Reference electrode management of battery cells

Abstract

A method for adjusting a state of charge of a reference electrode in a battery cell may include determining a reference electrode state of charge (SOC) of the reference electrode. The cell includes an anode, a cathode, and a reference electrode. And the reference electrode is arranged between the anode and the cathode. The method may also include comparing the reference electrode SOC to at least one predetermined low SOC threshold. The method may also include adjusting the reference electrode SOC in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold or the reference electrode SOC is greater than or equal to a predetermined high SOC threshold.

Description

Technical Field

[0001] The present disclosure relates to systems and methods for battery cell battery management, and more particularly to systems and methods for regulating the state of charge of a reference electrode in a battery cell. Background Art

[0002] In order to improve performance, ease of use and extend service life, it is advantageous to estimate the state of charge (SOC) of a battery cell. Typically, the SOC of a battery cell is not a directly measurable quantity because it depends on the electrochemical processes occurring within the battery cell. Therefore, mathematical models are used to estimate the SOC of a battery cell based on directly measurable quantities such as voltage and / or current. Mathematical models for estimating the SOC of a battery cell are typically highly nonlinear and may be subject to measurement errors. Therefore, it is advantageous to provide a reference electrode within the battery cell to provide a reference voltage for accurately measuring the voltage of the battery cell assembly. However, using a reference electrode for measurement may result in depletion of the SOC of the reference electrode and, therefore, changes in the reference voltage.

[0003] Therefore, while the battery management system and method achieve their intended purposes, a need remains for a new and improved system and method for regulating the state of charge of a reference electrode in a battery cell. Summary of the Invention

[0004] According to several aspects, a method for adjusting the state of charge of a reference electrode in a battery cell is provided. The method may include determining a reference electrode state of charge (SOC) of a reference electrode. The battery cell includes an anode, a cathode, and a reference electrode. The reference electrode is disposed between the anode and the cathode. The method may also include comparing the reference electrode SOC to at least one predetermined low SOC threshold. The method may also include adjusting the reference electrode SOC in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold or that the reference electrode SOC is greater than or equal to a predetermined high SOC threshold.

[0005] In another aspect of the present disclosure, determining the reference electrode SOC may further include charging the reference electrode and measuring the amount of charge added to the reference electrode. Determining the reference electrode SOC may further include measuring the cathode reference voltage while charging the reference electrode. Determining the reference electrode SOC may further include determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode.

[0006] In another aspect of the present disclosure, determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode may further include calculating a first-order reference electrode voltage slope. The first-order reference electrode voltage slope is a first-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode when the reference electrode is charged. Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode may further include determining the reference electrode SOC based at least in part on the first-order reference electrode voltage slope.

[0007] In another aspect of the present disclosure, determining the reference electrode SOC based at least in part on the first order reference electrode voltage slope may further include determining the reference electrode SOC to be less than or equal to a predetermined low SOC threshold in response to determining that the first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope low threshold and the first order reference electrode voltage slope is decreasing. Determining the reference electrode SOC based at least in part on the first order reference electrode voltage slope may further include determining the reference electrode SOC to be greater than or equal to a predetermined high SOC threshold in response to determining that the first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope high threshold and the first order reference electrode voltage slope is increasing.

[0008] In another aspect of the present disclosure, adjusting the reference electrode SOC may further include charging the reference electrode in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold. The reference electrode is charged until a first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope high threshold and the first order reference electrode voltage slope is increasing.

[0009] In another aspect of the present disclosure, determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode may further include calculating a second-order reference electrode voltage slope. The second-order reference electrode voltage slope is a second-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode when the reference electrode is charged. Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode may further include determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope.

[0010] In another aspect of the present disclosure, determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope may further include determining the reference electrode SOC to be less than or equal to a predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope is less than or equal to a predetermined second-order reference electrode voltage slope low threshold. Determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope may further include determining the reference electrode SOC to be greater than or equal to a predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined second-order reference electrode voltage slope high threshold.

[0011] In another aspect of the present disclosure, adjusting the reference electrode SOC may further include, in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold, charging the reference electrode. The reference electrode is charged until a second-order reference electrode voltage slope is greater than or equal to a predetermined second-order reference electrode voltage slope high threshold.

[0012] In another aspect of the present disclosure, determining the reference electrode SOC may further include tracking the elapsed time since the last charge process of the reference electrode. Determining the reference electrode SOC may further include calculating the amount of charge lost from the reference electrode based at least in part on the elapsed time and a predetermined reference electrode discharge rate of the reference electrode. Determining the reference electrode SOC may further include calculating the reference electrode SOC based at least in part on the amount of charge lost.

[0013] In another aspect of the present disclosure, adjusting the reference electrode SOC may further include charging the reference electrode in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold. The reference electrode is charged until the lost charge is returned to the reference electrode.

[0014] According to several aspects, a system for regulating the state of charge of a reference electrode in a battery cell is provided. The system may include a battery cell comprising an anode, a cathode, and a reference electrode. The reference electrode is disposed between the anode and the cathode. The system may also include a reference electrode management system electrically connected to the battery cell, the reference electrode management system comprising a controller. The controller is programmed to determine a reference electrode state of charge (SOC) of the reference electrode. The controller is further programmed to compare the reference electrode SOC with at least one predetermined low SOC threshold. The controller is further programmed to adjust the reference electrode SOC in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold or that the reference electrode SOC is greater than or equal to a predetermined high SOC threshold.

[0015] In another aspect of the present disclosure, to determine the reference electrode SOC, the controller is further programmed to charge the reference electrode by allowing current to flow between the cathode and the reference electrode. To determine the reference electrode SOC, the controller is further programmed to measure the amount of charge added to the reference electrode while charging the reference electrode. To determine the reference electrode SOC, the controller is further programmed to measure the anode reference voltage while charging the reference electrode. To determine the reference electrode SOC, the controller is further programmed to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode.

