Method for determining an open-circuit voltage characteristic of a battery

Abstract

Method for determining an open-circuit voltage characteristic curve (cold) of a battery, wherein the open-circuit voltage characteristic curve (cold) has a first characteristic curve section (KN1) and a second characteristic curve section (KN2) adjoining the first characteristic curve section (KN1), wherein the open-circuit voltage characteristic curve (cold) has a break point (X) and / or a plateau (P) in the transition region between the two characteristic curve sections (KN1, KN2), comprising the following steps: determining raw data open-circuit voltage points (P), determining a first region (B1) through which the first characteristic curve section (KN1) extends, and a second region (B2) through which the second characteristic curve section (KN2) extends, by determining the break point (X) or the plateau (PL) from the raw data open-circuit voltage points (P), determining the course of the first characteristic curve section (KN1) and the second characteristic curve section (KN2),wherein the first characteristic section (KN1) and the second characteristic section (KN2) are determined separately from one another, and composing the open-circuit voltage characteristic (cold) using the determined first characteristic section (KN1), the determined second characteristic section (KN2) and the determined break point (X) and / or the plateau (PL) in the transition region between the characteristic sections (KN1, KN2).,

Description

[0001] The invention relates to a method for determining an open-circuit voltage characteristic curve of a battery, wherein the open-circuit voltage characteristic curve has a first characteristic curve section and a second characteristic curve section adjoining the first characteristic curve section, wherein the open-circuit voltage characteristic curve has a break point and / or a plateau in the transition region between the two characteristic curve sections.

[0002] Batteries are used in a wide variety of applications in the state of the art to provide a supply of electrical energy. This is particularly true for electric vehicles, where the battery serves to provide electrical energy for at least one traction motor during driving. An electric vehicle can refer to a purely electrically powered motor vehicle or a hybrid vehicle. When driving, it is important to know the range of the electric vehicle, taking into account the current battery charge level.

[0003] For this purpose, it is known to determine the range based on the battery's state of charge. This typically takes advantage of the fact that the open-circuit voltage depends on the battery's state of charge. Therefore, an open-circuit voltage characteristic curve can be used, which makes it possible to determine the battery's current state of charge if a corresponding open-circuit voltage is measured or calculated. For example, the open-circuit voltage could be calculated from a measured terminal voltage.

[0004] Practical use has shown that, among other things, the actual available current capacity of the battery depends on the ageing of the battery and usually decreases with increasing age. This has an impact on the open circuit voltage characteristic. In many cases, the open circuit voltage characteristic is only available for a new, essentially unaged battery. Such an open circuit voltage characteristic is determined, for example, on a test bench. With increasing age, the open circuit voltage characteristic deviates from the original open circuit voltage characteristic of a new battery. This means that the battery state of charge determined from the open circuit voltage is determined to be higher or lower than it actually is. This gives rise to the problem that the available range of the electric vehicle is too high or too low.is estimated too small, resulting in the risk of not reaching the desired destination. In the worst case, this can lead to the electric vehicle breaking down. This is especially problematic if no charging station is available.

[0005] To counteract this problem, the current open-circuit voltage characteristic, i.e., an aged open-circuit voltage characteristic, can be determined online and after a certain mileage of the electric vehicle. However, such an open-circuit voltage characteristic, which reflects the aged condition of the battery, can only be estimated or determined with sufficient accuracy with considerable effort and only for certain types of battery cells.

[0006] The invention is therefore based on the object of simplifying the determination of an open-circuit voltage characteristic and improving the estimation accuracy of the open-circuit voltage characteristic.

[0007] The invention is solved by the features of claim 1.

