Method for regulating a state of charge of a reference electrode in a battery cell

Abstract

A method for regulating the 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 battery cell includes an anode, a cathode, and the reference electrode. The reference electrode is disposed between the anode and the cathode. The method may further include comparing the reference electrode SOC to at least one predetermined low SOC threshold. The method may further include adjusting 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.

Description

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

[0002] To increase performance, usability, and service life, it is advantageous to estimate the state of charge (SOC) of a battery cell. In general, the SOC of a battery cell is not a directly measurable quantity because it depends on electrochemical processes within the battery cell. Therefore, mathematical models are used to estimate the SOC of the battery cell based on directly measurable quantities such as voltages and / or currents. The mathematical models used to estimate the SOC of the battery cell are often highly nonlinear and can be influenced by measurement errors. Therefore, it is advantageous to provide a reference electrode within the battery cell to obtain a reference voltage for accurately measuring the voltages of the battery cell components.However, using the reference electrode to perform measurements can lead to depletion of the reference electrode SOC and thus to fluctuations in the reference voltage.

[0003] Although the battery management systems and methods serve their purpose, there is a need for a new and improved system and method for regulating the state of charge of a reference electrode in a battery cell.

[0004] DE 10 2022 120 004 A1 describes a vehicle as well as a balancing device and a method for regulating a state of charge of a reference electrode in a battery.

[0005] It can be considered an object to provide an alternative method for regulating the state of charge of a reference electrode in a battery cell, whereby the service life of the reference electrode and the battery cell can be extended. This object is achieved by the subject matter of claim 1.

[0006] The inventive method for regulating a state of charge of a reference electrode in a battery cell comprises determining a reference electrode state of charge (SOC) of the reference electrode. The battery cell comprises an anode, a cathode, and the reference electrode.

[0007] The reference electrode is arranged between the anode and the cathode. The method further includes comparing the reference electrode SOC to at least one predetermined low SOC threshold. The method further includes adjusting 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.

[0008] According to one embodiment, determining the reference electrode SOC further comprises charging the reference electrode and measuring an amount of charge supplied to the reference electrode. Determining the reference electrode SOC further comprises measuring an anode reference voltage during charging of the reference electrode. Determining the reference electrode SOC further comprises determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode.

[0009] According to one embodiment, determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode further comprises calculating a first-order reference electrode voltage slope. The first-order reference electrode voltage slope is a first derivative of the anode reference voltage during charging of the reference electrode with respect to the amount of charge supplied to the reference electrode. Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode further comprises determining the reference electrode SOC based at least in part on the first-order reference electrode voltage slope.

[0010] According to one embodiment, determining the reference electrode SOC based at least in part on the first-order reference electrode voltage slope further comprises determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to a predetermined low first-order reference electrode voltage slope threshold and that 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 further comprises determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to the predetermined high first-order reference electrode voltage slope threshold and that the first-order reference electrode voltage slope is increasing.

[0011] According to one embodiment, adjusting the reference electrode SOC further comprises charging the reference electrode 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 until the first-order reference electrode voltage slope is greater than or equal to the predetermined high first-order reference electrode voltage slope threshold and the first-order reference electrode voltage slope increases.

[0012] According to one embodiment, determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode further comprises calculating a second-order reference electrode voltage slope. The second-order reference electrode voltage slope is a second derivative of the anode reference voltage, during charging of the reference electrode, with respect to the amount of charge supplied to the reference electrode. Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode further comprises determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope.

[0013] According to one embodiment, determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope further comprises determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope is less than or equal to a predetermined low second-order reference electrode voltage slope threshold.Determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope further comprises determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined high second-order reference electrode voltage slope threshold.

[0014] According to one embodiment, adjusting the reference electrode SOC further comprises charging the reference electrode 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 until the second-order reference electrode voltage slope is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold.

[0015] According to one embodiment, determining the reference electrode SOC further comprises tracking an elapsed time since a previous charging event of the reference electrode. Determining the reference electrode SOC further comprises calculating a lost amount of charge 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 further comprises calculating the reference electrode SOC based at least in part on the lost amount of charge.

[0016] According to one embodiment, adjusting the reference electrode SOC further comprises charging the reference electrode 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 until the lost amount of charge is returned to the reference electrode.

