Method for balancing the performance of multiple battery modules

Abstract

The present invention relates to a method for improving the operating efficiency of a battery pack (100) including at least two battery modules (10, 10', 10''), each battery pack configured with a common gas space (29). The method includes the steps of acquiring (101) data regarding the battery modules (10, 10', 10'') (the data relating to the number of battery cells per battery module, the number of battery modules, the temperature of each battery module, and the energy capacity of the battery modules), acquiring internal resistances (R i1 , R i2 , R i3 ) (102), a step of determining the amount of oxygen to be charged into the battery pack when the difference in the indication parameter between any of the battery modules exceeds a first threshold (104), and a step of starting charging of the battery pack based on the determined amount of oxygen (107).

Description

[Technical Field]

[0001] The present invention relates generally to the field of batteries, particularly nickel-metal hydride (NiMH) batteries. The present invention relates to a method for adding hydrogen or oxygen gas or hydrogen peroxide to a battery pack to improve performance. Furthermore, the present invention relates specifically to the field of extending the life of a battery pack. [Background technology]

[0002] Nickel-metal hydride (NiMH) batteries have a long cycle life and rapid charge / discharge capabilities. During charging and discharging, hydrogen is transported between the electrodes in the form of water molecules, allowing the electrodes to interact with each other via the aqueous alkaline electrolyte. During discharging, hydrogen is released from the negative electrode and migrates to the positive electrode (nickel electrode), where it is absorbed. This bond releases energy. During charging, the hydrogen transport is reversed.

[0003] Nickel-metal hydride batteries, in particular, are designed to limit the nickel electrodes to a starved electrolyte state, which is done to prevent overcharge and overdischarge of the battery cell, and controls the battery cell's chemical reaction and state of charge via the gas phase.

[0004] When the battery cell is charged, hydrogen is transported from the nickel hydroxide to the metal hydride by water molecules in the alkaline electrolyte. When the battery cell is discharged, hydrogen is transported back to the nickel hydroxide electrode in the form of water molecules.

[0005] PCT Publication WO2017 / 069691 states that properly balancing the nickel electrode capacity with the metal hydride electrode capacity, with both the appropriate amount of overcharge reserve and overdischarge reserve, is essential for the battery module to function well and achieve stable, long-term charge and discharge performance. The addition of oxygen gas, hydrogen gas, or hydrogen peroxide can provide adequate overcharge and discharge reserve and replenish the electrolyte, extending the life of the battery module and increasing the number of cycles possible.

[0006] The addition of oxygen is preferably carried out when the battery module is not in operation, and therefore, in order to optimize the operation of the battery module, the oxygen filling is preferably carried out in a manner that optimizes not only the capacity of the battery module but also the operating time. Summary of the Invention

[0007] It is an object of the present invention to provide a method that includes improving the operating efficiency by adding oxygen to a battery pack that includes at least two battery modules, each battery module including at least one battery cell, that mitigates at least one of the drawbacks of the prior art.

[0008] This object is achieved by the method according to the independent claims.

[0009] Further advantages of the invention are provided by the features of the respective dependent claims.

[0010] According to a first aspect of the present invention, there is provided a method for improving the operating efficiency of a battery pack including at least two battery modules, each battery module including at least one battery cell. Each battery module has a casing containing the at least one battery cell and enclosing a gas space, and the gas spaces of the battery modules are connected to each other to form a common gas space. Each battery cell includes a first electrode, a second electrode, a porous separator, and an aqueous alkaline electrolyte disposed between the first electrode and the second electrode, the porous separator, the first electrode, and the second electrode configured to allow gas transfer between the electrodes to exchange hydrogen and oxygen. At least one of the casings includes a gas inlet for adding gas or liquid to the common gas space of the casing. The method includes acquiring data related to the battery modules, the data relating to the number of battery cells per battery module, the number of battery modules, the temperature of each battery module, and the energy capacity of each battery module. The method further comprises the steps of acquiring an indication parameter related to the internal resistance of at least two of the battery modules, and, when the difference in the indication parameter between any of the battery modules exceeds a predetermined first threshold, determining, based on the indication parameter and data related to the battery modules, the amount of oxygen to be charged into the battery modules in order to reduce the difference in the indication parameter between any two of the battery modules to a level below the first threshold.

[0011] The method may include initiating charging of the battery pack with the determined oxygen charge. This initiation may include directing a gas canister containing the correct charge of oxygen at the correct pressure to be delivered to the battery module. Alternatively, if the battery module is connected to an oxygen supply, this initiation may include initiating charging of oxygen from the oxygen supply.

[0012] The method according to the first aspect of the present invention can improve the operating efficiency of the battery compared to the method according to the prior art. Operating efficiency means that the life of the battery module is extended and, at the same time, the oxygen replenishment time is kept short. Properly balancing the internal resistance per battery cell of different battery modules can extend the life. Setting a threshold for the balance of the internal resistance per battery cell of different battery modules can avoid high internal resistance operation of the battery and, at the same time, avoid too short an interval between oxygen replenishments.

[0013] The method may be implemented in a control unit comprising a computer.

[0014] The step of obtaining an indicative parameter, such as data determining an internal resistance or a state of health (SOH), for at least two of the battery modules is preferably performed by receiving data from a measurement unit configured to obtain the indicative parameter for at least two of the battery modules. The number of cells in the battery module is obtained from the data regarding the battery module. The number of cells in this determination is subject to practical limitations. Typically, only the terminal contacts of the battery module are accessible. Thus, the indicative parameter, such as SOH or internal resistance, is determined for all battery cells in the battery module.

[0015] Obtaining data about the battery modules (this data relates to at least the number of battery cells in the battery modules and the energy capacity of the battery modules) can be accomplished in many different ways. One alternative is for the measurement unit to be configured to transmit data about the battery modules to a computer device executing the method. While the data may be transmitted from the measurement unit, it is preferred that the measurement unit transmit only the identification number to minimize the complexity of the measurement unit. Upon receiving the identification number from the measurement unit, the data can be retrieved, for example, from a memory. As described above, this data relates to at least the number of battery cells in the battery modules, the temperature of each battery module, and the energy capacity of the battery modules. This data is necessary for determining the amount of oxygen to be charged into the battery modules. However, the determination does not necessarily use the actual number of battery cells in the battery modules or the energy capacity of the battery modules. According to one alternative, the control unit may refer to a lookup table in memory to retrieve battery data corresponding to the identification number of the battery module. The battery data may, for example, be a model number identifying the type of battery. The control unit may then retrieve the required oxygen charge from another lookup table based on the determined indicator parameters, temperature, and model number. The required oxygen charge in the look-up table may be based on previous experiments with similar battery types. The model number defines a battery module having a given number of battery cells, a given energy capacity, and, if necessary, a given common gas space volume.

