. The first rate of change can, for example, fluctuate in a range of 3 mV to 10 mV per state of charge step. The state of charge is then assigned to the rate of change according to the type of the electrical energy storage cell.

[0033] Figure 2 A schematic diagram of the trend of a no-load voltage characteristic curve 20 according to one embodiment is shown. The no-load voltage characteristic curve 20 shows a typical trend for a lithium iron phosphate battery cell. Four points are representative here, at which the no-load voltage characteristic curve has a more drastic change. At the first point 21, the corresponding energy storage unit is charged to about 66%. At the second point 22, the energy storage unit is charged to about 25%. At the third point 23, the energy storage unit is charged to about 10%, and at the fourth point 24, the charge state is about 98%.

[0034] Figure 3 Shows Figure 2 Schematic diagram of the no-load voltage characteristic curve 20 shown in FIG. 3 with respect to the gradient 30 of the charge state trend. It is obvious here that Figure 2 At the points described in , there is a comparatively drastic change in the no-load voltage. These points are so-called extreme points. At the first point 21 and the second point 22, the predetermined first rate of change is in the range of, for example, 3 mV to 10 mV per charge state step. At the third point 23 and the fourth point 24, the predetermined second rate of change is in the range of 25 mV to 75 mV per charge state step.

[0035] By About Figure 2 and Figure 3 The description shows that, when the state of charge of the energy storage unit is roughly known, in particular for lithium iron phosphate battery cells, the state of charge of the energy storage unit can be accurately determined by the rate of change of the no-load voltage characteristic curve. For example, it is sufficient to know whether the energy storage unit has a state of charge greater than 50% or less than 50%. Even in the case of shorter relaxation times, the state of charge can still be roughly determined, for example, by evaluating the no-load voltage. The approximate state of charge can then be further roughly tracked by integrating the measured current, so-called coulomb counting.

[0036] Figure 4 A schematic diagram of a device 41 for determining the charge state of an electrical energy storage cell according to one specific embodiment is shown. The device 41 comprises an electronic computing unit 42, such as a battery management controller or a computer, which is configured to carry out the method according to the invention.

Claims

1. A method for determining a charge state of an electrical energy storage unit, the method comprising the following steps: - approximately determining the state of charge of the electrical energy storage unit, so that the state of charge is known at least within a predetermined tolerance; - transmitting a first control instruction for charging and / or discharging the electrical energy storage unit according to the determined approximate state of charge; - obtaining a rate of change of a no-load voltage of the electrical energy storage unit during the charging and / or discharging period; If the ascertained rate of change exceeds or falls below a predetermined first rate of change, ascertaining a first state of charge value as a function of the ascertained rate of change of the open circuit voltage during charging and / or discharging.

2. The method according to the preceding claim, wherein: The first state of charge value is obtained according to the extreme point of the rate of change of the no-load voltage.

3. A method according to any one of the preceding claims, wherein: During the charging and / or discharging, a predetermined current value is not exceeded.

4. The method according to the preceding claim, further comprising: - when a predetermined second rate of change is exceeded, determining a second state of charge value according to the determined rate of change of the no-load voltage; - ascertaining the current capacity of the electrical energy storage cell as a function of the first state of charge value, the second state of charge value and the charge quantity converted between the two state of charge values ​​during charging and / or discharging.

5. A method according to any one of the preceding claims, wherein: The predetermined first rate of change and the predetermined second rate of change correspond to a predetermined first state of charge and a predetermined second state of charge.

6. A method according to any one of the preceding claims, wherein: The first rate of change is in the range of 3 mV to 10 mV per state of charge step, and the second rate of change is in the range of 25 mV to 75 mV per state of charge step.

7. A device (41) for determining a charge state of an electrical energy storage cell, the device (41) comprising at least one component, in particular an electronic battery management control unit, which is designed to carry out the steps of the method according to any one of claims 1 to 5.

8. Computer program comprising commands causing the device (41) according to claim 7 to carry out all the steps of the method according to any one of claims 1 to 6.

9. A machine-readable storage medium on which the computer program according to claim 8 is stored.

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