GB2606178A

GB2606178A - Method for cell balancing of battery cells as well as a battery management system

Info

Publication number: GB2606178A
Application number: GB2106051.2A
Authority: GB
Inventors: Glossmann Tobias
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2021-04-28
Filing date: 2021-04-28
Publication date: 2022-11-02
Also published as: GB202106051D0

Abstract

The invention relates to a method for cell balancing of battery cells of a battery having a predefined cell chemistry, comprising registering 10 voltage and impedance data of each of the battery cells during specified normal operation of the battery, at a low state-of charge of the battery determining 12 an anode potential for anodes of all of the battery cells, calculating state-of-charge differences between the battery cells based on the anode potentials of the battery cells, and providing 16 cell balancing depending on the calculated state-of-charge differences.

Description

METHOD FOR CELL BALANCING OF BATTERY CELLS AS WELL AS A BATTERY

MANAGEMENT SYSTEM

FIELD OF THE INVENTION

[0001] The invention relates to a method for cell balancing of battery cells of a battery having a predefined cell chemistry. Moreover, the invention relates to a battery management system for balancing of battery cells of a battery having a predefined cell chemistry.

BACKGROUND INFORMATION

[0002] Generic methods and battery management systems are well known in the art. A battery cell is a galvanic cell, which is intended to be used as an electric energy storage. Usually, the battery cell has at least two electrodes, namely an anode and a cathode which are in contact with a respective electrolyte. Electric energy is stored in the battery cell via a chemical reaction and can be regained again by inverse chemical reaction. Each of both electrodes has a respective electrode potential, namely the anode potential and the cathode potential, respectively, between which a voltage is provided depending on the cell chemistry, an impedance of the cell, the SOC and/or the like.

[0003] Usually, generic battery cells are used in secondary batteries. Secondary batteries are used for providing electric energy, for example for mobile devices, uninterruptable energy supply and so on. Appliances using secondary batteries usually require a supply voltage that is higher than the voltage provided by a single battery cell. Therefore, at least partially, some battery cells are electrically connected in series so as to match the voltage requirement of the application.

[0004] During operation of the battery, the SOC of the battery cells connected in series tend to deviate from each other. This is cumbersome and causes some decrease of the electric capacity of the battery which substantially limited by the battery cell in the series connection which has the smallest capacity. Therefore, it is usual to provide a cell balancing in order to reduce this deficiency. In this regard, WO 2019/020303 Al discloses a device and a method for balancing of energy storage module. Moreover, US 7,193,391 B2 discloses a method for cell balancing for lithium battery systems. Further, US 2019/0148952 Al discloses cell balancing with local sensing and switching. Also, US 2018/0337536 Al discloses battery balancing and current control.

[0005] Battery cells in a series connection forming a battery pack should perform as equal as possible in terms of impedance and capacity or the pack performance suffers from the one single lowest performing battery cell as each battery cell has to stay in a strict voltage range. Most battery cells with silicon-based anodes and lithium-ions as the mobile cation suffer from side reactions that cause impedance increase and capacity loss at lower states of charge a lot more than at higher SOC. This phenomenon also applies for other battery chemistries such as SeOx or Sn and P-based anodes with lithium, sodium or other respective cations. As battery cell ageing is much more pronounced for these types of battery chemistry at lower SOC, or in other words, at a high anode potential versus lithium, optimizing the individual battery cell SOCs for anything but the lower end of their discharge curve will lead to unequal ageing of the battery cells.

[0006] State of the art cell balancing methods utilize the open circuit voltage of battery cells collected during operation and adjust the SOC of higher or lower charged battery cells with various methods with the goal of maximizing power or usable capacity such as US 7,193,391 B2. This approach collects data of battery cells during operation just like other methods. But this teaching takes the likelihood of occurrence of low single cell voltages in account more than other events to determine its state. A battery cell that experiences low cell voltage or high anode potential, respectively, more often, will age disproportionally more than the other battery cells. If one battery cell ages more, it will also show low voltage more often due to higher impedance, more thermal stress and lower capacity.

SUMMARY OF THE INVENTION

[0007] It is an object of the invention to improve balancing of battery cells of a battery, especially, when the battery cell are electrically connected in series.

[0008] According to the invention, a method and a battery management system according to the independent claims are proposed.

[0009] Preferable embodiments can be derived from the features of the dependent claims.

[0010] A method for cell balancing of battery cells of a battery having a predefined cell chemistry is proposed, comprising registering voltage and impedance data of each of the battery cells during specified normal operation of the battery; at a low state-of-charge of the battery, determining an anode potential for anodes of all of the battery cells; calculating state-of-charge differences between the battery cells based on the anode potentials of the battery cells; and providing cell balancing depending on the calculated state-of-charge differences.