[0016] Preferably, when the acquired indicator parameter is internal resistance, which means the internal resistance across multiple battery cells, an average internal resistance per battery cell is calculated at the measured temperature of the battery pack and / or battery module. The average internal resistance per battery cell is then compared with a resistance threshold value at the temperature value measured for a single battery cell. In principle, the difference in internal resistance between different battery modules can be compared with a resistance threshold value, which is equivalent to comparing the difference in internal resistance between battery cells from different battery modules with a resistance threshold value at the measured temperature. A disadvantage of comparing the difference in internal resistance between different battery modules with a resistance threshold value is that different resistance threshold values ​​must be provided depending on the number of battery cells in a battery module.

[0017] The method may include obtaining a voltage indication, such as an open circuit voltage (OCV) or a state of charge (SOC), at a measured temperature for at least one battery module; determining whether the voltage indication for any of the at least two battery modules is within a predetermined voltage range; and determining that it is safe to charge the battery pack with oxygen only if the obtained voltage indication for each battery module does not have a value outside the predetermined voltage range. The inventors have recognized that charging a battery module with oxygen when the voltage indication is outside the voltage range may pose a risk of fire. When OCV is used as the voltage indication, an average voltage per battery cell is preferably calculated from the voltage across each battery module. In this way, only one voltage threshold need be used.

[0018] The predetermined voltage range is defined by a lower voltage indication threshold and an upper voltage indication threshold, and the voltage indication may be an open circuit voltage (OCV) on the battery module or a state of charge (SOC) of the battery module.

[0019] The method may also include, if it is determined that it is unsafe to fill the battery pack with oxygen, initiating discharging or charging the battery modules to a voltage of the at least one battery module within a voltage range before initiating filling the battery pack with the determined amount of oxygen. According to one alternative, initiating discharging or charging may be sending a message to a battery operator to discharge or charge the battery. Alternatively, if the battery module is connected to an automatic discharging or charging device, initiating may include initiating automatic discharging or charging.

[0020] The charging of the battery pack with inert gas can be initiated in conjunction with, or simultaneously with, the initiation of charging the battery pack with oxygen. Charging with a combination of oxygen and inert gas can further reduce the risk of fire. If the battery module is connected to a gas supply, it is preferred that the gas supply contain the correct mixture of oxygen and inert gas.

[0021] The method may also include adding hydrogen gas to the common gas space before filling the battery packs with oxygen if it is determined that it is unsafe to fill the battery packs with oxygen, thereby further improving the operating efficiency of the battery modules.

[0022] The method may also include the step of adding hydrogen gas to the common gas space after filling the battery pack with oxygen, thereby further improving the operating efficiency of the battery module.

[0023] The method may also include the steps of measuring the temperature of the battery pack and / or battery modules after filling the battery pack with oxygen and obtaining post-fill parameters related to internal resistance for each of the battery modules; determining whether a difference in the post-fill parameters between any of the at least two battery modules at the measured temperature exceeds a predetermined second threshold; and if the difference in the post-fill parameters between any of the battery modules exceeds the second threshold, determining an additional charge of oxygen to be filled into the battery pack based on the post-fill parameters of the battery modules and data related to the battery modules at the measured temperature of the battery pack and / or battery module to further reduce the difference in the post-fill parameters between any two battery modules to a level below the second threshold; and starting to fill the battery pack with the determined additional charge of oxygen.

[0024] In one embodiment, the step of obtaining the indicative parameter includes obtaining the internal resistances of all battery modules in the battery pack at the measured battery pack and / or battery module temperature. The method further includes determining whether a difference in internal resistance per battery cell between any pair of battery modules in the battery pack at the measured temperature exceeds a predetermined first resistance threshold, and if the difference in internal resistance per battery cell between any pair of battery modules at the measured battery pack temperature and / or battery module temperature of the battery pack exceeds the predetermined first resistance threshold at a particular temperature, determining the amount of oxygen to be charged into the battery pack from the internal resistances of the different battery modules and the obtained battery pack data to reduce the difference in internal resistance of each battery module below the first resistance threshold. Obtaining the internal resistances of all battery modules ensures that the battery module with the largest difference in internal resistance to be compensated for the measured temperature can be detected.

[0025] The step of determining the amount of oxygen may determine the amount of oxygen charged to the battery modules to obtain a difference in internal resistance per battery cell between the battery modules that is less than a second resistance threshold, where the second resistance threshold is lower than the first resistance threshold.

[0026] According to a second aspect of the present invention, there is provided a computer program for improving the operating efficiency of a battery pack, the program comprising instructions which, when executed on at least one processor, cause the at least one processor to perform a method according to the first aspect of the present invention.

[0027] According to a third aspect of the present invention, there is provided a computer-readable recording medium carrying a computer program for improving the operating efficiency of a battery pack according to the second aspect of the present invention.

[0028] Preferably, the battery pack is made up of a plurality of battery modules each having at least one nickel-metal hydride battery cell.

[0029] Preferred embodiments of the present invention will now be described with reference to the drawings. [Brief explanation of the drawings]

[0030] [Figure 1] 1 illustrates a battery system for balancing between battery packs including two battery modules. [Figure 2] 1 shows a battery system for balancing among battery packs containing three battery modules. [Figure 3] 1 illustrates a flow diagram of a method for balancing battery modules, according to an embodiment. [Figure 4] 1 shows a plot of different measurements of resistance and voltage. [Figure 5] 3 shows how the voltage of the battery cells in the battery pack according to FIG. 2 changes during a cycle for battery qualification before oxygen filling. [Figure 6] 3 shows how the voltage of the battery cells of the battery pack according to FIG. 2 changes during a cycle for quality assurance of the battery after filling with oxygen. [Figure 7] A first example is given to illustrate how the internal resistance changes when balancing battery packs. [Figure 8]A second example is given to illustrate the change in internal resistance when balancing battery packs. DETAILED DESCRIPTION OF THE INVENTION

[0031] In the following description of the preferred embodiments, reference is made to the drawings. The drawings are not drawn to scale and some dimensions may be exaggerated to clearly show all features. The same reference numerals are used for similar features in different drawings.

[0032] The terminology used herein is for the purpose of describing particular embodiments of the disclosure only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly dictates otherwise.

[0033] In this application, the term "indicative parameter" related to the internal resistance of a battery module includes not only the internal resistance but also a measure of the state of health (SOH) of the battery module, which may include the internal resistance and other parameters important in determining the state of the battery module, such as internal gas pressure.