[0017] As an application of the method according to the invention, a system for regulating the state of charge of a reference electrode in a battery cell is provided. The system may comprise the battery cell with an anode, a cathode, and the reference electrode. The reference electrode is arranged between the anode and the cathode. The system may further comprise a reference electrode management system in electrical communication with the battery cell, wherein the reference electrode management system includes a control unit. The control unit is programmed to determine a reference electrode state of charge (SOC) of the reference electrode. The control unit 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 the predetermined low SOC threshold or that the reference electrode SOC is greater than or equal to a predetermined high SOC threshold.

[0018] In one embodiment, to determine the reference electrode SOC, the controller is further programmed to charge the reference electrode by allowing a current to flow between the cathode and the reference electrode. To determine the reference electrode SOC, the controller is further programmed to measure an amount of charge added to the reference electrode during charging of the reference electrode. To determine the reference electrode SOC, the controller is further programmed to measure an anode reference voltage during charging of 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.

[0019] In one embodiment, the controller is programmed to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, to calculate a first-order reference electrode voltage slope. The first-order reference electrode voltage slope is a first derivative of the anode reference voltage when charging the reference electrode with respect to the amount of charge supplied to the reference electrode.To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, the controller is further programmed to determine the reference electrode SOC to be less than or equal to the predetermined low SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to a predetermined low first-order reference electrode voltage slope threshold and that 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 to be greater than or equal to the predetermined high SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to the predetermined high reference electrode voltage slope threshold and that the first-order reference electrode voltage slope is increasing.

[0020] In one embodiment, the controller is programmed to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, to calculate a second-order reference electrode voltage slope. The second-order reference electrode voltage slope is a second derivative of the anode reference voltage when charging the reference electrode with respect to the amount of charge supplied to the reference electrode.To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, the controller is further programmed to determine the reference electrode SOC to be less than or equal to the 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 threshold.To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, the controller is further programmed to determine the reference electrode SOC to be greater than or equal to the predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined high second-order reference electrode voltage slope threshold.

[0021] In one embodiment, the reference electrode SOC adjustment controller is further programmed to charge the reference electrode 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 until at least one of the following occurs: (i) the first-order reference electrode voltage slope is greater than or equal to the predetermined first-order reference electrode voltage slope threshold and the first-order reference electrode voltage slope increases, and (ii) the second-order reference electrode voltage slope is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold.

[0022] In one embodiment, to determine the reference electrode SOC, the controller is further programmed to track the elapsed time since a previous charging event of the reference electrode. To determine the reference electrode SOC, the controller is further programmed to calculate a lost amount of charge 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 lost amount of charge.

[0023] In one embodiment, the reference electrode SOC adjustment controller is further programmed to charge the reference electrode 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 until the lost amount of charge is returned to the reference electrode.

[0024] As an application of the method according to the invention, a system for regulating the state of charge of a reference electrode in a battery cell in a vehicle is provided. The system may comprise the battery cell with an anode, a cathode, and the reference electrode. The reference electrode is arranged between the anode and the cathode. The system may further comprise a reference electrode management system that is in electrical communication with the battery cell. The reference electrode management system comprises a control unit. The control unit is programmed to charge the reference electrode by allowing a current to flow between the cathode and the reference electrode. The control unit is further programmed to measure an amount of charge supplied to the reference electrode when charging the reference electrode.The controller is further programmed to measure an anode reference voltage during charging of the reference electrode. The controller is further programmed to determine the SOC of the reference electrode based at least in part on the anode reference voltage and the amount of charge supplied 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.

[0025] In one embodiment, to determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied 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 derivative of the anode reference voltage when charging the reference electrode with respect to the amount of charge supplied to the reference electrode.To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, the controller is further programmed to determine the reference electrode SOC to be less than or equal to the 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 threshold.To determine the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode, the controller is further programmed to determine the reference electrode SOC to be greater than or equal to the predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope is greater than or equal to a predetermined high second-order reference electrode voltage slope threshold.

