Method and apparatus for determining the amount of gas in a battery cell of a battery pack

Abstract

The present invention relates to a method for determining the amount of gas (21) located in a battery cell (1) of a battery pack, wherein the battery cell (1) of the battery pack has an initial volume and the method has at least the following steps: a) immersing the battery cell (1) in a non-conductive liquid (3) having a defined density at a first ambient pressure (19); b) generating a lifting force directed contrary to the sinking force of the battery cell (1); c) changing the ambient pressure to a second ambient pressure (19) and measuring the buoyancy force of the battery cell (1) in the non-conductive liquid (3) depending on the ambient pressure (19); d) determining the amount of gas (21) located in the battery cell (1) taking into account the first ambient pressure and the second ambient pressure, the buoyancy force determined for the ambient pressure, the temperature of the non-conductive liquid (3) and the density of the liquid.

Description

Field of the Invention

[0001] The present invention relates to a method and a device for determining the amount of gas in a battery cell of a battery pack. Background Art

[0002] Battery cells, such as those used in the traction battery packs of electric vehicles, typically have a plurality of so-called pouch cells. These pouch cells are herein configured to permanently and tightly enclose a battery stack disposed therein using a resistive pouch film. To this end, the battery stack has, for example, an anode layer, a cathode layer, and a separator layer, an electrolyte, and conductor tabs (Ableiterfähnchen) in order to conduct the generated current through a conductor tab (Ableitertab) to a contact portion located outside the pouch cell.

[0003] In order to determine the long-term durability of a lithium-ion pouch cell, it is necessary to determine the gas formation in the pouch cell and the resulting pressure increase. Using the information obtained in this way, physical and empirical models can be created and equipped with data, which models allow predicting gas formation under different usage scenarios.

[0004] To this end, it is for example known to use the change in volume of a pouch cell, which is exhibited by the expansion of the pouch cell or a so-called airbag, for measuring gas formation. To this end, the Archimedes principle described, for example, in US 2019 / 0280333A1 is known. The disadvantage of this method is that once the pouch cell is completely filled with gas, only the pressure inside the pouch cell will increase further, while the volume no longer changes. Therefore, only the initial, i.e., the initial gas formation, can be measured using this method.

[0005] It is also known to determine the amount of gas located in a pouch cell by opening the pouch cell in a defined volume of vacuum in order to then derive the volume of the gas amount from the measured pressure increase during this process. In this second method for determining the amount of gas contained in a pouch cell, the pouch cell to be inspected has to be destroyed for measurement purposes. However, it is not possible thereby to observe the gas formation in the pouch cell over a longer period of time. Instead, if models for gas formation are to be created at different states of charge and temperatures, an extremely large number of pouch cells to be tested are required. In addition, the following disadvantage occurs in this method: the determined gas amount is significantly influenced by the solubility of the gas in the electrolyte used. The solubility of the gas in the electrolyte in turn depends to a large extent on the pressure present in the battery pack cell or the pouch cell and on the temperature, such that the gas amount to be determined also depends to a large extent on the temperature present at the time of performing the determination and on the subsequent measurement pressure. Summary of the Invention

[0006] Accordingly, the object of the present invention is to solve at least in part the problems arising in the prior art. In particular, a method and a device should be described, which can improvedly determine the amount of gas contained in a battery cell of a battery pack. In particular, the determination of the amount of gas should be carried out in a non-destructive manner, so that the amount of gas can be determined in a non-destructive manner and at different temperatures by repeatedly and distributed over a period of time measuring the battery cell of the battery pack.

[0007] A method having the features according to the invention helps to solve these tasks. Advantageous extensions are the subject of the following description. The individual features listed in the description can be combined with each other in a technically meaningful way and can be supplemented by explanatory facts from the description and / or details from the drawings, which show further implementation variants of the invention.