[0034] In this specification, the term "internal resistance" is generally used to mean the internal DC resistance of each battery module, and therefore of each battery cell. The internal resistance is obtained by measuring the voltage drop during controlled discharge using a predetermined discharge current. The internal resistance is then calculated based on the measured voltage drop and the discharge current. An example is described in the following standard, IEC 63115-1, Ed. 1.0 (2020-01), Chapter 7.6.3, "Measurement of Internal DC Resistance."

[0035] Some of the exemplary embodiments presented herein are directed to methods for balancing a plurality of battery cells, preferably having metal hydride (MH) electrodes, more preferably nickel-metal hydride battery cells. As part of the development of the exemplary embodiments presented herein, the problem is first identified and discussed.

[0036] In battery packs containing multiple battery modules, such as nickel-metal hydride battery modules (each battery module contains at least one battery cell), the performance of each battery cell deteriorates due to the drying of the electrolyte during charging and discharging. It has been found that adding oxygen gas restores the electrodes, promoting gas recombination and resulting in a reduction in internal pressure. This reduces the susceptibility to unintentional overcharging and over-discharging. A depleted electrolyte design means that only minimal electrolyte is available within the battery module. Loss of electrolyte leads to a decrease in performance, primarily due to an increase in internal resistance. Electrolyte drying is the primary cause of limited cycle life. This occurs primarily when the battery's internal pressure is too high, opening the safety valve and releasing oxygen or hydrogen during overcharging or over-discharging. Furthermore, the corrosion of the negative electrode consumes electrolyte, causing an increase in the pressure level within the battery. When two or more battery cells are connected by gas, electrolyte is lost unevenly from the battery cells. This also applies to battery modules with multiple cells.

[0037] The main cause of this is uneven charging, as battery cells are not 100% identical. Uneven charging causes some cells to heat up before others, causing water (in gaseous form) to move between the battery cells connected by gas, condensing in the lower temperature areas. This causes water to move within the battery module and even between battery modules. This causes the internal resistance of one of the multiple battery cells to increase faster than the other battery cells. This increase in internal resistance can lead to a shortened battery module lifespan. Uneven increases in internal resistance between battery modules can lead to uneven battery module lifespans, which in turn can shorten the battery pack lifespan.

[0038] FIG. 1 shows a battery system 50 including two battery modules 10, 10′ connected in series to form a battery pack 100. Each battery module 10, 10′ includes at least one battery cell 12 (preferably a nickel-metal hydride battery cell). Each battery module 10, 10′ has a casing 30 that houses the at least one battery cell and encloses a gas space. Each battery cell 12 in the battery modules 10, 10′ includes a first positive electrode, a second negative electrode, a porous separator, and an aqueous alkaline electrolyte disposed between the first and second electrodes. The separator, first and second electrodes are configured to allow gas transfer between the electrodes, thereby enabling hydrogen and oxygen exchange. Each battery module 10, 10′ includes a positive terminal 11 and a negative terminal 12 in electrical contact with at least one battery cell 12 in each battery module, and the battery cells are preferably connected in series. The positive terminal 11 of the first module 10 constitutes the positive terminal of the battery pack 100, as indicated by the plus sign, and the negative terminal 12 of the last module 10' constitutes the negative terminal of the battery pack 100, as indicated by the minus sign.

[0039] The battery pack 100 is also configured to connect the gas spaces of each battery module 10, 10′ to form a common gas space 29. The battery pack 100 further includes a gas inlet 25 for adding gas or liquid to the common gas space 29. FIG. 1 also shows a measurement unit 13 connected to the positive terminal 11 and the negative terminal 12 of each battery module and configured to acquire data necessary to calculate indicative parameters related to the internal resistance of the battery modules 10, 10′ between the positive terminal connector 11 and the negative terminal connector 12, as described above. The data acquired by the measurement unit 13 may include a voltage drop during discharge to determine the internal resistance, temperature, internal pressure, and current if a current detector is included in the measurement unit 13. The measurement unit 13 may also be configured to measure the open circuit voltage (OCV) between the positive terminal 11 and the negative terminal 12 in each battery module 10, 10′. Alternatively, the measurement unit 13 may be connected to acquire data for only one battery cell 12. However, manufacturing battery modules with this functionality would be very costly. An inlet valve 16 is connected to a gas inlet 25. In FIG. 1 , a gas container 17 is connected to the inlet valve 16, if necessary. A local control unit 20 is connected to the measurement unit 13 and the inlet valve 16, and may be configured to calculate an indication parameter based on data provided by the measurement unit 13. A safety valve 24, e.g., a burstable disc, is connected to the common gas space 29. The safety valve prevents the buildup of a dangerous gas pressure in the common gas space 29. A pressure sensor 23 may be attached to the safety valve 24 to measure the internal pressure in the common gas space 29. The pressure sensor 29 is also connected to the local control unit 20.

[0040] The local control unit 20 communicates with the control unit 14 by wire or wirelessly. It is also possible to provide one or more intermediate sections between the local control unit 20 and the control unit 14. It is also possible to omit the local control unit and connect the control unit 14 directly to the inlet valve 16 and the measurement unit 13. The control unit 14 may be located at a remote location, for example, at a battery module manufacturer. The central control unit 14 is connected to or includes a memory 26.

[0041] The control unit 14 is configured to initiate measurements of indicative parameters, such as temperature, pressure, voltage, and current, required for calculating the internal resistance between the positive connector 11 and the negative connector 12 of each battery module 10, 10′, at predetermined intervals using the measurement unit 13, and to transmit this information, along with the respective identification information of each battery module 10, 10′, to the control unit 14. To achieve this, the control unit 14 sends a request to the local control unit 20, which responds by returning information related to the current indicative parameters of the battery modules 10, 10′ and, if necessary, the open-circuit voltage of the battery modules 10, 10′. The internal resistance is not measured directly by the measurement unit 13. As described above, the measurement unit 13 measures the voltage drop during discharge with a predetermined discharge current and then calculates the internal resistance.

[0042] During use of the battery modules 10, 10', the battery modules are discharged and charged via the battery pack terminals, which are marked with positive and negative signs in the drawings. The internal resistance of the battery module increases with the number of charge / discharge cycles.

[0043] In a first embodiment (not shown), the battery module 10 is a stand-alone battery module having a casing that creates a gas space for all battery cells in the battery module, as disclosed in commonly assigned published applications WO 2006 / 104442 or WO 2007 / 093626. In a stand-alone battery module, the cells are bipolar. The cells in this example are connected in series using biplates to form a stack of battery cells. The battery module has a casing that houses the battery cells and encloses the gas space. Connecting tubes are required to connect the gas spaces of the battery modules to a common gas space.