[0026] In one embodiment, the reference electrode SOC adjustment controller is further programmed to charge the reference electrode 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 until the second-order reference electrode voltage slope is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold. Fig. 1 is a schematic diagram of a system for regulating the state of charge of a reference electrode in a battery cell according to an exemplary embodiment; Fig. 2 is a schematic diagram of the reference electrode according to an exemplary embodiment; Fig. 3 is a schematic diagram of a reference electrode management system according to an exemplary embodiment; Fig. 4 is a flowchart of a method for regulating the state of charge of a reference electrode in a battery cell according to an exemplary embodiment; Fig. 5 is a flowchart of a first method for determining the reference electrode state of charge according to an exemplary embodiment; Fig. 6 is a flowchart of a first method for determining the reference electrode state of charge based on an anode reference voltage and an amount of charge added during charging, according to an exemplary embodiment; Fig. 7 is a first exemplary graph showing an exemplary first-order reference voltage slope according to an exemplary embodiment; Fig. 8 is a flowchart of a second method for determining the reference electrode state of charge based on the anode reference voltage and the amount of charge added during the charging process, according to an exemplary embodiment; Fig. 9 is a second exemplary graph showing an exemplary second order reference voltage slope according to an exemplary embodiment; Fig. 10 is a flowchart of a second method for determining the reference electrode state of charge according to an exemplary embodiment; Fig. 11 is a flowchart of a method for charging the reference electrode according to an exemplary embodiment; Fig. 12 is a flowchart of a method for discharging the reference electrode according to an exemplary embodiment; and Fig. 13 is a schematic diagram of an exemplary vehicle with the system of Fig. 1, an electrical load and a battery management system according to an exemplary embodiment.

[0027] In aspects of the present description, it is advantageous to accurately and reliably estimate the state of charge of a battery cell. Estimating the state of charge may depend on measuring voltages within the battery cell. For example, battery cells may include a reference electrode to provide a reference voltage for measuring specific voltage potentials within the battery cell. However, the state of charge of the reference electrode may become depleted over the course of use, impacting performance. Accordingly, the present description provides a new and improved system and method for regulating the state of charge of a reference electrode in a battery cell.

[0028] In Fig. Figure 1 schematically illustrates a system 10 for regulating the state of charge of a reference electrode in a battery cell. The system 10 generally includes a battery cell 12 and a reference electrode management system 14.

[0029] The battery cell 12 serves to store electrical energy in the form of chemical energy. In an exemplary embodiment, the battery cell 12 is a lithium-ion battery cell (for example, a lithium cobalt oxide battery cell (LiCoO2), a lithium manganese oxide battery cell (LiMn2O4), a lithium iron phosphate battery cell (LiFePO4), a lithium nickel cobalt aluminum oxide battery cell (LiNiCoAlO2 or NCA), a lithium nickel manganese cobalt oxide battery cell (LiNiMnCoO2 or NMC), a lithium titanate battery cell (Li4Ti5O12), and / or the like). It is understood that the battery cell 12 may utilize other cell chemistries besides lithium-ion without exceeding the scope of the present description.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.

[0030] In one non-limiting example, the cathode 16 is composed of a mixed metal oxide of lithium, nickel, manganese, and cobalt. In one non-limiting example, the anode 18 is composed of graphite. In one non-limiting example, the electrolyte contains a lithium salt dissolved in a solvent (e.g., lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), and / or the like). The composition of the reference electrode 20 is discussed in more detail below.

[0031] In an exemplary embodiment, the cathode 16 is electrically connected to a positive terminal 22a of the battery cell 12. The anode 18 is electrically connected to a negative terminal 22b of the battery cell 12. The reference electrode 20 is electrically connected to a reference terminal 22c of the battery cell 12. The positive terminal 22a, the negative terminal 22b, and the reference terminal 22c enable the battery cell 12 to be connected to other systems to measure one or more conditions of the battery cell 12 and / or to power an external device, as explained in more detail below. The reference terminal 22c provides a reference electrode voltage used to measure 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 explained in more detail below.

[0032] As used herein, a state of charge (SOC) generally refers to an amount or concentration of lithium ions intercalated in a lithium ion intercalation material (i.e., a material capable of intercalating lithium ions) relative to a maximum capacity of the lithium ion intercalation material to intercalate lithium ions. The state of charge (SOC) of the battery cell 12 quantifies the current level of charge stored in the battery cell 12 relative to a maximum charge capacity of the battery cell 12. At the molecular level, the SOC of the battery cell 12 refers to the distribution of lithium ions between the cathode 16 and the anode 18. Specifically, the SOC of the battery cell 12 quantifies an amount or concentration of lithium ions intercalated in the anode 18 relative to a maximum capacity of the anode 18 to intercalate lithium ions.As a non-limiting example, the anode 18 is fully intercalated with lithium ions when the battery cell 12 is fully charged (i.e., the SOC of the battery cell 12 is 100%). As the battery cell 12 discharges, lithium ions migrate through the electrolyte from the anode 18 to the cathode 16, 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.