[0016] In another aspect of the present disclosure, to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to calculate a first-order reference electrode voltage slope. The first-order reference electrode voltage slope is a first-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode while charging the reference electrode. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as less than or equal to a predetermined low SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to a predetermined first-order reference electrode voltage slope low threshold and the first-order reference electrode voltage slope is decreasing. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as greater than or equal to a predetermined high SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to a predetermined first-order reference electrode voltage slope high threshold and the first-order reference electrode voltage slope is increasing.

[0017] In another aspect of the present disclosure, to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to calculate a second-order reference electrode voltage slope. The second-order reference electrode voltage slope is a second-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode while charging the reference electrode. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as less than or equal to a predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope is less than or equal to a predetermined second-order reference electrode voltage slope low threshold. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as greater than or equal to a predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined second-order reference electrode voltage slope high threshold.

[0018] In another aspect of the present disclosure, to adjust the reference electrode SOC, the controller is further programmed to, in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold, charge the reference electrode. The reference electrode is charged until at least one of the following conditions occurs: (i) the first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope low threshold and the first order reference electrode voltage slope is increasing, and (ii) the second order reference electrode voltage slope is greater than or equal to a predetermined second order reference electrode voltage slope high threshold.

[0019] In another aspect of the present disclosure, to determine the reference electrode SOC, the controller is further programmed to track the elapsed time since the last charge process of the reference electrode. To determine the reference electrode SOC, the controller is further programmed to calculate the amount of charge lost from the reference electrode based at least in part on the elapsed time and a predetermined reference electrode discharge rate of the reference electrode. To determine the reference electrode SOC, the controller is further programmed to calculate the reference electrode SOC based at least in part on the amount of charge lost.

[0020] In another aspect of the present disclosure, to adjust the reference electrode SOC, the controller is further programmed to charge the reference electrode in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold. The reference electrode is charged until the lost charge is returned to the reference electrode.

[0021] According to several aspects, a system for regulating the state of charge of a reference electrode in a vehicle battery cell is provided. The system may include a battery cell comprising an anode, a cathode, and a reference electrode. The reference electrode is disposed between the anode and the cathode. The system may also include a reference electrode management system in electrical communication with the battery cell, the reference electrode management system comprising a controller. The controller is programmed to charge the reference electrode by allowing current to flow between the cathode and the reference electrode. The controller is further programmed to measure an amount of charge added to the reference electrode during charging. The controller is further programmed to measure an anode reference voltage while charging the reference electrode. The controller is further programmed to determine a reference electrode state of charge (SOC) based at least in part on the anode reference voltage and the amount of charge added to the reference electrode. The controller is further programmed to compare the reference electrode SOC to at least one predetermined low SOC threshold. The controller is further programmed to adjust the reference electrode SOC in response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold or that the reference electrode SOC is greater than or equal to a predetermined high SOC threshold.

[0022] In another aspect of the present disclosure, to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to calculate a second-order reference electrode voltage slope. The second-order reference electrode voltage slope is a second-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode while charging the reference electrode. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as less than or equal to a predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope is less than or equal to a predetermined second-order reference electrode voltage slope low threshold. To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode, the controller is further programmed to determine the reference electrode SOC as greater than or equal to a predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined second-order reference electrode voltage slope high threshold.

[0023] In another aspect of the present disclosure, to adjust the reference electrode SOC, the controller is further programmed to charge the reference electrode in response to determining that the reference electrode SOC is less than or equal to a predetermined low SOC threshold. The reference electrode is charged until a second-order reference electrode voltage slope is greater than or equal to a predetermined second-order reference electrode voltage slope high threshold.

[0024] Further areas of applicability will become apparent from the description provided herein.It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

[0026] Figure 1 is a schematic diagram of a system for adjusting the state of charge of a reference electrode in a battery cell according to an exemplary embodiment;

[0027] Figure 2 is a schematic diagram of a reference electrode according to an exemplary embodiment;

[0028] Figure 3 is a schematic diagram of a reference electrode management system according to an exemplary embodiment;

[0029] Figure 4 is a flow chart of a method for adjusting the state of charge of a reference electrode in a battery cell according to an exemplary embodiment;

[0030] Figure 5 is a flow chart of a first method for determining a charge state of a reference electrode according to an exemplary embodiment;

[0031] Figure 6 is a flow chart of a first method for determining a charge state of a reference electrode based on an anode reference voltage and an amount of charge added during charging, according to an exemplary embodiment;

[0032] Figure 7 is a first exemplary graph illustrating an exemplary first order reference voltage slope according to an exemplary embodiment;

[0033] Figure 8 is a flow chart of a second method for determining a state of charge of a reference electrode based on an anode reference voltage and an amount of charge added during charging, according to an exemplary embodiment;

[0034] Figure 9 is a second exemplary graph having an exemplary second order reference voltage slope according to an exemplary embodiment;

[0035] Figure 10 is a flow chart of a second method for determining a charge state of a reference electrode according to an exemplary embodiment;

[0036] Figure 11 is a flow chart of a method for charging a reference electrode according to an exemplary embodiment;

[0037] Figure 12 is a flow chart of a method for discharging a reference electrode according to an exemplary embodiment; and

[0038] Figure 13 According to an exemplary embodiment, Figure 1 Schematic diagram of an exemplary vehicle, electrical load, and battery management system of a system. DETAILED DESCRIPTION

[0039] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

[0040] In various aspects of the present disclosure, it is advantageous to accurately and reliably estimate the state of charge of a battery cell. State of charge estimation can rely on measuring the voltage within the battery cell. Therefore, a battery cell may include a reference electrode to provide a reference voltage for measuring a specific voltage potential within the battery cell. However, the state of charge of the reference electrode may deplete during use, thereby degrading performance. Therefore, the present disclosure provides a new and improved system and method for regulating the state of charge of a reference electrode in a battery cell.