[0044] In a second embodiment (not shown), the battery modules are connected in series in a bipolar fashion. In this case, the battery modules are electrically connected by contacting the positive terminal of a first battery module 10 with the negative terminal of a second module 10', thereby using the entire surface of each single plate to minimize resistance when connecting the two battery modules in series. No connecting tubes are required. Instead, sealing O-rings are placed between the modules to ensure a tight seal between the gas spaces within each battery module. This type of battery module is disclosed in commonly assigned WO 2018 / 111182.

[0045] FIG. 2 shows a battery system 150 including a battery pack 120 according to another embodiment of the present invention. In the embodiment of FIG. 2, three battery modules 10, 10′, and 10″ are connected in series to form the battery pack 120. The battery pack 120 also includes a sealing ring (not shown) that connects the gas spaces of the battery modules 10, 10′, and 10″ to a common gas space 29. The battery pack 120 further includes a gas inlet for adding gas or liquid to the common gas space 29. An inlet valve 16 is connected to the gas inlet. An optional gas container 17 is connected to the inlet valve 16. A safety valve (not shown) may be connected to the common space 29, and a pressure sensor 23 may be provided to measure the internal pressure in the common gas space 29.

[0046] The battery system further includes a local control unit 20 connected to the measurement unit 13 and the inlet valve 16. The safety valve prevents dangerous gas pressure from building up in the common gas space 29. Each battery module 10, 10', 10" also includes a positive terminal and a negative terminal. Electrical connection between the different battery modules 10, 10', 10" is provided via contact plates located between the battery modules.

[0047] The local control unit 20 may be connected to the pressure sensor 23, and in this example communicates with the control unit 14 by wire. It is also possible to provide one or more intermediate sections between the local control unit 20 and the control unit 14. It is also possible to omit the local control unit and have the control unit 14 directly connected to the pressure sensor 23, the inlet valve 16 and the measurement unit 13. As mentioned above, the control unit 14 may be located at a remote location, for example at the battery module manufacturer. The central control unit 14 includes a memory 26 in this example. The measurement unit measures the first internal resistance R between the terminals of the first battery module 10. i1 And if necessary, the first open circuit voltage U1, the second internal resistance R between the positive terminals of the second battery module 10′ i2 and if necessary, the second open circuit voltage U2, the third internal resistance R between the positive terminals of the third battery module 10 ″ i3 and, if necessary, the third open circuit voltage U3. The internal resistance is calculated based on the voltage drop at the time of discharge current (as explained above). The measured internal resistance R i1 is divided by the number of first battery modules 10, i.e., 10 battery cells, to obtain the average internal resistance R per battery cell of the first battery module 10. ic1 The internal resistance R per battery cell for the second battery module 10′ and the third battery module 10″ is calculated as follows: ic2 , R ic3The parameter indicating the internal resistance is temperature dependent, and in order to correctly calculate the internal resistance, the measurement unit must measure the temperature of each module, or at least the pack temperature.

[0048] 3 is a flow diagram of a method for balancing multiple battery modules in a battery pack. The method includes a first step 101 of acquiring data for the battery pack. This step can be done in many different ways. One way of acquiring the data is for the local control unit to send a unique identification number to the control unit. The control unit may then acquire data about the battery pack from memory. In a second step 102, the internal resistance R of at least two modules of the battery pack is calculated. i The instruction parameter exemplified as, for example, R i1 and R i3 According to one embodiment, data for calculating the internal resistance is obtained from the measurement unit, and a control circuit (e.g., local control unit 20) determines the resistance between the positive terminals of each battery module. The local control unit then calculates the data related to the internal resistance, i.e., the calculated internal resistance R of each battery module. i1 , R i2 , R i3 , or transmits information about the measurement of the voltage drop at a certain discharge current to the control unit.

[0049] In a third step 103, the control unit 14 determines whether the difference in internal resistance between any of the two battery modules is greater than a first resistance threshold R per battery cell at the measured battery pack and / or battery module temperature. tc1 A predetermined first resistance threshold R corresponding to t1 The control unit determines whether the average internal resistance per battery cell R is exceeded, which is required when the battery module is composed of different numbers of battery cells. ic may be calculated for each battery module, but if the battery modules are composed of the same number of battery cells, this step may be omitted.

[0050] In some embodiments, the balancing process between the battery modules in the battery pack is performed by requiring the indicator parameter of each battery module to exceed a common threshold value, e.g., a level that is at least twice the internal resistance of the battery module when new. For example, if the internal resistance of the battery module when new is 6 mΩ, the common threshold value may be selected to be at least 12 mΩ.

[0051] Thus, for battery modules having the same number of battery cells, the determination in step 103 is to determine whether the absolute difference in internal resistance between two battery modules, e.g., modules 10 and 10″, is greater than or equal to a predetermined first resistance threshold R t1 , i.e., ΔR i =│R i1 -R i3 │>R t1 Alternatively, a comparison may be made between at least two battery modules 10, 10', 10'', and the internal resistance R i1 , R i2 , R i3 Based on this, any difference is determined to be equal to or greater than a predetermined first resistance threshold R t1 Determine whether it exceeds

[0052] If the battery modules have different numbers of battery cells, the average internal resistance per battery cell of each battery module, i.e., R ic1 , R ic2 , R ic3 Step 103 compares the absolute difference in average internal resistance between two battery modules, e.g., modules 10 and 10″, with a first predetermined cell resistance threshold Rtc1, e.g., ΔR ic =│R ic1 -R ic3 │>R tc1 Alternatively, the comparison is performed between at least two battery modules, each battery cell being calculated, and the average internal resistance R ic1 , R ic2 , R ic3, and determines whether any difference exceeds a predetermined first cell resistance threshold Rtc1.

[0053] First resistance threshold R t1 may be stored in the memory 26 or may be implemented in the method, i.e., in a computer program controlling the execution of the method. If the calculation in step 103 is performed on the average internal resistance in the battery module, the first cell resistance threshold R tc1 is a first resistance threshold value R for each battery cell along with data about the battery pack, i.e., the number of battery cells in each battery module. t1 More specifically, the control unit receives the identification number from the local control unit 20 and retrieves data about the battery modules from memory. This data may be that the battery pack includes 10 battery cells in each battery module, and that the battery pack includes three battery modules, such as battery pack 120 shown in FIG. 2.

[0054] Information regarding the energy capacity of each battery cell, the number of battery cells, and, if necessary, the volume of the common gas space, is transferred from the memory to the control unit, which then divides the obtained resistance value by the number of battery cells to obtain an average internal resistance R per battery cell for each of the two battery modules. ic1 , R ic2 The absolute value of the difference between the internal resistances from two different battery modules may reach a predetermined first resistance threshold R t1 or if the average internal resistance per battery cell from two different battery modules does not exceed a predetermined first cell resistance threshold R tc1 If it does not exceed , the control unit waits for the waiting time Tw for the next opportunity to obtain an updated value of the internal resistance of the battery module.