[0008] In the present case, a method for determining the amount of gas located in a battery cell of a battery pack is proposed, wherein the battery cell of the battery pack has an initial volume and the method has at least the following steps:

[0009] a) Immerse the battery cell of the battery pack in a non-conductive liquid having a defined density at a first ambient pressure;

[0010] b) Generate a lifting force (Hebekraft) directed oppositely to the driving force (Abtriebskraft) of the battery cell of the battery pack;

[0011] c) Change the ambient pressure to a second ambient pressure and measure the buoyancy force (Auftriebskraft) of the battery cell of the battery pack in the liquid depending on the ambient pressure;

[0012] d) Determine the amount of gas located in the battery cell of the battery pack taking into account the second ambient pressure, the buoyancy force determined for the second ambient pressure, the temperature of the liquid and the density of the liquid.

[0013] In this method, in the first step a), the battery cell of the battery pack to be inspected is immersed in a non-conductive liquid. This is done by completely immersing the battery cell of the battery pack in the liquid. The liquid itself is non-conductive, i.e. the liquid cannot conduct an electric current, and the liquid also has a known density, for example the density can be stated in grams per cubic centimeter.

[0014] Here, the first ambient pressure can for example be the hydrostatic air pressure at the location of the earth's atmosphere where the measurement is carried out. This pressure is generated (represented) by the gravity of the air column standing upright on the earth's surface or a body. Thus, for example, according to the standard, the average air pressure at sea level is 101325 [Pa]. However, the first ambient pressure can also be the air pressure actually currently present at the measurement location, or if particularly good comparability and reproducibility of different measurements all carried out at the same air pressure should be achieved, the first ambient pressure can be a standardized air pressure. In this case, for example, the average air pressure of the atmosphere can be 101325 [Pa]. In particular, for determining the gas quantity in the case of a fully filled battery cell of a battery pack that has reached its maximum volume, it is already meaningful to select the first ambient pressure such that it is greater than the gas pressure located in the battery cell of the battery pack.

[0015] In the next step b), a lifting force is generated that is directed oppositely to the sinking force generated by the battery cell of the battery pack. The lifting force is used to hold the battery cell of the battery pack in a suspended state in the liquid.

[0016] In the subsequent step c), the ambient pressure existing in the medium surrounding the liquid and the battery cell of the battery pack is increased to a second ambient pressure. During the increase of the ambient pressure to the second ambient pressure, the buoyancy force of the battery cell of the battery pack generated by the lifting force and the sinking force is measured by means of a single measurement or alternatively continuously.

[0017] Then, in the fourth step d) of the method, the gas quantity located in the battery cell of the battery pack is determined, where the first and second ambient pressures, the buoyancy forces determined for these ambient pressures, the temperature of the non-conductive liquid, and the density of the liquid are used.

[0018] Here, it is first assumed that the buoyancy force is calculated based on the general gas equation (Gasgleichung) according to the following formula:

[0019]

[0020] In particular, the following parameters can be determined or used here:

[0021] m = force [N]; for example m 浮力 ; m 下沉力 ; m 提升力 ; m 测量

[0022] n = amount of substance of the gas located in the battery cell of the battery pack [mol (mole)]

[0023] R = universal gas constant; [8.3144 ​

[0024] T = Temperature in Kelvin [K]

[0025] p = Ambient pressure [Pa]

[0026] V = Volume of the cells of the battery pack in the new state; volume of the receiving device

[0027] ρ 液体 = Specific density of the non-conductive liquid [g / cm 3 .

[0028] The receiving device can be used to fix the cells of the battery pack in a defined and stable position during immersion. In the case of cells of a battery pack constructed as a so-called soft pack battery, it can happen that these cells may already be deformed due to their own weight when immersed. In this case, the receiving device can provide a remedy and ensure that the cells of the battery pack are immersed without a greater degree of deformation. Here, as an exception among these deformations are those caused by a change in the volume of the gas contained in the cells of the battery pack. Such a deformation is possible even when using the receiving device. In particular, if an airbag is provided on the cells of the battery pack and filled with gas, the airbag may be deformed. During battery operation, the receiving device also ensures shape stability and reproducible electrical characteristic parameters. Otherwise, the gas formation and volume work of the active material may lead to a contact loss of the electrode plates in the battery.

[0029] For example, the temperature of the non-conductive liquid can be detected by a simple temperature sensor and continuously monitored if necessary.