[0041] refer to Figure 1 , a schematic diagram of a system 10 for regulating the state of charge of a reference electrode in a battery cell is shown. The system 10 generally includes a battery 12 and a reference electrode management system 14.

[0042] The battery cell 12 is used to store electrical energy in the form of chemical energy. In an exemplary embodiment, the battery cell 12 is a lithium-ion battery cell (e.g., a lithium cobalt oxide (LiCoO2) battery cell, a lithium manganese oxide (LiMn2O4) battery cell, a lithium iron phosphate (LiFePO4) battery cell, a lithium nickel cobalt aluminum oxide (LiNiCoAlO2 or NCA) battery cell, a lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC) battery cell, a lithium titanate (Li4Ti5O12) battery cell, etc.). It should be understood that the battery cell 12 can use other battery chemistries besides lithium ion without departing from the scope of the present disclosure. In an exemplary embodiment, the battery cell 12 includes a cathode 16, an anode 18, a reference electrode 20 disposed between the cathode 16 and the anode 18, and an electrolyte (not shown) in contact with the cathode 16, the anode 18, and the reference electrode 20.

[0043] In a non-limiting example, cathode 16 is made of a mixed metal oxide of lithium, nickel, manganese, and cobalt. In a non-limiting example, anode 18 is made of graphite. In a non-limiting example, the electrolyte includes a lithium salt dissolved in a solvent (e.g., lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), etc.). The composition of reference electrode 20 will be discussed in more detail below.

[0044] In an exemplary embodiment, the cathode 16 is electrically connected to the positive terminal 22a of the battery cell 12. The anode 18 is electrically connected to the negative terminal 22b of the battery cell 12. The reference electrode 20 is electrically connected to the reference terminal 22c of the battery cell 12. The positive terminal 22a, the negative terminal 22b, and the reference terminal 22c allow the battery cell 12 to be connected to other systems in order to measure one or more conditions of the battery cell 12 and / or to provide power to external devices, as will be discussed in more detail below. The reference terminal 22c provides a reference electrode voltage for measuring a cathode reference voltage measured between the positive terminal 22a and the reference terminal 22c and / or an anode reference voltage measured between the negative terminal 22b and the reference terminal 22c, as will be discussed in more detail below.

[0045] Within the scope of this disclosure, state of charge (SOC) generally refers to the number or concentration of lithium ions intercalated within a lithium-ion intercalation material (i.e., a material capable of intercalating lithium ions) relative to the maximum capacity of the lithium-ion intercalation material. The state of charge (SOC) of a battery cell 12 quantifies the current charge level stored in the battery cell 12 relative to the maximum charge capacity of the battery cell 12. At the molecular level, the SOC of a battery cell 12 refers to the distribution of lithium ions between the cathode 16 and the anode 18. More specifically, the SOC of a battery cell 12 quantifies the number or concentration of lithium ions intercalated within the anode 18 relative to the maximum capacity of the anode 18 to intercalate lithium ions. In a non-limiting example, when the battery cell 12 is fully charged (i.e., the SOC of the battery cell 12 is 100%), the anode 18 is fully intercalated with lithium ions. As the battery cell 12 discharges, lithium ions move from the anode 18 to the cathode 16 through the electrolyte, resulting in a decrease in the lithium ion concentration in the anode 18 and an increase in the lithium ion concentration in the cathode 16, thereby decreasing the SOC of the battery cell 12.

[0046] Overcharging or over-discharging of the battery cell 12 may damage components of the battery cell 12, such as the cathode 16 and / or the anode 18, resulting in a shortened overall useful life of the battery cell 12. Therefore, for battery management purposes, it is advantageous to determine the SOC of the battery cell 12. Generally speaking, the SOC of the battery cell 12 is not a directly measurable quantity, but must be estimated using a mathematical model of the electrochemical processes occurring within the battery cell 12. In a non-limiting example, the mathematical model is configured to determine or estimate the SOC of the battery cell 12 based at least in part on the open circuit voltage (OCV) of the battery cell 12. In another non-limiting example, the mathematical model is configured to determine or estimate the SOC of the battery cell 12 based at least in part on a cathode reference voltage measured between the positive terminal 22a and the reference terminal 22c and / or an anode reference measured between the negative terminal 22b and the reference terminal 22c.

[0047] refer to Figure 2 , a schematic diagram of the reference electrode 20 is shown. In an exemplary embodiment, the reference electrode 20 includes a separator sheet 30 that secures a reference strip 32 in place. In a non-limiting example, the separator sheet 30 is made of a thin, porous, electrically insulating material (e.g., polyethylene (PE), polypropylene (PP), etc.) that allows lithium ions to flow while preventing electrical short circuits. The reference strip 32 includes a conductive strip 34 connected to a reference material 36. In a non-limiting example, the reference strip 32 is a flexible structure made of a metal foil (e.g., copper, aluminum, etc.) or a polymer film (e.g., polyimide, polyethylene terephthalate, etc.). The conductive strip 34 is used to provide an electrical connection between the reference material 36 and the reference terminal 22c. In a non-limiting example, the conductive strip 34 is made of a conductive material (e.g., gold, silver, platinum, copper, etc.).

[0048] The reference material 36 is an active material that determines the reference electrode voltage of the reference electrode 20. In a non-limiting example, the reference material 36 is made of a lithium compound that has a relatively stable and relatively reproducible electrochemical potential over a relatively wide range of lithium concentrations. For example, the reference material 36 may include lithium iron phosphate (LiFePO4), lithium titanate (Li4Ti5O12), lithium cobalt oxide (LiCoO2), and the like. As described above, the state of charge (SOC) generally refers to the number or concentration of lithium ions embedded in a lithium ion embedding material (i.e., a material capable of embedding lithium ions) relative to the maximum capacity of the lithium ion embedding material. Therefore, within the scope of the present disclosure, the reference electrode SOC is defined as the number or concentration of lithium ions embedded in the reference material 36 relative to the maximum capacity of the reference material 36 to embed lithium ions.