[0055] Difference in internal resistance ΔR between any two different battery modules i The absolute value of t1If the internal resistance of the battery module R i1 , R i2 , R i3 , and the amount of oxygen to be filled into the battery pack is determined based on the battery pack data (step 104), and the internal resistance R between any two battery modules is calculated. i1 , R i2 , R i3 to a level below the first resistance threshold Rt1, or preferably below a predetermined second resistance threshold R t2 where the second threshold is lower than the first threshold (R t1 >R t2 ). This is done using the average internal resistance when battery modules contain different numbers of battery cells. The data used to determine the amount of oxygen required is preferably information regarding the energy capacity of each battery cell and the number of battery cells in each battery module, as well as information regarding the volume of the common gas space, if required. The amount of oxygen required can be determined in a number of different ways, as explained below.

[0056] According to one alternative, the control unit relies on previous measurements to obtain the required amount of oxygen to be filled into the common space of the battery pack. The control unit may refer to a lookup table in memory to obtain battery pack data corresponding to the battery pack's identification number. The battery pack data may, for example, be a type number identifying the type of battery pack. The control unit may then retrieve the required amount of oxygen from another lookup table based on the measured resistance value and model number. The required amount of oxygen in the lookup table may be based on previous experiments using similar battery pack types.

[0057] According to another alternative, the control unit obtains the data required to calculate the amount of oxygen from a look-up table, which may be the number of battery cells in each battery module, the number of battery modules in the battery pack, and the energy capacity of each battery cell, as well as the volume of the common gas space, if necessary.

[0058] The method comprises: n This may include an optional step 105 of obtaining a voltage indication for each of the battery modules (n equals the number of battery modules, e.g., U1, U2, U3, in the battery pack). The voltage indication may be the open circuit voltage (OCV) of the battery module at the measured temperature or a state of charge (SOC) indicating that it is safe to add oxygen to the battery module. In this example, OCV is used and the determination in step 105 is the open circuit voltage U of the battery module. n The voltage of each battery module (U t0 n t1 ) is performed. Alternatively, step 105 may be performed to determine whether the battery cells U ci In this case, the control unit 14 determines the average battery cell voltage and determines the predetermined cell voltage threshold U ct To compare this, we must have information about the number of battery cells included in the voltage measurement.

[0059] As shown in the embodiments of Figures 1 and 2, voltage measurements are typically performed only across the battery module. n is a predetermined voltage threshold U t If it is determined that the battery module voltage is not within the voltage range, it is determined that it is safe to fill the battery pack with oxygen. Conversely, if the battery module voltage is not within the voltage range, an optional step 106 is performed in which the battery pack is charged or discharged to adjust the voltage of the battery module before repeating step 105. That is, if the battery module voltage is equal to or greater than the upper voltage indication threshold (U​​n ≧U t0 ), the battery pack is discharged (step 106a) and the battery module voltage is below the lower voltage indication threshold (U n ≦U t0 ), the battery pack is charged (step 106b). Performing these optional steps 105, 105a, and 106 is advantageous in reducing the risk of fire if oxygen fills the battery pack when the battery cell voltage is too high, which can be caused by the fact that the oxygen recombination rate becomes too high when the voltage on the battery cells is too high. If the battery module voltage is too low, oxygen reacts directly with the negative electrode, which is not protected from occluded hydrogen.

[0060] Figure 4 shows the battery module resistance R at room temperature, i.e., 20°C ± 2°C. in and the corresponding n-th battery module (U n The data in Figure 4 is for a nickel-metal hydride battery module with 10 battery cells. The voltage threshold of the module, U t is the resistance value R of the battery module as shown in Figure 4. in The four circled points 27 indicate measurements where the voltage was too high to allow oxygen to be charged.

[0061] As described above, if it is determined that it is unsafe to charge the battery pack with oxygen, the method may include the optional intermediate step 106 of adjusting the voltage of the battery modules. This is done by charging or discharging the battery pack to a voltage that ensures that the open circuit voltage on each battery module of the battery pack is within an indicated voltage range, before initiating step 107 of charging the battery pack with the determined amount of oxygen. As an example, at a temperature of +20°C ± 2°C, the upper voltage indication threshold is 1.39 V / cell and the lower voltage indication threshold is 1.3 V / cell. The upper voltage indication threshold, like the lower voltage indication threshold, is temperature dependent and can be normalized to a predetermined temperature range (e.g., room temperature) to ensure that the OCV is within the voltage range of 1.3 to 1.39 V / cell. Otherwise, thresholds for different temperatures would need to be available to determine whether it is safe to charge the battery modules with oxygen.

[0062] When determining whether it is safe to fill a battery module with oxygen based on the SOC, the upper threshold of the SOC is 95% and the lower threshold is 50%.

[0063] The battery modules 10, 10', 10" may be filled with an inert gas at the same time as filling the battery modules with oxygen, thereby reducing the risk of fire during filling. Filling is illustrated in FIG. 1 as being performed using a gas container 17 connectable to a gas inlet 25 via an inlet valve 16. The control unit 14 may be configured to initiate filling by initiating delivery of the container 17 to the site of the battery modules 10, 10', 10".

[0064] In some embodiments, the step of initiating charging of the battery packs may include adding hydrogen gas to the common gas space before charging the battery packs with oxygen, which may further improve the operating efficiency of the battery modules, but this step may only be performed if the voltage indication is within the voltage indication range and it is safe for the battery modules to be charged with oxygen.

[0065] After filling the battery modules with oxygen in step 107 as a precaution, the method includes an optional eighth step 108 in which the control unit 14 obtains post-fill parameters related to the internal resistances of the at least two battery modules 10, 10', 10" in the battery pack after filling. In an optional ninth step 109, it is determined whether the difference in internal resistance between any two battery modules 10, 10', 10" exceeds a predetermined second threshold, e.g., a second resistance threshold Rt2. In this case, the method returns to step 104 to determine an additional amount of oxygen to be filled into the battery pack in order to reduce the difference in the post-fill parameters between two of the at least two battery modules in the battery pack 100 to a level below the second resistance threshold Rt2. This additional amount is the amount of oxygen filled into the battery pack in step 107. These optional steps are performed to determine whether the battery modules R i1 , R i2 , R i3 This provides a more robust method since it allows more cycles of the battery before the internal resistance of the battery again exceeds the first resistance threshold Rt1. The optional feedback loop from step 109 to step 104 should not be necessary in principle, but if the battery pack needs to be filled with any additional oxygen, the battery pack is filled with the determined amount of oxygen. For this step to be meaningful, it is necessary that the oxygen filling occur more or less instantaneously. If the container 17 has to be sent for oxygen filling, there could be a delay of several hours or even days before the battery pack is filled with oxygen.