[0030] The buoyancy force still required to determine the gas quantity can be determined in step d) of the method according to the following formula:

[0031] m 浮力 = m 下沉力 - m 提升力 + m 测量

[0032] The sinking force is here composed of the gravity of the cells of the battery pack plus, if necessary, the gravity of the holding device for accommodating the cells of the battery pack in the liquid.

[0033] Here, the lifting force is a force generated by a force generating device and directed opposite to the sinking force.

[0034] Furthermore, the measured resultant force [m 测量 is the measured resultant force composed of the sinking force and the lifting force. This resultant force is the force exerted by the force generating device minus the counteracting sinking force.

[0035] It is particularly advantageous here that the force generating device acts against the force measuring device. In this case, the resultant force can be directly read from the force measuring device as a measured value. In a particularly simple embodiment, the force generating device can be a defined weight, a pre-tensioned spring, a hydraulic cylinder or a pneumatic cylinder or a similar device suitable for applying a force. The force measuring device can for example be a balance that detects the resultant force acting on it.

[0036] If all the necessary data is available after performing the above steps, the amount of substance (n) of the gas in the battery cells of the battery pack can be calculated after conversion using the above equation.

[0037] For this purpose, steps a) to d) can be performed at least once in the order described here. These steps can be performed at different frequencies and / or at least partially overlapping in time.

[0038] Step c) can also be performed multiple times during the increase in ambient pressure, so as to determine and record the time course of the buoyancy force according to the corresponding ambient pressure.

[0039] As an alternative to this method, the amount of gas in the battery cells of the battery pack can also be determined by determining and recording the product that changes with the increase in ambient pressure in order to derive a graph therefrom, for example, by means of a graphical method or a mathematical function. For this purpose, for example, this product can be plotted on the Y-axis of a two-dimensional coordinate system, and the reciprocal value of the ambient pressure can be plotted on the X-axis of this two-dimensional coordinate system. The amount of substance of the gas in the battery cells of the battery pack is directly obtained from the slope of the graph within the following value range, in which the change in ambient pressure causes a change in the value of this product. Specifically, this means that as long as the ambient pressure is less than or equal to the internal air pressure in the battery cells of the battery pack, the value of this product remains constant. Here, this internal air pressure is the air pressure present in the gas in the battery cells of the battery pack or in the airbag. Once the ambient pressure rises above the internal air pressure of the battery cells of the battery pack, the volume of the gas or the volume of the airbag changes, and the value of this product begins to decrease with the increase in ambient pressure. This should be explained as follows: due to the increase in external pressure, the gas volume is compressed, thereby reducing the buoyancy force generated.

[0040] In particular, it can be stipulated that, when carrying out the method, the non-conductive liquid and the battery cells of the battery pack are maintained at a defined temperature during steps a) to d). Although the method can be carried out without problems at various temperatures (for example below room temperature), for the actual operation of the battery cells of the battery pack, it is generally desirable to more accurately check and determine the characteristics of these battery cells with regard to gas formation at higher temperatures. Of interest here is: in particular higher temperatures, for example temperatures between 40 °C and 70 °C. Such temperatures can occur, for example, when the battery cells of the battery pack are operating in a very hot environment or under summer operating conditions or during rapid charging. Of particular importance here is to more accurately understand the behavior of the battery cells with regard to gas formation in order to ensure the long-term safe and reliable operation of the battery cells of the battery pack.

[0041] In particular, in the method, it can also be stipulated that a pressure higher than the internal air pressure in the battery cells of the battery pack is selected as the second ambient pressure. Thereby, even in the case where the battery cells of the battery pack have formed gas on a large scale and a very high internal air pressure relative to the ambient pressure has been formed in the battery cells of the battery pack, it is possible to carry out: the determination of the amount of gas present in the battery cells of the battery pack. In the previously known methods, it is no longer possible or at least not possible non-destructively to determine the amount of gas present in the battery cells of the battery pack in this state, while by using a second ambient pressure higher than the internal air pressure present in the battery cells of the battery pack, it is possible to carry out non-destructively in these cases: the determination of the amount of substance or gas contained.