[0033] Overcharging or overdischarging the battery cell 12 can damage components of the battery cell 12 such as the cathode 16 and / or the anode 18, resulting in a reduction in the overall useful life of the battery cell 12. Therefore, it is beneficial to determine the SOC of the battery cell 12 for battery management purposes. Generally, the SOC of the battery cell 12 is not a directly measurable quantity and must instead be estimated using a mathematical model of the electrochemical processes occurring within the battery cell 12. In one 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 an 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 the cathode reference voltage measured between the positive terminal 22a and the reference terminal 22c and / or the anode reference voltage measured between the negative terminal 22b and the reference terminal 22c.

[0034] In Fig. Figure 2 shows a schematic representation of the reference electrode 20. In one exemplary embodiment, the reference electrode 20 includes a separator film 30 that holds a reference strip 32. In one non-limiting example, the separator film 30 is made of a thin, porous, electrically insulating material that allows the flow of lithium ions while preventing electrical short circuits (e.g., polyethylene (PE), polypropylene (PP), and / or the like). The reference strip 32 includes a conductive strip 34 connected to a reference material 36. In one non-limiting example, the reference strip 32 is a flexible structure made of a metal foil (e.g., copper, aluminum, and / or the like) or a polymer film (e.g., polyimide, polyethylene terephthalate, and / or the like).The conductive strip 34 is used to establish an electrical connection between the reference material 36 and the reference terminal 22c. In one non-limiting example, the conductive strip 34 is made of a conductive material (e.g., gold, silver, platinum, copper, and / or the like).

[0035] The reference material 36 is an active material that determines the reference electrode voltage of the reference electrode 20. In one non-limiting example, the reference material 36 consists of a lithium compound that exhibits a relatively stable and relatively reproducible electrochemical potential over a relatively wide range of lithium concentrations. The reference material 36 may include, for example, lithium iron phosphate (LiFePO4), lithium titanate (Li4Ti5O12), lithium cobalt oxide (LiCoO2), and / or the like. As explained above, a state of charge (SOC) generally refers to an amount or concentration of lithium ions intercalated within a lithium ion intercalation material (i.e., a material capable of intercalating lithium ions) relative to a maximum capacity of the lithium ion intercalation material to intercalate lithium ions.Therefore, in the context of the present description, a reference electrode SOC is defined as an amount or concentration of lithium ions intercalated in the reference material 36, relative to a maximum capacity of the reference material 36 for intercalating lithium ions.

[0036] While the reference electrode voltage of reference electrode 20 is ideally stable over a wide range of lithium concentrations (i.e., the reference electrode SOC), variations in the reference electrode SOC can lead to variations in the reference electrode voltage, thereby reducing the accuracy of the cathode reference voltage and / or anode reference voltage measurements. In one non-limiting example, variations in the reference electrode SOC can be caused by electrochemical reactions (also referred to as side reactions) occurring within the battery cell 12. In another non-limiting example, variations in the reference electrode SOC can be caused by charging and / or discharging of the reference electrode material induced by external measurement circuitry when performing cathode reference voltage and / or anode reference voltage measurements.Therefore, it is advantageous to regulate the reference electrode SOC and maintain the reference electrode SOC within a known and / or ideal range, as explained in more detail below.

[0037] As in Fig. 1, the reference electrode management system 14 is used to regulate the reference electrode SOC. As shown in Fig. 1, the reference electrode management system 14 is in electrical connection 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 attached to the battery cell 12 so that the reference electrode management system 14 can function even when the battery cell 12 is not incorporated into another device or module (e.g., a battery pack). While the reference electrode management system 14 in Fig. 1 as being attached to an outer casing of the battery cell 12, the reference electrode management system 14 may be integrated into the outer casing of the battery cell 12, located within the battery cell 12 with internal connections to the cathode 16, anode 18, and reference electrode 20, or otherwise integrated into the battery cell 12 without departing from the scope of the present description.