[0066] When the method aims to obtain a difference in internal resistance between any two different battery modules, for example, 10 and 10 ″, that is less than a second resistance threshold, the absolute difference ΔR of the internal resistance per battery module between the two different battery modules 10 and 10 ″ is i =│R i1 -R i3 │ is the second resistance threshold R t2 Lower (ΔR i <R t2 ) must be.

[0067] The determination step 109 may be replaced by a QA step, since the internal resistance is determined as part of the QA step.

[0068] It is contemplated that the control unit may perform step 101 of acquiring battery pack data differently depending on the length of time that has elapsed since the last time the data was acquired. The data may also be stored in the working memory of the control unit 14 for a short period of time.

[0069] The above description illustrates how the control unit 14 may implement this method. The control unit may include at least one processor 14' (FIG. 1). The processor may be programmed with a computer program including a plurality of instructions that, when executed by the at least one processor, cause the processor to implement the above-described method for improving the operational efficiency of a battery pack. This method in the control unit may be computer-implemented. Example

[0070] FIG. 5 shows how the voltage of the battery cells changes in the battery pack according to FIG. 2 during a cycle for battery quality assurance before oxygen filling.

[0071] FIG. 6 shows how the voltage of the battery cells changes in the battery pack according to FIG. 2 during a cycle for quality assurance of the battery after filling with oxygen.

[0072] The table below includes details on charging the battery pack with oxygen (an example of a step to start charging the battery pack, step 107 in FIG. 4) and details on the internal resistance per battery cell for different battery modules. [Table 1]

[0073] In Table 1 above, the resistance per battery cell of different battery modules 1-3 is shown at different times during balancing between battery packs 120. As can be seen from the first column, oxygen is added in four steps. Before the first step, the internal resistance per battery cell R ic1 , R ic2 , R ic3 is obtained in the battery modules connected to the battery pack. The difference in the fifth column is the internal resistance R per battery cell between the first battery module 10 and the third battery module 10″. ic1 , R ic2 , R ic3 The reason for this selection is that the internal resistance R per battery cell after the fourth oxygen charge ic1 , R ic2 , R ic3 This is because the difference between the first and third battery modules 1 and 3, which correspond to the battery modules 10 and 10'' in FIG.

[0074] Before the first oxygen filling, a quality assurance cycle is performed as shown in Figure 5. The first voltage curve 31 of the first battery module 1 is clearly separated from the second voltage curve 32 of the second battery module 2 and the third voltage curve 33 of the third battery module 3. This is because the average internal resistance R per battery cell of the first battery module 1 ic1 and the average internal resistance R per battery cell of the second and third battery modules ic2 , R ic3 The second voltage curve 32 and the third voltage curve 33 are different in average internal resistance R per battery cell. ic2 , R ic3 are similar and close to each other.

[0075] After the fourth filling of oxygen into the battery pack 120, the internal resistance R per battery cell between different battery modules ic1 , R ic2 , R ic3The difference between the voltages is quite small, as is evident from the last row of the table above. This small difference is also evident from Figure 6, where the first voltage curve 31, the second voltage curve 32, and the third voltage curve 33 are very close to each other. The step labeled Repeat QA is a repeated quality assurance step that includes charging and discharging the battery module.

[0076] Figure 7 is a first example illustrating how internal resistance changes during battery pack balancing. In Table 2 below, the battery pack resistance measurements used to determine the charge required for battery balancing are expressed as an average K factor and its standard deviation. The K factor reflects the increase in average internal resistance from the initial average resistance value of the battery pack. For example, a K factor of 2.5 represents an increase in internal resistance from 4 mΩ to 10 mΩ.

[0077] Furthermore, equation (1) is used to determine the required oxygen charge. Y=(C1 x-C2) M (1) where Y is the oxygen charge (liters), C1 and C2 are constants, x is the average K coefficient of the battery pack, and M is the number of modules in the battery pack. The constants are determined based on the battery module data, such as the number of battery cells in each battery module and the battery capacity. In this example, C1 = 5.8958, C2 = 5.3106, and M = 12, and equation (2) is obtained. Y=(5.8958·x-5.3106)·12 (2) [Table 2]

[0078] The process of charging the battery pack involves the difference in internal resistance (ΔR i =R max -R min ) is greater than a predetermined value, or when the standard deviation of the internal resistance exceeds a predetermined value. iWhen the resistance was >3.5 mΩ (as shown by curve 40 in Figure 7), the process was initiated and the amount of oxygen required was calculated. In this example, the maximum oxygen charge was limited to 72 liters, so it was performed in two stages. The results after the first charge of 72 liters are shown by curve 41, and after the first charge (37.9 liters), the additional amount of oxygen was calculated and a second charge of 40 liters was performed. The results after the second charge of 40 liters are shown by curve 42. From Table 2, it can be determined that the standard deviation of the internal resistance has decreased to a level below the initial standard deviation of 0.020, indicating that the battery packs are balanced.

[0079] Figure 8 shows a second example of the change in internal resistance when balancing battery packs. The amount of oxygen to be charged is calculated using equation (2), and Table 3 shows the measured resistance of the battery packs, the calculated charge amount for balancing the battery packs, the actual charge amount, and the deviation of the internal resistance. [Table 3]

[0080] In this example, ΔR i When the resistance was >2.5 mΩ (as shown in curve 50 of FIG. 8), the process was initiated and the amount of oxygen required was calculated. Because the calculated volume of 107.8 liters was greater than the capacity of the filling device, the filling process was performed in two separate steps, with the volume filled in the first step being approximately half of the calculated volume, i.e., 53 liters. The results after the first fill of 53 liters are shown in curve 51. After the first fill, an additional volume of oxygen (39.9 liters) was calculated, and a second fill of 40 liters was performed. The results after the second fill of 40 liters are shown in curve 52. From Table 3, it can be determined that after the second fill, the standard deviation of the internal resistance had decreased to a level below the initial standard deviation of 0.040, indicating that the battery packs were balanced.

[0081] After that, the battery pack was repeatedly charged and discharged, and the internal resistance within the battery pack increased, and the difference in internal resistance between the battery modules also increased to over 2.5 mΩ (shown by curve 55 in Figure 8). The amount of oxygen required to balance the battery packs was 67.6 liters. The results after the third charge of 68 liters are shown by curve 56. From Table 3, it can be determined that the standard deviation of internal resistance after the third charge fell to below 0.030, and the battery packs were balanced.