[0049] Although the reference electrode voltage of the reference electrode 20 ideally remains stable over a wide range of lithium concentrations (i.e., reference electrode SOC), changes in the reference electrode SOC may result in changes in the reference electrode voltage, thereby reducing the accuracy of the cathode reference voltage and / or anode reference voltage measurements. In a non-limiting example, changes in the reference electrode SOC may be caused by electrochemical reactions (also known as side reactions) occurring within the battery cell 12. In another non-limiting example, changes in the reference electrode SOC may be caused by charging and / or discharging of the reference electrode material caused by an external measurement circuit during the measurement of the cathode reference voltage and / or anode reference voltage. Therefore, it is advantageous to adjust the reference electrode SOC and maintain the reference electrode SOC within a known and / or ideal range, as will be discussed in more detail below.

[0050] Reference again Figure 1 , the reference electrode management system 14 is used to adjust the reference electrode SOC. Figure 1 As shown, the reference electrode management system 14 is in electrical communication with the positive terminal 22a, the negative terminal 22b, and the reference terminal 22c. In an exemplary embodiment, the reference electrode management system 14 is fixed to the battery cell 12 so that the reference electrode management system 14 can operate even when the battery cell 12 is not installed in another device or module (e.g., a battery pack). Figure 1 1 as being secured to the housing of the battery cell 12, it should be understood that the reference electrode management system 14 may be integrated into the housing of the battery cell 12, or may be located within the battery cell 12, internally connected to the cathode 16, anode 18, and reference electrode 20, or otherwise integrated with the battery cell 12 without departing from the scope of the present disclosure.

[0051] In another exemplary embodiment, the reference electrode management system 14 is a modular component configured to be easily installed, removed, and replaced on the battery cells 12. In another exemplary embodiment, the reference electrode management system 14 is located remotely from the battery cells 12 and is electrically connected to the battery cells 12. In another exemplary embodiment, the reference electrode management system 14 is configured to adjust the reference electrode SOC of a plurality of battery cells that are part of a battery system (e.g., a multi-cell battery module / pack) and is electrically connected to the plurality of battery cells.

[0052] refer to Figure 3 , a schematic diagram of the reference electrode management system 14 is shown. In an exemplary embodiment, the reference electrode management system 14 includes at least a controller 40 and an interface circuit 42.

[0053] The controller 40 is used to implement the method 100 for regulating the charge state of a reference electrode in a battery cell, as described below. The controller 40 includes at least one processor 44 and a non-transitory computer-readable storage device or medium 46. The processor 44 can be a custom or commercially available processor, a central processing unit (CPU), a graphics processing unit (GPU), an auxiliary processor among several processors associated with the controller 40, a semiconductor-based microprocessor (in the form of a microchip or chipset), a macroprocessor, a field programmable gate array (FPGA), an application-specific integrated circuit (ASIC), a combination thereof, or a device generally used to execute instructions.

[0054] The computer-readable storage device or medium 46 may include, for example, volatile and non-volatile memory in a read-only memory (ROM), a random access memory (RAM), and a keep-alive memory (KAM). KAM is a persistent or non-volatile memory that can be used to store various operating variables when the processor 44 is shut down. The computer-readable storage device or medium 46 may be implemented using a plurality of storage devices, such as a PROM (programmable read-only memory), an ePROM (electrical PROM), an EEPROM (electrically erasable PROM), a flash memory, or other electrical, magnetic, optical, or combined storage device capable of storing data, some of which represent executable instructions used by the controller 40 to control the reference electrode management system 14. The controller 40 may also be composed of a plurality of controllers electrically connected to each other. The controller 40 may also include additional components and / or modules, such as a real-time clock (RTC) module for measuring real-time channels. In an exemplary embodiment, the controller 40 is powered by connection to the positive terminal 22a and the negative terminal 22b of the battery cell 12.

[0055] The controller 40 is in electrical communication with the interface circuit 42. In an exemplary embodiment, the electrical communication is performed using, for example, general purpose input / output (GPIO) pins, an inter-integrated circuit (I2C) bus, a serial peripheral interface (SPI) bus, a parallel communication bus, etc. It should be understood that various additional communication protocols for communicating with the controller 40 are within the scope of the present disclosure.

[0056] The interface circuit 42 is used to connect the controller 40 to the positive terminal 22 a, the negative terminal 22 b, and the reference terminal 22 c. In an exemplary embodiment, the interface circuit 42 includes a measurement circuit 48 and a charging circuit 50.

[0057] The measurement circuit 48 is used to measure the anode reference voltage, the cathode reference voltage, and the current flowing into / out of the reference electrode 20. In a non-limiting example, the measurement circuit 48 includes, for example, an analog-to-digital converter (ADC). The measurement circuit 48 may also include additional components that support voltage measurement, including, for example, a voltage follower, an input buffer, a multiplexer, etc. The measurement circuit 48 also includes components that allow the controller 40 to measure the current flowing into / out of the reference electrode 20, including, for example, a shunt resistor, an electromagnetic current sensor, an ADC, etc.

[0058] The charging circuit 50 is used to charge and / or discharge the reference electrode 20. In an exemplary embodiment, the charging circuit 50 includes switching electronics that allow the controller 40 to connect and disconnect the reference terminal 22c between the positive terminal 22a or the negative terminal 22b to charge or discharge the reference electrode 20. In a non-limiting example, the charging circuit 50 includes, for example, a relay, a contactor, a transistor, etc. It should be understood that the measurement circuit 48 and / or the charging circuit 50 of the interface circuit 42 may also include additional passive or active analog and / or digital electronics, such as resistors, capacitors, inductors, filters, amplifiers, power electronics, digital-to-analog converters (DACs), etc. In an exemplary embodiment, the interface circuit 42 is powered by connecting to the positive terminal 22a and the negative terminal 22b of the battery cell 12. The interface circuit 42 is electrically connected to the controller 40, as described above.