[0038] 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 cell 12. In another exemplary embodiment, the reference electrode management system 14 is located remotely from the battery cell 12 and is electrically connected to the battery cell 12. In another exemplary embodiment, the reference electrode management system 14 is configured to regulate the reference electrode SOC of a plurality of battery cells as part of a battery system (e.g., a multi-cell battery module / pack) and is electrically connected to the plurality of battery cells.

[0039] In Fig. Figure 3 shows a schematic diagram of the reference electrode management system 14. In an exemplary embodiment, the reference electrode management system 14 includes at least one control unit 40 and an interface circuit 42.

[0040] The control unit 40 is used to implement a method 100 for regulating the state of charge of a reference electrode in a battery cell, as described below. The control unit 40 includes at least one processor 44 and a non-transferable, computer-readable device or medium 46. The processor 44 may be a custom or off-the-shelf processor, a central processing unit (CPU), a graphics processing unit (GPU), an auxiliary processor among a plurality of processors connected to the control unit 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 generally an instruction-executing device.

[0041] The computer-readable device or medium 46 may include volatile and non-volatile memory, for example, read-only memory (ROM), random access memory (RAM), and keep-alive memory (KAM). KAM is persistent or non-volatile memory that can be used to store various operating variables while the processor 44 is powered off. The computer-readable storage device or media 46 may be implemented using a variety of storage devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or other electrical, magnetic, optical, or combination storage devices capable of storing data, some of which may be executable instructions used by the controller 40 to control the reference electrode management system 14.The control unit 40 may also consist of multiple control units that are electrically connected to one another. The control unit 40 may also contain additional elements and / or modules, such as a real-time clock (RTC) module for measuring the passage of time. In an exemplary embodiment, the control unit 40 is powered by connection to the positive terminal 22a and the negative terminal 22b of the battery cell 12.

[0042] The control unit 40 is in electrical communication with the interface circuit 42. In an exemplary embodiment, the electrical communication is established, for example, via GPIO (General Purpose Input / Output) pins, an I2C (Inter-Integrated Circuit) bus, an SPI (Serial Peripheral Interface) bus, a parallel communication bus, or the like. It is understood that various additional communication protocols for communicating with the control unit 40 are within the scope of the present description.

[0043] The interface circuit 42 is used to connect the control unit 40 to the positive terminal 22a, the negative terminal 22b, and the reference terminal 22c. In an exemplary embodiment, the interface circuit 42 includes a measuring circuit 48 and a charging circuit 50.

[0044] The measurement circuit 48 is used to measure the anode reference voltage, the cathode reference voltage, and a current flow into / out of the reference electrode 20. In one 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 to support the voltage measurement, for example, a voltage follower, an input buffer, a multiplexer, and / or the like. The measurement circuit 48 further includes components that enable the control unit 40 to measure the current flow into / out of the reference electrode 20, for example, a shunt resistor, an electromagnetic current sensor, an ADC, and / or the like.

[0045] 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 enable the control unit 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 one non-limiting example, the charging circuit 50 includes, for example, relays, transformers, transistors, and / or the like. It is understood that the measuring circuit 48 and / or the charging circuit 50 of the interface circuit 42 may further include additional passive or active analog and / or digital electronics, such as resistors, capacitors, inductors, filters, amplifiers, power electronics, digital-to-analog converters (DACs), and / or the like.In an exemplary embodiment, the interface circuit 42 is powered by connection to the positive terminal 22a and the negative terminal 22b of the battery cell 12. The interface circuit 42 is in electrical communication with the control unit 40, as described above.

[0046] In Fig. 4 shows a flowchart of the method 100 for regulating the state of charge of a reference electrode in a battery cell. The method 100 begins at block 102 and proceeds to block 104. In block 104, the controller 40 determines the reference electrode SOC of the reference electrode 20. The methods for determining the reference electrode SOC are explained in more detail below. After block 104, the method 100 proceeds to block 106.

[0047] In block 106, the control unit 40 compares the reference electrode SOC determined in block 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 block 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 transitions to a standby state in block 110. If the reference electrode SOC is greater than or equal to the predetermined high SOC threshold, the method 100 transitions to block 112, as explained in more detail below.