[0082] The present disclosure relates to a method for improving the operating efficiency of a battery pack including at least two battery modules 10, 10', 10'', each battery module including at least one battery cell. Each battery module has a casing containing the at least one battery cell and enclosing a gas space, the gas spaces of the battery modules being connected to each other to form a common gas space 29. Each battery cell includes a first electrode, a second electrode, a porous separator, and an aqueous alkaline electrolyte disposed between the first electrode and the second electrode, the porous separator, the first electrode, and the second electrode being configured to allow gas transfer between the electrodes to exchange hydrogen and oxygen. At least one of the casings includes a gas inlet for adding gas or liquid to the common gas space. The method includes obtaining data related to the number of battery cells and battery modules per battery module, the temperature of each battery module, and the energy capacity of the battery modules (10, 10', 10''); determining an internal resistance R for at least two of the battery modules (10, 10', 10''); i1 , R i2 , R i3 the step of acquiring an indication parameter relating to the battery modules; if the difference in the indication parameter between the battery modules exceeds a predetermined first threshold, a step 104 of determining, based on the indication parameter and the data relating to the battery modules, the amount of oxygen to be charged into the battery modules so that the difference in the indication parameter between any two battery modules becomes less than the first threshold; and a step of starting to charge oxygen into the battery pack based on the determined amount of oxygen to be charged.

[0083] In some embodiments, the indicator parameter is the internal resistance R of at least two of the battery modules 10, 10′, 10″. i1 , R i2 , R i3 The first threshold is selected to be a value that indicates whether or not oxygen charging into the battery pack reduces the difference in internal resistance between any two or more of the battery modules 10, 10′, 10″ to a value equal to or greater than the first resistance threshold R t1 the first resistance threshold R t1 is.

[0084] In some embodiments, the indication parameter relates to the state of health (SOH) of the battery module.

[0085] In some embodiments, the indication parameters and the first threshold for the battery modules 10, 10', 10'' are determined for each battery cell based on the indication parameters of each battery module and the obtained number of battery cells in each battery module.

[0086] In some embodiments, the method includes step 105 obtaining a voltage indication U1, U2, U3 across each of the at least two battery modules 10, 10', 10'', and determining whether the voltage indication U1, U2, U3 for any of the at least two battery modules is greater than or equal to a predetermined upper voltage indication threshold U1. t1 and determining whether the obtained voltage indication of each of the at least two battery modules exceeds a predetermined upper voltage indication threshold U before step 105a of determining whether the obtained voltage indication of each of the at least two battery modules exceeds a predetermined upper voltage indication threshold U t1 If it is lower, performing a filling step 107.

[0087] In some embodiments, a predetermined voltage indication threshold U t1 is the internal resistance R of at least two battery modules 10, 10', 10''. i1 , R i2 , R i3 is a function of an instruction parameter related to

[0088] In some embodiments, the method further comprises determining whether the obtained voltage indication across any of the at least two battery modules is greater than or equal to a predetermined upper voltage indication threshold U t1 In this case, the method further includes a step 106a of discharging the battery pack to reduce the voltages of at least two battery modules 10, 10', 10'' to a level below a predetermined upper voltage indication threshold before starting to fill 107 the battery pack with the determined amount of oxygen.

[0089] In some embodiments, the method further comprises determining whether the voltage indication of any of the at least two battery modules 10, 10′, 10″ is below a predetermined lower voltage indication threshold U t0 and determining 105a whether the obtained voltage indication for each of the at least two battery modules is below a predetermined lower voltage indication threshold (U t0 ) and performing step 107 of initiating filling when

[0090] In some embodiments, the method further comprises determining whether the obtained voltage indication of any of the at least two battery modules 10, 10', 10'' is below a predetermined lower voltage indication threshold U t0 Before performing step 107 of starting to fill the battery modules with the determined amount of oxygen, the battery pack is charged to lower the voltage of the at least two battery modules 10, 10', 10'' to a lower voltage indication threshold U t0 The method further comprises step 106b of raising the concentration of the ionized water to a level exceeding 106c.

[0091] In some embodiments, the initiating charging step 107 further includes charging the battery pack with hydrogen before charging the at least two battery modules with oxygen.

[0092] In some embodiments, the voltage indication is selected to be the open circuit voltage of the at least two battery modules, and the upper and lower voltage indication thresholds are temperature dependent.

[0093] In some embodiments, the voltage indication is related to the State of Charge (SOC) of the battery module.

[0094] In some embodiments, charging the battery pack 100, 150 with an inert gas is performed in conjunction with charging the battery pack 100, 150 with oxygen.

[0095] According to some embodiments, the inert gas is selected to be any combination of argon, nitrogen, helium and / or air.

[0096] In some embodiments, the step 107 of initiating filling further includes the step of initiating preparation of the container 17 with the determined amount of oxygen to reduce the difference in the indicator parameters between at least two battery modules. The pressure of the gas in the container depends on the volume of the container and the amount of gas in the container. For small containers, the amount of gas in the container will be approximately the same as the oxygen filling amount. However, after filling the container with oxygen from the container, a residual amount of oxygen will always remain in the container. The flow of gas from the container to the battery modules continues until the pressure in the container is the same as the pressure in the common gas space of the battery modules. Therefore, the amount of gas in the container needs to be slightly larger than the filling amount.

[0097] In some embodiments, after filling the battery pack with oxygen, the post-filled internal resistance R i1 , R i2 , R i3 a step 108 of acquiring post-fill parameters related to the at least two battery modules 10, 10′, 10″; a step 109 of determining whether a difference in the post-fill parameters between any of the at least two battery modules 10, 10′, 10″ exceeds a predetermined second threshold; and if the difference in the post-fill parameters between any of the battery modules exceeds the second threshold, calculating the difference in the post-fill parameters between any two battery modules based on the post-fill parameters of the battery modules and the data related to each battery module, based on the second resistance threshold R. t2 determining an additional charge of oxygen to be charged into the battery pack to reduce the oxygen level to a level below the predetermined level; and charging the battery pack with the determined additional charge of oxygen.

[0098] According to some embodiments, the method is performed on a battery pack including nickel-metal hydride (NiMH) battery cells.

[0099] The present disclosure also relates to a computer program for improving the operating efficiency of a battery pack, the computer program comprising instructions that, when executed on at least one processor 14', cause the at least one processor 14' to perform a method as described above.

[0100] The present disclosure also relates to a computer-readable storage medium having a computer program thereon for improving the operating efficiency of such a battery pack.

[0101] The present disclosure also relates to a balancing container 17 between battery packs 100, 150. The container is filled with at least a predetermined amount of pressurized oxygen for charging the battery pack with the charge of oxygen to carry out the method described above.