[0008] To estimate the open-circuit voltage characteristic curve simply and with high reliability and accuracy, raw data open-circuit voltage points are first determined from the measured raw data. Specifically, the battery terminal voltage is measured online in the electric vehicle, for example, after a predefined total driving distance, and the open-circuit voltage is calculated from this. Based on this, the raw data open-circuit voltage points or a raw data open-circuit voltage characteristic curve are created using well-known methods. The raw data open-circuit voltage points exhibit significant deviations due to measurement inaccuracies and other inaccuracies. A first and a second range of an open-circuit voltage characteristic curve are then determined.The first characteristic curve section extends through the first area and the second characteristic curve section extends through the second area, whereby the characteristic curve sections have different profiles and a transition area between the two characteristic curve sections has a break point and / or a plateau. The determination of the individual areas is carried out by detecting or determining the break point or the plateau, whereby the raw data open-circuit voltage points are evaluated or processed for this purpose. The exact profiles of the first characteristic curve section and the second characteristic curve section are then determined individually from the raw data using generally known methods. Finally, the open-circuit voltage characteristic is calculated using the individually determined characteristic curve sections, ieThe first characteristic curve section and the second characteristic curve section, and the determined inflection point and / or the plateau in the transition region between the characteristic curve sections, are combined. By determining the profiles of the characteristic curve sections and the transition region determined using the determined inflection point or the determined plateau, the open-circuit voltage characteristic can be unambiguously determined and reconstructed.

[0009] By reconstructing the open-circuit voltage characteristics in this way, the voltage can be determined easily, reliably, and with relatively high accuracy. This allows, for example, the range of an electric vehicle to be determined with high accuracy.

[0010] Preferably, the battery has a graphite and silicon-containing anode and / or cathode, wherein the first region is a silicon region, the second region is a graphite region, the first characteristic section is a silicon characteristic section and the second characteristic section is a graphite characteristic section.

[0011] In some battery types, it can be observed that the shape of the open circuit voltage potentials of the cathode and anode do not change significantly with aging. Therefore, the open circuit voltage characteristics are adjusted as aging progresses so that the open circuit voltage potentials are shifted and / or scaled along the state of charge axis. In a battery with a silicon-containing anode and / or cathode, the silicon continuously dissolves as the anode and / or cathode ages, while the graphite ages more slowly, thereby changing the shape of the open circuit voltage potentials of the anode and / or cathode. The dissolution of the silicon becomes apparent in the first region and at the first section of the characteristic curve, whereby the age-related change in the open circuit voltage characteristics is particularly noticeable in the first region, i.e., in the silicon region.In this case, scaling or shifting the open circuit voltage potentials of the cathode and / or anode is not useful.

[0012] Using the previously explained method, i.e., the reconstruction of the open-circuit voltage characteristic, an open-circuit voltage characteristic can be reliably provided for a battery with a silicon-containing anode and / or cathode, and the range of the electric vehicle can be determined with a relatively high degree of accuracy. The described method can also be applied to batteries with anodes and / or cathodes, which lead to a comparable effect and course of the open-circuit voltage characteristics, in order to determine the open-circuit voltage characteristics.

[0013] Preferably, the gradients between the raw data open-circuit voltage points are determined, and based on this, the inflection point and / or the plateau are determined. In the simplest case, two best-fit lines are created from the raw data open-circuit voltage points, i.e., a first best-fit line in the first region and a second best-fit line in the second region, wherein the lines have differing gradients or slopes. The inflection point is at the point where the lines intersect. The plateau can also be determined in a similar way, wherein, based on the determined gradients between the raw data open-circuit voltage points, a region is determined where the slope is almost zero. In this way, the transition region between the two regions can be determined simply and reliably.

[0014] In a preferred embodiment, a hysteresis-affected open-circuit voltage characteristic curve has a charging section and a discharging section that differs from the charging section due to hysteresis. One of the two charging sections or the discharging section is determined, and the other section is estimated based on a predefined hysteresis behavior. This allows a hysteresis-affected open-circuit voltage characteristic curve to be easily determined.

[0015] Here, for example, the charging section of the open-circuit voltage characteristic curve is determined using the method already explained. In addition, the discharging section could now also be determined by repeating the method, which would result in a complete hysteresis-affected open-circuit voltage characteristic curve. In principle, the hysteresis behavior of the open-circuit voltage of a battery, in particular a battery with a silicon-containing anode and / or cathode, is known from measurements on a test bench. For example, in batteries with a silicon-containing anode and / or cathode, it is known that the hysteresis exists essentially in the first region, i.e. in the silicon region. Based on this, the discharging section can be determined such that the already determined charging section is adapted using the known hysteresis behavior in the first region.This eliminates the need to repeat the previously explained procedure for estimating the characteristic curve.