[0059] refer to Figure 4 , a flow chart of a method 100 for adjusting the state of charge of a reference electrode in a battery cell is shown. Method 100 begins at block 102 and proceeds to block 104. At block 104, controller 40 determines the reference electrode SOC of reference electrode 20. Methods for determining the reference electrode SOC will be discussed in more detail below. After block 104, method 100 proceeds to block 106.

[0060] At box 106, the controller 40 compares the reference electrode SOC determined at box 104 to a predetermined low SOC threshold (e.g., 10%) and a predetermined high SOC threshold (e.g., 90%). If the reference electrode SOC is less than or equal to the predetermined low SOC threshold, the method 100 proceeds to box 108. If the reference electrode SOC is greater than the predetermined low SOC threshold and less than the predetermined high SOC threshold, the method 100 proceeds to enter a standby state at box 110. If the reference electrode SOC is greater than or equal to the predetermined high SOC threshold, the method 100 proceeds to box 112, as will be discussed in more detail below.

[0061] At block 108, in response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, the controller 40 charges the reference electrode 20 using the interface circuit 42 to adjust the reference electrode SOC. Methods for charging the reference electrode 20 will be discussed in more detail below. After block 108, the method 100 proceeds to a standby state at block 110.

[0062] At block 112, in response to determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold, the controller 40 discharges the reference electrode 20 using the interface circuit 42 to adjust the reference electrode SOC. Methods for discharging the reference electrode 20 will be discussed in more detail below. After block 112, the method 100 proceeds to enter a standby state at block 110.

[0063] refer to Figure 5 , a flow chart of a first exemplary embodiment 104a of method 100 (i.e., a first method for determining reference electrode SOC) at block 104 is shown. The first exemplary embodiment 104a begins at blocks 502, 504, and 506. At block 502, the controller 40 charges the reference electrode 20 using the interface circuit 42. In the exemplary embodiment, to charge the reference electrode 20, the controller 40 connects the positive terminal 22a to the reference terminal 22c using the charging circuit 50, thereby allowing current to flow from the cathode 16 to the reference electrode 20. In the exemplary embodiment, the charging circuit 50 allows the current to flow for a predetermined duration. In a non-limiting example, the predetermined duration depends at least in part on the cross-sectional area and / or volume of the reference electrode 20. In another exemplary embodiment, the charging circuit 50 allows the current to flow until the reference electrode SOC increases by a predetermined amount (e.g., 5%). After block 502, the first exemplary embodiment 104a proceeds to block 508, as discussed in more detail below.

[0064] At block 504, the controller 40 uses the measurement circuit 48 to measure the current flowing into the reference electrode 20 during the charging process at block 502. At block 502, based on the predetermined duration, the controller 40 determines the amount of charge added to the reference electrode 20 during the charging process. Following block 504, the first exemplary embodiment 104a proceeds to block 508, as will be discussed in greater detail below.

[0065] At block 506 , the controller 40 uses the measurement circuit 48 to measure the anode reference voltage during the charging process at block 502 . After block 506 , the first exemplary embodiment 104 a proceeds to block 508 .

[0066] At block 508, the controller 40 determines the reference electrode SOC based at least in part on the anode reference voltage during charging and the amount of charge added to the reference electrode 20 during charging. Methods for determining the reference electrode SOC based at least in part on the anode reference voltage during charging and the amount of charge added to the reference electrode 20 during charging are discussed in more detail below. After block 508, the first exemplary embodiment 104a ends and the method 100 continues as described above.

[0067] refer to Figure 6 , a flow chart of a first exemplary embodiment 508a of a first exemplary embodiment 104 of a method 100 (i.e., a first method for determining a reference electrode SOC based on an anode reference voltage and an amount of charge added during charging) at block 508 is shown. The first exemplary embodiment 508a begins at block 602. At block 602, the controller 40 calculates a first-order reference electrode voltage slope. Within the scope of the present disclosure, the first-order reference electrode voltage slope is the first-order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode 20 (as determined at block 506) while charging the reference electrode 20.

[0068] refer to Figure 7 , shows a first exemplary graph 70 having exemplary first order reference voltage slopes 72 determined over a range of reference electrode SOCs. The x-axis 74 of the first exemplary graph 70 represents the reference electrode SOC. The y-axis 76 of the first exemplary graph 70 represents the values ​​of the first order reference voltage slopes. The first dashed line of the first exemplary graph 70 represents a predetermined first order reference electrode voltage slope low threshold 78a. The second dashed line of the first exemplary graph 70 represents a predetermined first order reference electrode voltage slope high threshold 78b. In an exemplary embodiment, the predetermined first order reference electrode voltage slope low and high thresholds 78a, 78b are selected based at least in part on a predetermined low SOC threshold and / or a predetermined high SOC threshold.

[0069] It should be understood that the exemplary first-order reference voltage slope 72 is exemplary in nature and that the shape and value of the first-order reference electrode voltage slope may vary based on, for example, the application-specific characteristics of the reference electrode 20 and / or the battery cell 12. It should also be understood that the values ​​of the predetermined first-order reference electrode voltage slope low threshold 78a and high threshold 78b shown in the first exemplary graph 70 are merely exemplary in nature. In some embodiments, the values ​​of the predetermined first-order reference electrode voltage slope low threshold 78a, 78b may be equal. Furthermore, the first exemplary graph 70, the predetermined first-order reference electrode voltage slope low threshold 78a, 78b, and the exemplary first-order reference voltage slope 72 are not necessarily to scale.