[0048] In block 108, the controller 40 uses the interface circuit 42 to charge the reference electrode 20 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. Methods for charging the reference electrode 20 are discussed in more detail below. After block 108, the method 100 transitions to the standby state in block 110.

[0049] In block 112, the controller 40 uses the interface circuit 42 to discharge the reference electrode 20 to adjust the reference electrode SOC in response to determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold. Methods for discharging the reference electrode 20 are discussed in more detail below. After block 112, the method 100 transitions to the standby state in block 110.

[0050] Fig. 5 shows a flowchart of a first exemplary embodiment 104a of block 104 of method 100 (i.e., a first method for determining the reference electrode SOC). The first exemplary embodiment 104a begins with blocks 502, 504, and 506. In block 502, the controller 40 uses the interface circuit 42 to charge the reference electrode 20. In an exemplary embodiment, to charge the reference electrode 20, the controller 40 uses the charging circuit 50 to connect the positive terminal 22a to the reference terminal 22c, thereby enabling current to flow from the cathode 16 to the reference electrode 20. In an exemplary embodiment, the charging circuit 50 enables current to flow for a predetermined duration. In one non-limiting example, the predetermined duration depends at least in part on a cross-sectional area and / or a volume of the reference electrode 20.In another exemplary embodiment, charging circuit 50 allows current to flow until the reference electrode SOC has increased by a predetermined amount (e.g., five percent). After block 502, the first exemplary embodiment 104a proceeds to block 508, as explained in more detail below.

[0051] In block 504, the control unit 40 uses the measurement circuit 48 to measure the current flow into the reference electrode 20 during the charging process in block 502. Based on the predetermined duration, the control unit 40 determines an amount of charge to be supplied to the reference electrode 20 during the charging process in block 502. After block 504, the first exemplary embodiment 104a proceeds to block 508, as explained in more detail below.

[0052] In block 506, the control unit 40 uses the measurement circuit 48 to measure the anode reference voltage during the charging process in block 502. After block 506, the first exemplary embodiment 104a proceeds to block 508.

[0053] In block 508, the controller 40 determines the reference electrode SOC based at least in part on the anode reference voltage during the charging process and the amount of charge supplied to the reference electrode 20 during the charging process. Methods for determining the reference electrode SOC based at least in part on the anode reference voltage during the charging process and the amount of charge supplied to the reference electrode 20 during the charging process are discussed in more detail below. After block 508, the first exemplary embodiment 104a is complete, and the method 100 continues as described above.

[0054] Fig. 6 shows a flowchart of a first exemplary embodiment 508a of block 508 of the first exemplary embodiment 104a of block 104 of the method 100 (i.e., a first method for determining the reference electrode SOC based on the anode reference voltage and the amount of charge added during charging). The first exemplary embodiment 508a begins in block 602. In block 602, the controller 40 calculates a first-order reference electrode voltage slope. For the purposes of the present description, the first-order reference electrode voltage slope is a first derivative of the anode reference voltage during charging of the reference electrode 20 (as determined in block 506) with respect to the amount of charge added to the reference electrode 20 (as determined in block 508).

[0055] In Fig. 7 illustrates a first exemplary graph 70 with an exemplary first-order reference voltage slope 72 determined over a range of reference electrode SOCs. An x-axis 74 of the first exemplary graph 70 represents the reference electrode SOC. A y-axis 76 of the first exemplary graph 70 represents a value of the first-order reference voltage slope. A first dashed line of the first exemplary graph 70 represents a predetermined lower threshold 78a for the first-order reference electrode voltage slope. A second dashed line of the first exemplary graph 70 represents a predetermined high threshold 78b for the first-order reference electrode voltage slope.In an exemplary embodiment, the predetermined low and high thresholds 78a, 78b for the first order reference electrode voltage slope are selected based at least in part on the predetermined low SOC threshold and / or the predetermined high SOC threshold.

[0056] It should be understood that the exemplary first-order reference voltage slope 72 is only exemplary, and that the shape and values ​​of the first-order reference electrode voltage slope may vary, for example, based on application-specific characteristics of the reference electrode 20 and / or the battery cell 12. It should further be understood that the value of the predetermined low and high thresholds 78a, 78b for the first-order reference electrode voltage slope illustrated in the first example graph 70 are merely exemplary. In some embodiments, the predetermined thresholds 78a, 78b for the low and high first-order reference electrode voltage slope may have the same value.Furthermore, the first exemplary diagram 70, the predetermined low and high thresholds for the first-order reference electrode voltage slope (78a, 78b) and the exemplary first-order reference voltage slope (72) are not necessarily to scale.