[0011] A battery management system for balancing of battery cells of a battery having a predefined cell chemistry is proposed, the battery management system comprising an apparatus that is configured to register voltage and impedance data of each of the battery cells during specified normal operation of the battery; determine, at a low state-of-charge of the battery, an anode potential for anodes of all of the battery cells; calculate state-ofcharge differences between the battery cells based on the anode potentials of the battery cells; and provide cell balancing depending on the calculated state-of-charge differences.

[0012] The invention considers that a voltage change over state-of-charge (SOC) change is larger at lower SOC then at higher SOC. This is true for an anode potential as well as a cathode potential.

[0013] Contrary to the state of the art, the invention teaches to provide cell balancing of battery cells not only depending from other actual state of the art, but generally based on the consideration that the anode potential has a clear and distinct effect on battery cell ageing. Therefore, the inventive concept uses determining the anode potential for the anode of the battery cells in order to get information whether balancing shall be provided. For this purpose, the anode potential can be calculated and/or estimated from the registered data. In this regard, the invention further considers that the anode potential provides a sharp voltage drop with regard to a reference electrode which is basically considering the cell chemistry. Therefore, the sharp voltage drop of the anode potential with regard to the reference electrode is a clear indication that the respective cell reaches its capacity limit. Since this consideration is based on basic physical and chemical constitutions, the inventive method allows more precise determination when balancing should be provided, than the state of the art.

[0014] Contrary to the invention, in the state of the art, balancing is usually provided during the fully loaded battery. The deficiencies of the state of the art can be overcome. The invention allows providing balancing at any time independent from a current SOC based on the previously determined SOC differences based on the anode potential differences determined at low SOC of the battery. Therefore, the invention lends especially to lithium based battery cells.

[0015] The invention further considers that the anode potential, especially at high anode potentials (corresponding to low SOC of the battery cell), appears to be responsible for unwanted mechanisms on the anode side. Those are especially pronounced for alloy anodes such as silicon-based anodes in lithium-ion batteries. It is crucial for equal aging of battery cells that lower anode potential that have a disproportionally higher impact on cell ageing are kept to be equal between the battery cells. That is true for open circuit potential (battery cell at rest) and for battery cell potentials underload.

[0016] For this reason, it is suggested according to the invention that the typical battery cell balancing, as described above, tries to equalize the SOC between battery cells and usually uses the open circuit potential, when the battery is at rest, is not adequate. Rather than balancing the SOC according to the state of the art, the inventive battery management system and method, respectively, deals with this finding to improve a cell life of battery cells.

[0017] Further advantages, features, and details of the invention derive from the following description of preferred embodiments as well as from the drawings. The features and feature combinations previously mentioned in the description as well as the features and feature combinations mentioned in the following description of the figures and/or shown in the figures alone can be employed not only in the respectively indicated combination but also in any other combination or taken alone without leaving the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings show in: [0019] Fig. 1 a schematic diagram showing a voltage between an anode versus a Li-metal reference, here comprising a Si/graphite composite, following an SOC-dependent curve; [0020] Fig. 2 a schematic diagram showing a voltage between a cathode versus a Li-metal reference following an SOC-dependent curve, [0021] Fig. 3 a schematic flow chart showing a method according to the invention and [0022] Fig. 4 a schematic diagram showing an open circuit voltage (OCV) of the anode during charging of the battery.

[0023] Fig. 5 a schematic diagram showing an OCV of the anode during discharging of the battery.

[0024] Fig. 6 a schematic diagram showing an OCV of the cathode during charging of the battery.

[0025] Fig. 7 a schematic diagram showing an OCV of the cathode during discharging of the battery.

[0026] Fig. 8 a schematic diagram showing an OCV of the anode and cathode during charging and discharging of the battery.

[0027] In the figures the same elements or elements having the same function are indicated by the same reference signs.

DETAILED DESCRIPTION

[0028] In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration". Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

[0029] While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawing and will be described in detail below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.

[0030] The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion so that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a system or apparatus preceded by "comprises" or "comprise' does not or do not, without more constraints, preclude the existence of other elements or additional elements in the system or method.

[0031] In the following detailed description of the embodiment of the disclosure, reference is made to the accompanying drawing that forms part hereof, and in which is shown by way of illustration a specific embodiment in which the disclosure may be practiced. This embodiment is described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.