[0016] An embodiment of the invention is explained in more detail with reference to the drawings. Fig. 1 shows an open circuit voltage characteristic of a battery in a new condition of the battery and in an aged condition of the battery, and Fig. 2 shows an open circuit voltage characteristic curve of a battery reconstructed according to a method according to the invention.

[0017] The Fig. 1 shows an open circuit voltage-state of charge diagram in which an open circuit voltage characteristic K neu a new type of battery and an open circuit voltage characteristic K altof an aged battery. In addition, the open-circuit voltage-state-of-charge diagram shows an example of an open-circuit voltage U0, which can be determined, for example, from a measured terminal voltage after a certain mileage of an electric vehicle. The intersection point between one of the open-circuit voltages U0 and the two open-circuit voltage characteristic curves K neu , K alt The state of charge (SOC) can be determined. It is crucial that the charge levels determined in this way are related to the open circuit voltage characteristic curve K neu and the open circuit voltage characteristic K alt differ considerably. Therefore, it is useful to use the aged open-circuit voltage characteristic K altto know as precisely as possible. The open-circuit voltage is a determined or measured open-circuit voltage. The open-circuit voltage characteristic is a set of points or several open-circuit voltages that together form a characteristic curve.

[0018] Basically, it is known that the no-load voltage characteristics consist of a first area B1 and a second area B2, which adjoins the first area B1, whereby the curves in the different areas B1, B2 differ from each other. As in Fig. As shown in Figure 1, the range B1, B2 changes with the aging of the battery. Fig. 1 it can be seen that the area B1 alt an aged battery compared to range B1 neu a new type of battery is smaller. Accordingly, the range B2 alt of the aged battery is greater than the range B2 neuof the new battery. This type of behavior can be observed, for example, in batteries with an anode or cathode containing graphite and silicon. Here, the first region B1 represents the behavior or capacity of the silicon, and the second region B2 represents the behavior or capacity of graphite. Therefore, the first region B1 forms a silicon region, and the second region B2 a graphite region. The reason for the change in regions B1 and B2 is that the silicon dissolves with increasing use of the battery, i.e., with progressive aging.

[0019] The Fig. 2 also shows an open circuit voltage-state of charge diagram, whereby this open circuit voltage-state of charge diagram is intended to explain how the aged open circuit voltage-state of charge diagram K alt is determined.

[0020] According to the invention, the open-circuit voltage characteristic K altAn aged battery is composed of two characteristic curve sections KN1, KN2, with a first characteristic curve section KN1 running through the first region B1 and the second characteristic curve section KN2 running through the second region B2. Thus, the first characteristic curve section KN1 forms a silicon characteristic curve section, and the second characteristic curve section KN2 forms a graphite characteristic curve section.

[0021] When determining the open circuit voltage characteristic K altFirst, raw data open-circuit voltage / state-of-charge points P are determined from the measured raw data. Specifically, the battery terminal voltage is measured online in the electric vehicle, for example, after a predefined total driving distance, and the open-circuit voltage is calculated from this. Based on this, the raw data open-circuit voltage / state-of-charge points P or a raw data open-circuit voltage / state-of-charge characteristic curve are created using well-known methods. However, the raw data open-circuit voltage / state-of-charge points P exhibit significant deviations due to measurement inaccuracies and other inaccuracies.

[0022] Subsequently, the first range B1 and the second range B2 are determined by determining a break point X between the two characteristic curve sections KN1, KN2, i.e., a transition region between the two ranges B1, B2. For this purpose, the raw data open-circuit voltage / state of charge points P are processed in such a way that two best-fit lines G1, G2, each with a gradient m1, m2, result, which intersect at the break point X.