[0042] In particular, a pressure lower than the internal air pressure in the battery cells of the battery pack can be selected as the second ambient pressure. This method can be applied if the battery cells of the battery pack have only a very small filling volume, in which case the gas present in the battery cells of the battery pack is only at a very low pressure. In this case, by means of the reduced second pressure, the gas volume present in the battery cells of the battery pack can first be increased, thereby making the determination of the buoyancy easier and more precise. If necessary, the second ambient pressure can be reduced until the battery cells of the battery pack reach their maximum possible volume expansion due to the increase in gas volume. By using a second ambient pressure lower than the first ambient pressure, it is possible to determine the amount of gas contained even in the case of battery cells in which the gas is only at a very low pressure and in particular in which the battery cells of the battery pack have not reached their maximum volume expansion.

[0043] In particular, when implementing this method, it can also be stipulated that the lifting force is selected such that the sum of the lifting force and the buoyancy force is greater than or equal to the sinking force of the battery cells of the battery pack. This achieves the following: The battery cells of the battery pack immersed in the non-conductive liquid and, if necessary, together with the housing device connected to the battery cells are completely immersed in the liquid, are reliably kept suspended in the liquid, and at the same time do not sink to the bottom of the liquid container.

[0044] It is also particularly advantageous to additionally take into account in step d) the temperature-dependent solubility of the gas in the liquid present in the battery cells. It has been shown that in order to dissolve the gas also present in the battery cells, the solubility of the liquid present in the battery cells decreases significantly with increasing temperature. The liquid is usually an electrolyte and may be moisture formed or accumulated in the battery cells. If this is the case, not taking into account the solubility that drops sharply with increasing temperature will lead to significant inaccuracies in determining the amount of gas released. If the solubility of the liquid is now taken into account in the knowledge of the temperature actually present during the measurement, the amount of the substance can be determined with significantly improved accuracy.

[0045] In particular, the measurement of the parameters can be carried out in a closed pressure chamber during steps a), b) and c). The ambient pressure can be set in a particularly accurately controlled manner in the closed pressure chamber. This has two advantages. On the one hand, it is also possible to achieve a higher second ambient pressure in a particularly simple manner, which is significantly higher than the internal air pressure present in the battery cells of the battery pack. As already mentioned above, it is thus also possible to first check the battery cells with a particularly high internal air pressure to determine the amount of gas present therein. Here, it is even possible to first reduce the first ambient pressure before the start of the measurement in order to first increase the possibly present and only very small gas volume. In the case of a subsequent increase in the ambient pressure, the volume change of these very small gas volumes can then also be detected, the corresponding buoyancy value can be determined, and finally the amount of the gas can be determined. Finally, the ambient pressure applied separately can be measured with particularly high precision in the defined pressure chamber, and thus the amount of gas contained can also be calculated with high precision. In addition, the measurement can be reproduced particularly well in this way.

[0046] In particular, in order to determine the amount of the substance, at least one temporal pressure change process of the ambient pressure, the temperature change process of the non-conductive liquid or the change process of the buoyancy force can be continuously detected. Preferably, the mentioned change processes are detected and stored simultaneously and during the increase of the ambient pressure to the second ambient pressure for subsequent evaluation for the purpose of determining the amount of the gas.

[0047] This method is particularly suitable for being carried out in combination with an aged battery cell of a battery pack, wherein the airbag (i.e., Gas - Pocket) of the aged battery cell of the battery pack is bulged, and the internal air pressure in the airbag is greater than the hydrostatic air pressure, and in particular greater than 1 bar.

[0048] Furthermore, the present invention also provides a device for determining the amount of gas located in a battery cell of a battery pack. The device respectively has at least one pressure chamber, a liquid container arranged in the pressure chamber, a pressure measuring device for measuring the ambient pressure, a device for generating a lifting force, a measuring device for measuring the generated buoyancy force, and a control device for detecting and processing the detected measured values and for determining the amount of gas.

[0049] Here, the ambient pressure is measured in the pressure chamber and in the environment immediately adjacent to the battery cell to be inspected. The pressure measuring device is suitable for measuring a first ambient pressure and a large number of other higher second ambient pressures.

[0050] Here, the control device can be functionally connected to at least one measuring device, sensor or other device, which is suitable for or is set to perform the steps described herein of the proposed method, in particular steps a) to d) of the method claims.