[0032] Fig. 1 shows a voltage between an anode and a Li-metal reference based on Si/graphite composite the following SOC-dependent curve indicated by the reference character 20 in the diagram. An ordinate is allocated to a potential (V vs. Li/Lit). An abscissa is allocated to an exchanged capacity (mAh cm-2). As can be seen from Fig. 1, the anode potential vs. Li/Lit rises sharply in the delithiated anode (see Fig. 1; > 0.75 V is typical for the last 20 % capacity).

[0033] Fig. 2 shows a schematic diagram, wherein the voltage between a cathode vs. the Li-metal reference following a SOC-dependent curve 22 is depicted. As can be seen from Fig. 2, the cathode potential also sharply changes at low SOC that is when the cathode is highly lithiated. However, battery cells are designed with oversized anodes in order to prevent metal plating and there are lithium inventory losses during the formation of the cell. All lithium inventory originates in the cathode material. Therefore, the anode will surely be delithiated while the cathode is only fully lithiated in a new battery cell and, consequently, operates outside of the sharp voltage change range.

[0034] From Fig. 1 and 2, it can be derived that the voltage change over SOC change is larger at lower SOC than at higher SOC. That is true for the anode potential of a specific battery cell as well as the cathode potential of the same battery cell. Therefore, the detection of single battery cell SOC differences is more accurate at lower SOC than at higher SOC.

[0035] The anode potential of a specific one of the battery cells, especially at high anode potentials, which corresponds to low battery cell SOC, is responsible for unwanted mechanisms of the anode side. Those are especially pronounced for anodes such as silicon based anodes in Li-ion batteries. It is crucial for equal aging of battery cells that lower anode potential differences that have a disproportionally higher impact on battery cell aging are kept between the battery cells. That is true for open circuit potential, when the battery cell is at rest, and for cell potential under load.

[0036] Battery packs consist of battery cells connected in series, parallel or a combination of both. A typical pack for an electric vehicle may consist of a series of 100 battery cells in series. All battery cells need to have a SOC that is supposed to be equal for all battery cells as far as possible, in order to allow full power load charging and discharging. The reason lies in the fact that all battery cells have to stay within a strictly defined voltage range otherwise electrochemical reactions will damage the electrolyte quickly and cause growth of passivating, unwanted layers of decomposition products on the electrode surfaces. SOC and open circuit voltage (OCV), sometimes also called open circuit potential (OCR), are linked in most battery cells.

[0037] Li-ion and similar battery cells do not show significant reversible side reactions at higher SOC that could self-balance the system. Therefore, such battery packs use single cell voltage monitoring and balancing electronics to keep the battery cells at the same voltage level that correlate to SOC.

[0038] Redox shuttle molecules could be used to achieve cell balancing without electronic circuit if such molecules were found for the given cell voltage range. However, the cell voltage of a specific battery cell still has to be monitored as impedance differences may cause some battery cells to hit higher or lower than allowed voltage levels under load.

[0039] For this reason, it is suggested by the invention that the typical battery cell balancing, provided by many methods of the art, that tries to equalize the SOC between the battery cells and usually uses the open circuit potential when the battery is at rest is not adequate. Rather than balancing the SOC, the invention teaches a battery manage system and a method that calculates a figure for each cell that represents its tendency to reach low anode voltages or potentials in operation. This figure should be based on impedance as well as SOC and is likely application-dependent. The method of the invention will ensure equal aging of the battery cells, especially when electrically connected in series.

[0040] The calculated figure for each battery cell will then be utilized to decide which battery cells need to be discharged or charged more using the various charge balancing methods available in the market but without using the method of those technologies used to determine the required charge change in each battery cell which is typically the difference in open circuit voltage.

[0041] Fig. 4 to Fig. 8 further detail the advantages of the invention according to Fig. 1. Fig. 4 comprises a plot that shows a curve 24 corresponding to the SoC of Fig. 1. In particular, Fig. 4 shows the anode during charging of the battery. Fig. 6 shows the curve 28 corresponding to the SoC of Fig. 1. In particular, Fig. 6 shows the cathode during charging of the battery. In an embodiment, the anode may dominate voltage change over capacity change at the empty end of charging. In an embodiment, the battery cell may be full of charge when the anode is not completely lithiated but the cathode is almost fully delithiated.

[0042] Fig. 5 shows a curve 26 corresponding to the SoC of Fig. 1. In particular, Fig. 5 shows the anode during discharging of the battery. Fig. 7 shows a curve 30 corresponding to the SoC of Fig. 1. In particular, Fig. 7 shows the cathode during discharging of the battery. In an embodiment, the anode may dominate voltage change over capacity change at the empty end of discharging. Fig. 8 shows the curves 24 through 30 in an overlaid manner in a single plot diagram.