[0057] With further reference to Fig. 6 and with continued reference to Fig. 7, the first exemplary embodiment 508a proceeds to block 604 after block 602. In block 604, the control unit 40 compares the first-order reference electrode voltage slope determined in block 602 with the predetermined low and high first-order reference electrode voltage slope thresholds 78a, 78b. If the first-order reference electrode voltage slope determined in block 602 is greater than or equal to the predetermined low first-order reference electrode voltage slope threshold 78a and decreases as charge is applied to the reference electrode 20, the first exemplary embodiment 508a proceeds to block 606.If the first-order reference electrode voltage slope determined in block 602 is greater than or equal to the predetermined high first-order reference electrode voltage slope threshold 78b and increases as charge is applied to the reference electrode 20, the first exemplary embodiment 508a proceeds to block 608, as explained in more detail below.

[0058] In block 606, it is determined that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the first-order reference electrode voltage slope determined in block 602 is greater than or equal to the predetermined low first-order reference electrode voltage slope threshold 78a and decreases as charge is applied to the reference electrode 20. After block 606, the first exemplary embodiment 508a is completed, and the first exemplary embodiment 104a continues as described above.

[0059] In block 608, it is determined that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the first-order reference electrode voltage slope determined in block 602 is greater than or equal to the predetermined high first-order reference electrode voltage slope threshold 78b and increases as charge is applied to the reference electrode 20. After block 608, the first exemplary embodiment 508a is completed, and the first exemplary embodiment 104a continues as described above.

[0060] Fig. 8 shows a flowchart of a second exemplary embodiment 508b of block 508 of the first exemplary embodiment 104a of block 104 of the 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). It should be understood that the first exemplary embodiment 508a, the second exemplary embodiment 508b, or any combination thereof may be used to perform block 508 of the first exemplary embodiment 104a within the scope of the present description. The second exemplary embodiment 508b begins in block 802. In block 802, the controller 40 calculates a second-order reference electrode voltage slope.For the purposes of this description, the second order reference electrode voltage slope is a second derivative of the anode reference voltage during charging of the reference electrode 20 (as determined in block 506) with respect to the amount of charge supplied to the reference electrode 20 (as determined in block 508).

[0061] In Fig. 9 illustrates a second exemplary graph 80 with an exemplary second-order reference voltage slope 82 determined over a range of reference electrode SOCs. An x-axis 84 of the second exemplary graph 80 represents the reference electrode SOC. A y-axis 86 of the second exemplary graph 80 represents a value of the second-order reference voltage slope. A first dashed line of the second exemplary graph 80 represents a predetermined lower threshold 88a for the second-order reference electrode voltage slope. A second dashed line of the second exemplary graph 80 represents a predetermined high threshold 88b for the second-order reference electrode voltage slope.In an exemplary embodiment, the predetermined low and high second order reference electrode voltage slope thresholds 88a, 88b are selected based at least in part on the predetermined low SOC threshold and / or the predetermined high SOC threshold.

[0062] It should be understood that the exemplary second-order reference voltage slope 82 is only exemplary, and that the shape and values ​​of the second-order reference electrode voltage slope may vary, for example, based on application-specific characteristics of the reference electrode 20 and / or the battery cell 12. It should further be understood that the value of the predetermined low and high second-order reference electrode voltage slope thresholds 88a, 88b illustrated in the second exemplary diagram 80 are merely exemplary. In some embodiments, the predetermined thresholds 88a, 88b for the low and high second-order reference electrode voltage slope may be equal.Furthermore, the second exemplary diagram 80, the predetermined second-order reference electrode voltage slope with low and high threshold values ​​88a, 88b, and the exemplary second-order reference voltage slope 82 are not necessarily to scale.

[0063] With further reference to Fig. 8 and with continued reference to Fig. 9, the second exemplary embodiment 508b proceeds to block 804 after block 802. In block 804, the control unit 40 compares the second-order reference electrode voltage slope determined in block 802 with the predetermined low and high second-order reference electrode voltage slope thresholds 88a, 88b. If the second-order reference electrode voltage slope determined in block 802 is less than or equal to the predetermined low second-order reference electrode voltage slope threshold 88a, the second exemplary embodiment 508b proceeds to block 806. If the second-order reference electrode voltage slope determined in block 802 is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold 88b, the second exemplary embodiment 508b proceeds to block 808, as explained in more detail below.