[0051] Furthermore, the device can have a heating device for heating a non - conductive liquid located in the liquid container and a temperature measuring device for measuring the temperature of the liquid.

[0052] In addition, a computer program is provided, which includes instructions that cause the proposed device to perform the method steps according to the independent method claims.

[0053] Finally, a computer - readable medium is also provided, on which the computer program is stored.

[0054] It should be noted in advance: The numerals used here ("first", "second",...) are mainly (only) used to distinguish multiple objects, variables or processes of the same type, that is, in particular, they do not compulsorily pre - specify the dependencies and / or order between these objects, variables or processes. If the dependencies and / or order are necessary, they will be clearly stated here or will be obvious to those skilled in the art when studying the specifically described design. Description of the Drawings

[0055] The present invention and the technical field thereof will be explained in more detail below based on the drawings. It should be noted that the present invention should not be limited by the described embodiments. In particular, unless otherwise explicitly stated, partial aspects of the facts explained in the drawings can also be extracted and combined with other components and knowledge from this specification. In particular, it should be noted that the drawings and especially the shown dimensional relationships are only schematic.

[0056] Figure 1 A schematic view of a device for determining the amount of gas in a battery cell of a battery pack is shown; and

[0057] Figure 2 A graph created within the scope of the method for determining the amount of gas is shown. Detailed Description of the Invention

[0058] In Figure 1 a possible embodiment of a device for performing a method for determining the amount of gas 21 in a battery cell 1 of a battery pack according to the present invention is shown in a schematic side view. The battery cell 1 of the battery pack has air bags 2 at both ends, which are determined to accommodate the gas 21 that may be formed within the battery cell 1 during operation. The battery cell 1 is completely immersed in a non-conductive liquid 3 held in a liquid container 4. The battery cell 1 is held below the surface of the non-conductive liquid 3 by a downward-acting sinking force [m 下沉力 . This sinking force acts in opposition to a lifting force [m 提升力 generated by a force generating device 14. For this purpose, the generated lifting force is transmitted to the battery cell 1 via a rope 5, which is deflected by two pulleys 7. The lifting force is dimensioned such that the lifting force is greater than the sinking force. Thereby, the battery cell 1 is reliably held in a floating state within the liquid container 4. The resultant force [m 测量 composed of the lifting force and the sinking force acts downward in the vertical direction on a force measuring device 6. The force measuring device 6 is implemented as a precision balance, for example, and can detect this resultant force very accurately.

[0059] The above-mentioned components are arranged together in a pressure chamber 15. In addition, a pressure measuring device 8 and a temperature measuring device 9 are arranged in the pressure chamber 15. If the temperature of the non-conductive liquid 3 drops below a pre-given temperature value, the non-conductive liquid 3 can be heated by a heating device 10 also arranged in the pressure chamber 15, and thereby the pre-given temperature can be maintained. The pressure chamber 15 has a door 16 on its side, which is determined to completely enclose the pressure chamber 15 with respect to the environment in the closed state and to allow access to the components or devices located therein in the open state.

[0060] The pressure measuring device 8 is connected to a display or recording device 11 for pressure measurement. The force measuring device 6 is on its side connected to a display or recording device 12 for force measurement, and the temperature measuring device 9 is connected to a display or recording device 13 for temperature measurement. All the display or recording devices 11, 12, 13 are connected to a control device 17 which can call up and further process the stored measurement data for pressure, temperature and the measured force. In order to avoid deformation of the battery pack cell 1 during measurement, the battery pack cell 1 is held in a receiving device 18. In the present embodiment, the receiving device 18 consists of two clamping plates (Spanplatten), and the battery pack cell 1 is arranged between these two clamping plates, wherein these clamping plates are connected to each other by means of a plurality of screws.

[0061] Thus, in the illustrated embodiment, the sinking force is calculated from the gravity of the battery pack cell 1 and the gravity of the receiving device 18 including the clamping plates and screws used. The buoyancy force experienced by the battery pack cell 1 and the receiving device 18 is determined by the volume of these two components and the specific density of the non-conductive liquid 3.