[0102] Aspects of the present disclosure are described with reference to drawings, e.g., block diagrams and / or flow diagrams. It will be understood that some entities in the drawings, e.g., blocks of the block diagrams, and combinations of entities in the drawings, can be implemented by computer program instructions, which can be stored in a computer-readable memory and loaded into a computer or other programmable data processing apparatus. Such computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, and / or other programmable data processing apparatus to produce a machine, such that the instructions, executed by the processor of the computer and / or other programmable data processing apparatus, create means for performing the function(s) / act(s) specified in the block diagram(s) and / or flow diagram(s) block(s).

[0103] Depending on the implementation and aspects of the present disclosure, the functions or steps noted in each block may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially simultaneously, or the blocks may sometimes be executed in the reverse order, depending on the functions / acts involved. Also, the functions or steps noted in each block may be executed sequentially in a loop, according to some aspects of the present disclosure.

[0104] In the drawings and specification, illustrative embodiments of the present disclosure have been disclosed. However, many variations and modifications can be made to these embodiments without substantially departing from the principles of the present disclosure. Accordingly, the present disclosure is to be regarded as illustrative rather than restrictive, and is not limited to the particular embodiments described above. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A method for improving the operating efficiency of a battery pack (100, 150) including at least two battery modules (10, 10', 10") that are nickel-metal hydride (NiMH) battery modules (10, 10', 10"), each battery module including at least one battery cell, each battery module (10, 10', 10") having a casing containing the at least one battery cell and enclosing a gas space, the gas spaces of the battery modules (10, 10', 10") being connected to one another to form a common gas space (29), each battery cell including a first electrode, a second electrode, a porous separator, and an aqueous alkaline electrolyte disposed between the first electrode and the second electrode, the porous separator, the first electrode, and the second electrode being configured to transfer gas between the electrodes to enable exchange of hydrogen and oxygen, and at least one of the casings having a gas inlet for adding gas or liquid to the common gas space, the method comprising: obtaining (101) data relating to the number of battery cells per battery module (10, 10', 10''), the number of battery modules (10, 10', 10''), the temperature of each battery module, and the energy capacity of each battery module (10, 10', 10''); obtaining (102) the internal resistances (Ri1, Ri2, Ri3) of at least two of the battery modules (10, 10', 10''); When the difference in internal resistance between any of the battery modules exceeds a predetermined first resistance threshold (Rt1), determining (104) based on the internal resistance and data on the battery modules, an amount of oxygen to be charged into the battery modules (10, 10', 10'') to reduce the difference in internal resistance between any two battery modules to a level less than the first resistance threshold (Rt1); and initiating charging of the battery pack based on the determined oxygen charge (107).

2. 2. The method of claim 1, wherein the internal resistance and the first resistance threshold (Rt1) of the battery module (10, 10', 10'') are determined for each battery cell based on the internal resistance of each battery module and the number of battery cells of each battery module obtained.

3. The method comprises: obtaining (105) a voltage indication (U1, U2, U3) of each of said at least two battery modules (10, 10', 10''); a step (105a) of determining whether a voltage indication (U1, U2, U3) of any of the at least two battery modules (10, 10', 10'') exceeds a predetermined upper voltage indication threshold (Ut1); 3. The method according to claim 1, further comprising, before the step (107) of starting to fill the battery pack with the determined amount of oxygen, a step of performing the filling step (107) if the acquired voltage indication of each of the at least two battery modules is lower than the predetermined upper voltage indication threshold (Ut1).

4. 4. The method according to claim 3, wherein said predetermined voltage indication threshold (Ut1) is a function of said internal resistances (Ri1, Ri2, Ri3) of said at least two battery modules (10, 10', 10'').

5. When the acquired voltage instruction applied to any of the at least two battery modules is equal to or greater than the predetermined upper limit voltage instruction threshold (Ut1), 5. The method according to claim 3 or 4, further comprising, before the step (107) of starting to fill the battery pack with the determined amount of oxygen, a step (106a) of discharging the battery pack to reduce the voltage of the at least two battery modules (10, 10', 10'') to a level below the predetermined upper voltage indication threshold (Ut).

6. determining (105a) whether the voltage indication of any of the at least two battery modules (10, 10', 10'') is less than or equal to a predetermined lower voltage indication threshold (Ut0); 6. The method according to claim 3, further comprising: performing a step (107) of initiating charging when the acquired voltage indication of each of the at least two battery modules (10, 10′, 10″) exceeds the predetermined lower voltage indication threshold (Ut0).

7. If the acquired voltage indication of any of the at least two battery modules (10, 10', 10'') is equal to or less than the predetermined lower voltage indication threshold (Ut0), 7. The method of claim 6, further comprising, before performing the step (107) of starting to fill the battery modules with the determined amount of oxygen, charging the battery pack (106b) to increase the voltage of the at least two battery modules (10, 10', 10'') to a level above the predetermined lower voltage indication threshold (Ut0).

8. 8. The method of claim 6 or 7, wherein the step of initiating charging (107) further comprises charging the battery pack with hydrogen before charging the at least two battery modules with oxygen.

9. 9. The method of claim 6, wherein the voltage indication is selected to be an open circuit voltage of the at least two battery modules, and the predetermined upper and lower voltage indication thresholds are temperature dependent.

10. 9. The method of any one of claims 3 to 8, wherein the voltage indication relates to the state of charge, SOC, of ​​the battery module.

11. 11. The method according to any one of claims 1 to 10, characterized in that the charging of the battery pack (100, 150) with an inert gas is performed in conjunction with the charging of the battery pack (100, 150) with oxygen.

12. 12. The method of claim 11, wherein the inert gas is selected to be any combination of argon, nitrogen, helium and / or air.

13. 13. The method of claim 1, wherein the step of initiating filling (107) further comprises the step of initiating preparation of a container (17) with the determined amount of oxygen to reduce the difference in internal resistance between the at least two battery modules.

14. After filling the battery pack with oxygen, Obtaining (108) post-charge parameters related to the internal resistances (Ri1, Ri2, Ri3) of the battery pack after charging; determining (109) whether a difference in the post-fill parameter between any of the at least two battery modules (10, 10', 10'') exceeds a predetermined second threshold; When the difference in the post-filling parameter between any of the battery modules (10, 10', 10'') exceeds the second threshold, determining, based on the post-filling parameter of the battery module and data related to each battery module, an additional charge amount of oxygen to be charged into the battery pack to reduce the difference in the post-filling parameter between any two battery modules (10, 10', 10'') to a level below a second resistance threshold (Rt2); 14. The method of claim 1, further comprising the step of charging the battery pack with the determined additional amount of oxygen.

15. 15. A computer program for improving the operating efficiency of a battery pack, comprising instructions that, when executed on at least one processor (14'), cause the at least one processor (14') to perform the method of any one of claims 1 to 14.

16. A computer-readable recording medium storing a computer program for improving the operating efficiency of the battery pack according to claim 15.

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