[0043] Fig. 3 shows a schematic flow chart using the inventive method. In this embodiment a predefined a cell chemistry of the battery cells which are electrically connected in series is used, which is in this exemplary embodiment based on lithium-ionbattery cells. Based on the cell chemistry anode potentials of the battery cells are estimated considering a reference electrode which is in the present case a lithium-electrode. Estimating further considers that a sharp voltage drop between an anode and the reference electrode appears at low SOC. The sharp voltage drop allows determining the SOC of any battery cell more precise than with the state of the art.

[0044] During operations, in a step 10, voltage and impedance data of each of the battery cells are registered. Based on this data, in a second step 12, the anode potentials of all of the battery cells are estimated.

[0045] In step 14, the estimated anode potentials of the battery cells is compared with the sharp voltage drop by considering the reference electrode. If the anode potential is not in the range of the sharp voltage drop, the method proceeds with step 10.

[0046] However, if any of the anode potentials is in the range of the sharp voltage drop, the method proceeds with step 16. In step 16, a balance electronic is started to match a predefined anode potential. After having finished balancing, the method proceeds with step 10.

[0047] The above-mentioned procedure will be preferably executed for all of the battery cells, especially those ones which are electrically connected in series.

[0048] In a further exemplary embodiment, which is based on the previous described embodiment, step 14 also includes a comparison step, where a major anode potential imbalance is detected. If no major anode potential imbalance is detected, the method proceeds with step 10. However, if a major anode potential imbalance is detected, the method proceeds with step 16. This further comparison step can be executed at the same time, when the previous mentioned comparison step 14 is executed.

[0049] Moreover, the step of estimating may include calculating a figure for the at least one battery cell which may dependent on the SOC. The figure may also represent the tendency of the battery cell to reach low anode potential in operation which figure can be considered during the step of comparing. This may simplify the step of comparing.

[0050] The figure may be further based on an impedance and/or a state-of-charge of the at least one battery cell.

[0051] Preferably, the method may also comprise applying the method to the at least one battery cell only if the at least one battery cell is utilized in the range of the state-ofcharge which is smaller than 70 %. This considers the worst case of each single battery cell and even then the likelihood for unequal aging is still present due to increased aging at lower anode voltages.

[0052] The invention has also the advantage that battery pack assemblies can be operated using silicon based and other similarly behaving technologies with minimized balance "runaway" using the invention. Without the technology, a battery pack using this type of anodes may quickly encounter single battery cell failure. The invention avoids that a specific battery cell or a few battery cells that hit low voltages, will quickly age even more, a self-accelerating effect.

[0053] Balancing itself can be provided at any time. Balancing can be based on the data received by estimation. The use a software algorithm to control battery cell balancing hardware such that high single cell anode potential which usually appeared at low SOC, is the same for all the battery cells, especially when they are electrically connected in series.

[0054] Outside of the low SOC range, balancing need not be started except in case of major battery cell imbalance, a condition that should only be relevant in special situations such as pack assembly.

[0055] The embodiments discussed above shall not be regarded as limiting the scope of the claims.

List of reference signs step 12 second step 14 comparison step 16 step reference character 22 curve 24 curve 26 curve 28 curve curve

Claims

CLAIMS1. A method for cell balancing of battery cells of a battery having a predefined cell chemistry, comprising: registering (10) voltage and impedance data of each of the battery cells during specified normal operation of the battery; at a low state-of-charge of the battery, determining (12) an anode potential for anodes of all of the battery cells; calculating state-of-charge differences between the battery cells based on the anode potentials of the battery cells; and providing (16) cell balancing depending on the calculated state-of-charge differences.
2. The method according to claim 1, wherein the low state-of-charge of the battery is less than 35%, preferably less than 20%, most preferably less than 15%.
3. The method according to any one of the preceding claims, wherein, for each of the battery cells, an electrode capacity of the anode is oversized with regard to an electrode capacity of the cathode, and the anode potential is determined based on an open circuit voltage of the battery cell.
4. The method according to any one of the preceding claims, further comprising applying the method to the battery only if the battery is utilized in the range of the state-of-charge which is smaller than 70%.
5. The method according to any one of the preceding claims, further comprising providing cell balancing of the at least one battery cell if a major anode potential imbalance is detected.
6. A battery management system for balancing of battery cells of a battery having a predefined cell chemistry, comprising an apparatus that is configured to register (10) voltage and impedance data of each of the battery cells during specified normal operation of the battery; determine, at a low state-of-charge of the battery, an anode potential for anodes of all of the battery cells; calculate state-of-charge differences between the battery cells based on the anode potentials of the battery cells; and provide (16) cell balancing depending on the calculated state-of-charge differences.