[0062] Since the volume of the battery pack cell 1 has changed due to the formation of the gas to be inspected compared to the initial volume defined during manufacturing, the buoyancy force cannot be calculated but must be determined by measurement. For this purpose, the resultant force exerted on the force measuring device 6 by the force generating device 14 is measured by means of the force measuring device 6. In this particularly simple embodiment, the force generating device 14 is configured as a defined weight. Using the values known in this way, the buoyancy force m can be calculated using the following formula 浮力 :

[0063] m 浮力 =(m 电池组电池 +m 夹紧板 +m 螺钉 )-m 提升力 +m 测量

[0064] Here, a first measurement of the buoyancy force is carried out at a first ambient pressure 19, which for example corresponds to the normal hydrostatic air pressure at the measurement location. Then the ambient pressure 19 in the pressure chamber 15 is gradually increased, and the respective buoyancy force is determined for each set increased ambient pressure 19.

[0065] The following formula, for its part, describes the relationship (Verhältnis) between the buoyancy force and the amount of substance of the gas 21 present in the battery pack cell 1. Using the buoyancy forces now determined for different ambient pressures, the amount of substance of the included gas 21 can be calculated by transforming the following equation.

[0066]

[0067] This formula can also be transformed into

[0068]

[0069] wherein for the constant C it holds that:

[0070]

[0071] From this, the first buoyancy force for the first ambient pressure and the second buoyancy force for the second ambient pressure can now be calculated.

[0072]

[0073] If the difference between the first buoyancy force and the second buoyancy force is now calculated, the constant C can be eliminated and the formula is obtained:

[0074]

[0075] This formula can then be transformed as follows to calculate the amount of substance n.

[0076]

[0077] Thus, the amount of substance n can be determined, where in addition to the known values R, T, and ρ 液体 it is only necessary to determine the difference between the buoyancy forces and the difference between the first ambient pressure and the second ambient pressure.

[0078] As Figure 2 shown, the determination of the amount of substance can alternatively be done with the aid of a graph or a function.

[0079] For this, the function

[0080]

[0081] is plotted in a coordinate system above the reciprocal of the ambient pressure. At the start of the measurement, there is initially the first ambient pressure 19 and the initial buoyancy force. As the pressure increases, the graph first moves (wanders) to the left until the ambient pressure 19 set in the pressure chamber 15 is the same as the internal air pressure 20 in the battery cell 1 of the battery pack. During the increase of the ambient pressure from the first ambient pressure 19 to the internal air pressure 20, the buoyancy force remains constant at first. Once the ambient pressure increases to a value greater than the internal air pressure 20, the buoyancy force starts to decrease.

[0082] Using the graph thus obtained, the amount of substance of the gas 21 located inside the battery cell 1 of the battery pack is determined from the slope of the curve shown in the left part of the graph. In this region of the graph, the ambient pressure is greater than the internal air pressure 20 in the battery cell 1 or the airbag 2.

[0083] Thus, the method described here provides a non-destructive measurement of the amount of gas in the airbag 2 of the battery cell 1 of the battery pack. This method can also be applied even in the case of a strongly aged battery cell of the battery pack, where the airbag of the battery cell is already completely filled and inflated, and furthermore the battery cell also has a strongly increased internal air pressure 20. Optionally, the temperature dependence of the solubility of the gas 21 in the liquid 22 located within the battery cell 1 can also be taken into account here. For example, the liquid 22 can be the electrolyte used or a mixture with other liquids that may be present. For this purpose, for example, the non-conductive liquid 3 can be heated to an elevated temperature, since the solubility of the liquid 22 decreases at higher temperatures. It is assumed here that the determination of the amount of substance of the gas 21 present in the case of the elevated temperature basically represents a more critical scenario. At a higher temperature, the solubility of the liquid 22 is significantly lower and a relatively large amount of the gas 21 is present in the gaseous form. This exists, for example, in the application case of very fast charging of an electric vehicle with a battery pack. For this reason, for example, the measurement of the gaseous gas amount at a temperature of 60 °C is particularly important.

[0084] For this purpose, the temperature of the non-conductive liquid 3 can optionally be adapted by means of the heating device 10.