[0070] Reference again Figure 6 And continue to refer to Figure 7 , after block 602, the first exemplary embodiment 508a proceeds to block 604. At block 604, the controller 40 compares the first order reference electrode voltage slope determined at block 602 to predetermined first order reference electrode voltage slope low threshold 78a and high threshold 78b. If the first order reference electrode voltage slope determined at block 602 is greater than or equal to the predetermined first order reference electrode voltage slope low threshold 78a and decreases as charge is added to the reference electrode 20, the first exemplary embodiment 508a proceeds to block 606. If the first order reference electrode voltage slope determined at block 602 is greater than or equal to the predetermined first order reference electrode voltage slope high threshold 78b and increases as charge is added to the reference electrode 20, the first exemplary embodiment 508a proceeds to block 608, as will be discussed in greater detail below.

[0071] At block 606, in response to determining that the first order reference electrode voltage slope determined at block 602 is greater than or equal to the predetermined first order reference electrode voltage slope low threshold 78a and is decreasing as charge is added to reference electrode 20, the reference electrode SOC is determined to be less than or equal to the predetermined low SOC threshold. After block 606, the first exemplary embodiment 508a ends and the first exemplary embodiment 104a proceeds as described above.

[0072] At block 608, in response to determining that the first order reference electrode voltage slope determined at block 602 is greater than or equal to the predetermined first order reference electrode voltage slope high threshold 78b and increases as charge is added to the reference electrode 20, the reference electrode SOC is determined to be greater than or equal to the predetermined high SOC threshold. After block 608, the first exemplary embodiment 508a ends and the first exemplary embodiment 104a proceeds as described above.

[0073] refer to Figure 8, a flow chart of a second exemplary embodiment 508b of the first exemplary embodiment 104a of method 100 (i.e., a second method for determining the reference electrode SOC based on the anode reference voltage and the amount of charge added during charging) at block 508 is shown. It should be understood that within the scope of the present disclosure, the first exemplary embodiment 508a, the second exemplary embodiment 508b, or any combination thereof can be used to perform block 508 of the first exemplary embodiment 104a. The second exemplary embodiment 508b begins at block 802. At block 802, the controller 40 calculates a second-order reference electrode voltage slope. Within the scope of the present disclosure, the second-order reference electrode voltage slope is the second-order derivative of the anode reference voltage (as determined at block 506) with respect to the amount of charge added to the reference electrode 20 while charging the reference electrode 20 (as determined at block 508).

[0074] refer to Figure 9 , shows a second exemplary graph 80 having exemplary second order reference voltage slopes 82 determined within a range of reference electrode SOCs. The x-axis 84 of the second exemplary graph 80 represents the reference electrode SOC. The y-axis 86 of the second exemplary graph 80 represents the values ​​of the second order reference voltage slopes. The first dashed line of the second exemplary graph 80 represents a predetermined second order reference electrode voltage slope low threshold 88a. The second dashed line of the second exemplary graph 80 represents a predetermined second order reference electrode voltage slope high threshold 88b. In an exemplary embodiment, the predetermined second order reference electrode voltage slope low threshold 88a and high threshold 88b are selected based at least in part on a predetermined low SOC threshold and / or a predetermined high SOC threshold.

[0075] It should be understood that the exemplary second order reference voltage slope 82 is exemplary in nature and that the shape and value of the second order reference electrode voltage slope can vary based on, for example, the application-specific characteristics of the reference electrode 20 and / or the battery cell 12. It should also be understood that the values ​​of the predetermined second order reference electrode voltage slope low threshold 88a and high threshold 88b shown in the second exemplary graph 80 are merely exemplary in nature. In some embodiments, the values ​​of the predetermined second order reference electrode voltage slope low threshold 88a and high threshold 88b can be equal. Furthermore, the second exemplary graph 80, the predetermined second order reference electrode voltage slope low threshold 88a and high threshold 88b, and the exemplary second order reference voltage slope 82 are not necessarily to scale.

[0076] Reference again Figure 8 And continue to refer to Figure 9After block 802, the second exemplary embodiment 508b proceeds to block 804. At block 804, the controller 40 compares the second order reference electrode voltage slope determined at block 802 to predetermined second order reference electrode voltage slope low threshold 88a and high threshold 88b. If the second order reference electrode voltage slope determined at block 802 is greater than or equal to the predetermined second order reference electrode voltage slope low threshold 88a, the second exemplary embodiment 508b proceeds to block 806. If the second order reference electrode voltage slope determined at block 802 is greater than or equal to the predetermined second order reference electrode voltage slope high threshold 88b, the second exemplary embodiment 508b proceeds to block 808, as will be discussed in greater detail below.

[0077] At block 806, in response to determining that the second order reference electrode voltage slope determined at block 802 is less than or equal to the predetermined second order reference electrode voltage slope low threshold 88a, the reference electrode SOC is determined to be less than or equal to the predetermined low SOC threshold. After block 806, the second exemplary embodiment 508b ends and the first exemplary embodiment 104a continues as described above.

[0078] At block 808, in response to determining that the second order reference electrode voltage slope determined at block 802 is greater than or equal to the predetermined second order reference electrode voltage slope high threshold 88b, the reference electrode SOC is determined to be greater than or equal to the predetermined high SOC threshold. After block 808, the second exemplary embodiment 508b ends and the first exemplary embodiment 104a continues as described above.

[0079] refer to Figure 10 , a flow chart of a second exemplary embodiment 104b of the method 100 (i.e., a second method for determining the reference electrode SOC) at block 104 is shown. It should be understood that the first exemplary embodiment 104a, the second exemplary embodiment 104b, or any combination thereof may be used to perform the method 100 at block 104 within the scope of the present disclosure. The second exemplary embodiment 104b begins at block 1002. At block 1002, the controller 40 tracks the elapsed time since the reference electrode 20 was previously charged (i.e., the elapsed time since the previous execution of the method 100 at block 108). In an exemplary embodiment, the controller 40 uses a real-time clock (RTC) module to track the elapsed time. After block 1002, the second exemplary embodiment 104b proceeds to block 1004.