[0023] In a subsequent step, the curves of the first and second characteristic curve segments are determined separately from the raw data using well-known methods. The exact curves of the characteristic curve segments KN1 and KN2 can be directly estimated using existing methods. Alternatively, the open-circuit potentials of the anode and cathode can be estimated first, and the exact curves of the first characteristic curve segments KN1 and KN2 can be determined from these.

[0024] Finally, the cold open circuit voltage-state of charge characteristic curve is compiled using the separately determined characteristic curve sections KN1, KN2 and the determined break point X between the characteristic curve sections KN1, KN2.

[0025] By reconstructing the open circuit voltage-state of charge characteristics K alt This can be determined easily, reliably, and with relatively high accuracy. This allows, for example, the range of an electric vehicle to be determined with high accuracy.

[0026] The battery with a graphite and silicon-containing anode and / or cathode has a hysteresis-affected open-circuit voltage-state-of-charge characteristic curve, wherein the course of the open-circuit voltage-state-of-charge characteristic curve exhibits hysteresis in the first region B1 depending on whether a charging or discharging process is taking place. When determining the open-circuit voltage-state-of-charge characteristic curve K alt according to Fig. 2 the open circuit voltage-state of charge characteristic curve K alt during a charging process of the battery, so that the characteristic curve section KN1 has a charging section KN1 L To determine a hysteresis-related discharge section KN1 E the determined charging section KN1 L adjusted using a known hysteresis behavior determined through laboratory tests. Alternatively, the procedure described above could be applied again for the discharge process.

Claims

[1] Method for determining an open-circuit voltage characteristic (K alt ) of a battery, where the open circuit voltage characteristic (K alt ) has a first characteristic section (KN1) and a second characteristic section (KN2) adjoining the first characteristic section (KN1), wherein the open-circuit voltage characteristic (K alt ) has a break point (X) and / or a plateau (P) in the transition area between the two characteristic curve sections (KN1, KN2), with the following steps: Determination of raw data open circuit voltage points (P), Determining a first area (B1) through which the first characteristic section (KN1) extends and a second area (B2) through which the second characteristic section (KN2) extends by determining the break point (X) or the plateau (PL) from the raw data open-circuit voltage points (P), Determining the course of the first characteristic curve section (KN1) and the second characteristic curve section (KN2), wherein the first characteristic curve section (KN1) and the second characteristic curve section (KN2) are determined separately from one another, and Composition of the open circuit voltage characteristic (K alt ) using the determined first characteristic curve section (KN1), the determined second characteristic curve section (KN2) and the determined break point (X) and / or the plateau (PL) in the transition region between the characteristic curve sections (KN1, KN2). [2] Method according to claim 1, characterized by that the battery has a graphite and silicon-containing anode and / or cathode, wherein the first region (B1) is a silicon region, the second region (B2) is a graphite region, the first characteristic section (KN1) is a silicon characteristic section and the second characteristic section (KN2) is a graphite characteristic section. [3] Method according to claim 1 or 2, characterized by that the gradients between the raw data open circuit voltage points (P) are determined and based on this the break point (X) and / or the plateau (PL) is determined. [4] Method according to one of the preceding claims, characterized by that the courses of the first characteristic section (KN1) and the second characteristic section (KN2) are estimated directly or that the open-circuit potentials of the anode and the cathode are first estimated and from this the exact courses of the first characteristic section (KN1) and the second characteristic section (KN2) are determined. [5] Method according to one of the preceding claims, characterized by that a hysteresis-affected open circuit voltage-state of charge characteristic (K alt ) a loading section (KN1 L ) and a hysteresis-related from the charging section (KN1 L ) different unloading section (KN1 E ), wherein one of the two charging sections (KN1 L) or the unloading section (KN1 E ) is determined and the other section (KN1 L , KN1 E ) is estimated based on a predefined hysteresis behavior. [6] Method according to one of claims 1 to 4, characterized by that a hysteresis-affected open circuit voltage-state of charge characteristic (K alt ) a loading section (KN1 L ) and a hysteresis-related one from the charging section (KN1 L ) different unloading section (KN1 E ), wherein the charging section (KN1 L ) and the unloading section (KN1 E ) are determined separately.

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