[0085] List of reference numerals

[0086] 1 Battery cell

[0087] 2 Airbag

[0088] 3 Non-conductive liquid

[0089] 4 Liquid container

[0090] 5 Rope

[0091] 6 Force measuring device

[0092] 7 Deflection pulley

[0093] 8 Pressure measuring device

[0094] 9 Temperature measuring device

[0095] 10 Heating device

[0096] 11 Display or recording device for pressure measurement

[0097] 12 Display or recording device for force measurement

[0098] 13 Display or recording device for temperature measurement

[0099] 14 Force generating device

[0100] 15 Pressure chamber

[0101] 16 Door

[0102] 17 Control device

[0103] 18 Containment device

[0104] 19 First and second ambient pressures

[0105] 20 Internal air pressure

[0106] 21 Gas

[0107] 22 Liquid in battery of battery pack

Claims

1. A method for determining the amount of gas (21) in a battery cell (1) of a battery pack, wherein the battery cell (1) has an initial volume and the method has at least the following steps: a) Immerse the battery cell (1) in a non-conductive liquid (3) having a defined density under a first ambient pressure; b) Generate a lifting force directed opposite to the sinking force of the battery cell (1); c) Measure the buoyancy force of the battery cell (1) in the liquid (3) depending on the first ambient pressure, change the first ambient pressure to a second ambient pressure, and measure the buoyancy force of the battery cell (1) in the liquid (3) depending on the second ambient pressure; d) Determine the amount of gas (21) in the battery cell (1) taking into account the first ambient pressure and the second ambient pressure, the buoyancy forces determined for the first and second ambient pressures, the temperature of the non-conductive liquid (3) and the density of the liquid; wherein, During steps a), b) and c), the measurement of the parameters is performed in a closed pressure chamber (15).

2. The method according to claim 1, wherein, During steps a) to d), the non-conductive liquid (3) and the battery cell (1) are maintained at a defined temperature.

3. The method according to claim 1, wherein, A pressure higher than the internal air pressure (20) in the battery cell (1) is selected as the second ambient pressure.

4. The method according to claim 1, wherein, A pressure lower than the internal air pressure (20) in the battery cell (1) is selected as the second ambient pressure.

5. The method according to any one of claims 1 - 4, wherein, The lifting force is selected such that the lifting force together with the buoyancy force is greater than or equal to the sinking force of the battery cell (1).

6. The method according to any one of claims 1 - 4, wherein, In step d), additionally consider the temperature-dependent solubility of the gas (21) in the liquid (22) present in the battery cell (1).

7. The method according to any one of claims 1 - 4, wherein, In order to determine the amount of substance of the gas (21), at least one temporal pressure change process of the first ambient pressure, the temperature change process of the non-conductive liquid (3) or the change process of the buoyancy force is continuously detected.

8. The method according to any one of claims 1 - 4, wherein, The battery cell (1) has an aged battery, the airbag (2) of the aged battery is inflated, and the internal air pressure (20) in the airbag is greater than the hydrostatic air pressure.

9. The method according to claim 8, wherein, The internal air pressure (20) is greater than 1 bar.

10. An apparatus for determining the amount of gas (21) located in a battery cell (1) of a battery pack according to the method of any one of claims 1 - 9, the apparatus respectively having at least one pressure chamber (15), a liquid container (4) arranged in the pressure chamber (15), a pressure measuring device (8) for measuring the ambient pressure (19), a force generating device (14) for generating a lifting force, a force measuring device (6) for measuring the generated buoyancy force, and a control device (17) for detecting and processing the detected measured values and for determining the amount of the gas (21).

11. The apparatus according to claim 10, characterized in that the apparatus has a heating device (10) for heating a non - conductive liquid (3) located in the liquid container (4) and a temperature measuring device (9) for measuring the liquid temperature of the non - conductive liquid (3).

12. A computer program product having a computer program including instructions that cause the apparatus according to claim 10 or 11 to perform the steps of the method according to any one of claims 1 - 9.

13. A computer - readable medium having stored thereon a computer program including instructions that cause the apparatus according to claim 10 or 11 to perform the steps of the method according to any one of claims 1 - 9.

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