[0080] At block 1004, the controller 40 calculates the reference electrode SOC. In an exemplary embodiment, the controller 40 first calculates the amount of charge lost from the reference electrode 20 during the elapsed time determined at block 1002. In a non-limiting example, the controller 40 multiplies the elapsed time by a predetermined reference electrode discharge rate for the reference electrode. Within the scope of the present disclosure, the predetermined reference electrode discharge rate quantifies the amount of charge lost from the reference electrode 20 per unit time during normal operation of the reference electrode management system 14. In an exemplary embodiment, the predetermined reference electrode discharge rate is determined using computer simulations and / or physical experiments. The predetermined reference electrode discharge rate is then stored in the medium 46 of the controller 40 for use during the second exemplary embodiment 104b.

[0081] The controller 40 calculates the reference electrode SOC based on the amount of charge lost. In an exemplary embodiment, the reference electrode SOC is proportional to the previous charge on the reference electrode 20 (e.g., determined using the measurement circuit 48 of the interface circuit 42 based on a previous charging process of the reference electrode 20) minus the amount of charge lost. After block 1004, the second exemplary embodiment 104b ends, and the method 100 continues as described above.

[0082] refer to Figure 11 , a flow chart of an exemplary embodiment 108a of method 100 (i.e., a method for charging reference electrode 20) at block 108 is shown. Exemplary embodiment 108a begins at block 1102. At block 1102, controller 40 charges reference electrode 20. In the exemplary embodiment, to charge reference electrode 20, controller 40 uses charging circuit 50 of interface circuit 42 to connect positive terminal 22a (i.e., cathode 16) to reference terminal 22c (i.e., reference electrode 20), allowing current to flow into reference electrode 20. In the exemplary embodiment, controller 40 simultaneously uses measurement circuit 48 to measure the magnitude of the current flowing into reference electrode 20 over time in order to calculate the charge added to reference electrode 20. Controller 40 also uses measurement circuit 48 to measure the anode reference voltage while charging reference electrode 20. After block 1102, exemplary embodiment 108a proceeds to block 1104.

[0083] At block 1104, the controller 40 evaluates a charge stop condition. In the first exemplary embodiment, the charge stop condition is satisfied when the first order reference electrode voltage slope (as determined with reference to the first exemplary embodiment 508a at block 508 and the method 100 described with reference to the first exemplary embodiment 104a at block 104) is greater than or equal to a predetermined first order reference electrode voltage slope high threshold 78b and the first order reference electrode voltage slope is increasing. In other words, the charge stop condition is satisfied when the reference electrode SOC reaches a predetermined high SOC threshold.

[0084] In the second exemplary embodiment, the charge stop condition is satisfied when the second order reference electrode voltage slope (as determined with reference to the second exemplary embodiment 508b at block 508 and the method 100 described with reference to the first exemplary embodiment 104a at block 104) is greater than or equal to a predetermined second order reference electrode voltage slope high threshold 88b. In other words, the charge stop condition is satisfied when the reference electrode SOC reaches a predetermined high SOC threshold.

[0085] In the third exemplary embodiment, the charge stop condition is met when the amount of lost charge (as determined using method 100 described with reference to method 100 at block 104 of the second exemplary embodiment 104b) is returned to reference electrode 20. If the charge stop condition is not met, exemplary embodiment 108a returns to block 1102 to continue charging reference electrode 20. If the charge stop condition is met, exemplary embodiment 108a ends and method 100 continues as described above.

[0086] refer to Figure 12 , a flow chart of an exemplary embodiment 112a of method 100 (i.e., a method for discharging reference electrode 20) at block 112 is shown. Exemplary embodiment 112a begins at block 1202. At block 1202, controller 40 discharges reference electrode 20. In the exemplary embodiment, to discharge reference electrode 20, controller 40 uses charging circuit 50 of interface circuit 42 to connect negative terminal 22b (i.e., anode 18) to reference terminal 22c (i.e., reference electrode 20), allowing current to flow out of reference electrode 20. In the exemplary embodiment, controller 40 simultaneously uses measurement circuit 48 to measure the magnitude of the current flowing out of reference electrode 20 over time for calculating the charge removed from reference electrode 20. Controller 40 also uses measurement circuit 48 to measure the anode reference voltage while discharging reference electrode 20. After block 1202, exemplary embodiment 112a proceeds to block 1204.

[0087] At block 1204, the controller 40 evaluates a discharge stop condition. In the first exemplary embodiment, the discharge stop condition 100 is satisfied when the first order reference electrode voltage slope (as determined with reference to the first exemplary embodiment 508a at block 508 and the method 10 described with reference to the first exemplary embodiment 104a at block 104) is greater than or equal to a predetermined first order reference electrode voltage slope high threshold 78b and the first order reference electrode voltage slope is increasing. In other words, the discharge stop condition is satisfied when the reference electrode SOC reaches a predetermined high SOC threshold.

[0088] In the second exemplary embodiment, the discharge stop condition 100 is satisfied when the second-order reference electrode voltage slope (as determined with reference to the second exemplary embodiment 508b at block 508 and the method 100 described with reference to the first exemplary embodiment 104a at block 104) is greater than or equal to the predetermined second-order reference electrode voltage slope high threshold 88b. In other words, the discharge stop condition is satisfied when the reference electrode SOC reaches the predetermined high SOC threshold. If the discharge stop condition is satisfied, the exemplary embodiment 112a ends and the method 100 continues as described above.

[0089] refer to Figure 13 , an exemplary vehicle 90 is shown having system 10, electrical loads 92, and a battery management system 94. Electrical loads 92 and battery management system 94 are in electrical communication with system 10. In the exemplary embodiment, electrical loads 92 include electrical and / or electromechanical components or systems of vehicle 90 that require electrical energy to operate. In a non-limiting example, electrical loads 92 include battery management system 94, a vehicle controller, an infotainment system, one or more lights, a vehicle propulsion system including an electric motor, and the like. Battery cells 12 of system 10 are configured to provide electrical energy to electrical loads 92.

[0090] The battery management system 94 is used to monitor the battery cells 12, provide information about the status of the battery cells 12 to external systems (e.g., a vehicle controller), and optimize the use of the battery cells 12 to extend the useful life of the battery cells and protect the battery cells 12 from damage. In an exemplary embodiment, the battery management system 94 facilitates the electrical connection between the system 10 and the electrical load 92 and can disconnect the system 10 from the electrical load 92 to protect the battery cells 12. In an exemplary embodiment, the battery management system 94 performs measurements of the anode reference voltage and / or cathode reference voltage (using, for example, the measurement circuit 48 of the interface circuit 42 of the reference electrode management system 14) and executes a mathematical model to estimate the state of charge (SOC) of the battery cells 12. The SOC of the battery cells 12 is used by other vehicle systems (e.g., a vehicle controller) to provide vehicle range and / or battery status information to occupants of the vehicle 90.

[0091] The system 10 and method 100 of the present disclosure have several advantages. Using the first exemplary embodiment 104a of the method 100 at block 104 (i.e., the first method for determining the reference electrode SOC), the reference electrode SOC can be determined regardless of the SOC of the battery cell 12. Therefore, the reference electrode SOC can be frequently monitored and adjusted to prevent excessive discharge of the reference electrode 20. Using the second exemplary embodiment 104b of the method 100 at block 104 (i.e., the second method for determining the reference electrode SOC), the reference electrode SOC can be determined regardless of the SOC of the battery cell 12 and the equilibrium state of the battery cell 12 (i.e., when the battery cell 12 is under any load condition). Therefore, the reference electrode SOC can be frequently monitored and adjusted to prevent excessive discharge of the reference electrode 20, even when the battery cell 12 is under frequent load. In an exemplary embodiment, the first exemplary embodiment 104a and the second exemplary embodiment 104b are used in combination to provide a balance between accuracy and frequency of reference electrode SOC measurement. In addition, by securing the reference electrode management system 14 to the battery cell 12 or otherwise integrating it with the battery cell 12, the reference electrode management system 14 can operate to regulate the reference electrode SOC even when the battery cell 12 is not installed in a host application (e.g., if the battery cell 12 is in storage or shipping), thereby extending the useful life of the reference electrode 20 and the battery cell 12.

[0092] The description of the present disclosure is merely exemplary in nature, and variations that do not depart from the gist of the present disclosure are intended to fall within the scope of the present disclosure. Such variations should not be regarded as a departure from the spirit and scope of the present disclosure.

Claims

1. A method for adjusting the state of charge of a reference electrode in a battery cell, the method comprising: determining a reference electrode state of charge (SOC) of the reference electrode, wherein the battery cell includes an anode, a cathode, and the reference electrode, wherein the reference electrode is disposed between the anode and the cathode; comparing the reference electrode SOC to at least one predetermined low SOC threshold; and In response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold or the reference electrode SOC is greater than or equal to a predetermined high SOC threshold, the reference electrode SOC is adjusted.

2. The method according to claim 1, wherein Determining the reference electrode SOC further includes: charging the reference electrode and measuring the amount of charge added to the reference electrode; measuring an anode reference voltage while charging the reference electrode; and The reference electrode SOC is determined based at least in part on the anode reference voltage and the amount of charge added to the reference electrode.

3. The method according to claim 2, wherein Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode further comprises: calculating a first order reference electrode voltage slope, wherein the first order reference electrode voltage slope is a first order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode when charging the reference electrode; and The reference electrode SOC is determined based at least in part on the first order reference electrode voltage slope.

4. The method according to claim 3, wherein Determining the reference electrode SOC based at least in part on the first order reference electrode voltage slope further comprises: responsive to determining that the first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope low threshold and the first order reference electrode voltage slope is decreasing, determining the reference electrode SOC to be less than or equal to the predetermined low SOC threshold; and In response to determining that the first order reference electrode voltage slope is greater than or equal to a predetermined first order reference electrode voltage slope high threshold and the first order reference electrode voltage slope is increasing, determining the reference electrode SOC to be greater than or equal to the predetermined high SOC threshold.

5. The method according to claim 4, wherein Adjusting the reference electrode SOC further includes: In response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, charging the reference electrode, wherein the reference electrode is charged until the first-order reference electrode voltage slope is greater than or equal to the predetermined first-order reference electrode voltage slope high threshold and the first-order reference electrode voltage slope is increasing.

6. The method according to claim 2, wherein Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge added to the reference electrode further comprises: calculating a second order reference electrode voltage slope, wherein the second order reference electrode voltage slope is a second order derivative of the anode reference voltage with respect to the amount of charge added to the reference electrode when the reference electrode is charged; and The reference electrode SOC is determined based at least in part on the second order reference electrode voltage slope.

7. The method according to claim 6, wherein Determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope further comprises: In response to determining that the second order reference electrode voltage slope is less than or equal to a predetermined second order reference electrode voltage slope low threshold, determining the reference electrode SOC to be less than or equal to the predetermined low SOC threshold; and In response to determining that the second order reference electrode voltage slope is greater than or equal to a predetermined second order reference electrode voltage slope high threshold, the reference electrode SOC is determined to be greater than or equal to the predetermined high SOC threshold.

8. The method according to claim 7, wherein Adjusting the reference electrode SOC further includes: In response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, the reference electrode is charged, wherein the reference electrode is charged until the second order reference electrode voltage slope is greater than or equal to the predetermined second order reference electrode voltage slope high threshold.

9. The method according to claim 1, wherein Determining the reference electrode SOC further includes: tracking the elapsed time since the last charging process of the reference electrode; calculating an amount of charge lost from the reference electrode based at least in part on the elapsed time and a predetermined reference electrode discharge rate of the reference electrode; and The reference electrode SOC is calculated based at least in part on the amount of charge lost.

10. The method according to claim 9, wherein Adjusting the reference electrode SOC further includes: In response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, the reference electrode is charged, wherein the reference electrode is charged until the lost amount of charge returns to the reference electrode.

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