[0064] In block 806, it is determined that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope determined in block 802 is less than or equal to the predetermined low second-order reference electrode voltage slope threshold 88a. After block 806, the second exemplary embodiment 508b is completed, and the first exemplary embodiment 104a continues as described above.

[0065] In block 808, it is determined that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the second-order reference electrode voltage slope determined in block 802 is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold 88b. After block 808, the second exemplary embodiment 508b is completed, and the first exemplary embodiment 104a continues as described above.

[0066] Fig. 10 shows a flowchart of a second exemplary embodiment 104b of block 104 of the method 100 (i.e., a second method for determining the reference electrode SOC). It should be understood that the first exemplary embodiment 104a, the second exemplary embodiment 104b, or any combination thereof may be used to perform block 104 of the method 100 within the scope of the present description. The second exemplary embodiment 104b begins in block 1002. In block 1002, the controller 40 tracks an elapsed time since the reference electrode 20 was previously charged (i.e., an elapsed time since a previous execution of block 108 of the method 100). In one 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.

[0067] In block 1004, the controller 40 calculates the SOC value of the reference electrode. In an exemplary embodiment, the controller 40 first calculates a lost amount of charge lost from the reference electrode 20 over the elapsed time determined in block 1002. In one non-limiting example, the controller 40 multiplies the elapsed time by a predetermined reference electrode discharge rate of the reference electrode. For the purposes of the present description, the predetermined reference electrode discharge rate quantifies an 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 through computer simulation and / or physical experiments.The predetermined reference electrode discharge rate is then stored in the media 46 of the control unit 40 for use during the second exemplary embodiment 104b.

[0068] The control unit 40 then calculates the reference electrode SOC based on the amount of charge lost. In an exemplary embodiment, the reference electrode SOC is proportional to a previous amount of charge on the reference electrode 20 (as determined, for example, based on the previous charging of the reference electrode 20 using the measurement circuit 48 of the interface circuit 42) less the amount of charge lost. After block 1004, the second exemplary embodiment 104b is complete, and the method 100 continues as described above.

[0069] In Fig. 11 illustrates a flowchart of an exemplary embodiment 108a of block 108 of method 100 (i.e., a method for charging reference electrode 20). The exemplary embodiment 108a begins at block 1102. At block 1102, controller 40 charges reference electrode 20. In an exemplary embodiment, to charge reference electrode 20, controller 40 uses charging circuitry 50 of interface circuitry 42 to connect positive terminal 22a (i.e., cathode 16) to reference terminal 22c (i.e., reference electrode 20), thereby allowing current to flow into reference electrode 20. In an exemplary embodiment, controller 40 concurrently uses measurement circuitry 48 to measure the magnitude of current flow into reference electrode 20 over time to calculate the charge delivered to reference electrode 20.The control unit 40 also uses the measurement circuit 48 to measure the anode reference voltage during charging of the reference electrode 20. After block 1102, the exemplary embodiment 108a proceeds to block 1104.

[0070] In block 1104, the control unit 40 evaluates a charge stop condition. In a first exemplary embodiment, the charge stop condition is met when the first-order reference electrode voltage slope (as determined using the method described with reference to the first exemplary embodiment 508a of block 508 of the first exemplary embodiment 104a of block 104 of the method 100) is greater than or equal to the predetermined high reference electrode voltage slope threshold 78b and the first-order reference electrode voltage slope is increasing. In other words, the charge stop condition is met when the reference electrode SOC reaches the predetermined high SOC threshold.

[0071] In a second exemplary embodiment, the charge stop condition is met when the second-order reference electrode voltage slope (as determined using the method described with reference to the second exemplary embodiment 508b of block 508 of the first exemplary embodiment 104a of block 104 of the method 100) is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold 88b. In other words, the charge stop condition is met when the reference electrode SOC reaches the predetermined high SOC threshold.

[0072] In a third exemplary embodiment, the charge stop condition is met when the amount of lost charge (as determined using the method described with reference to the second exemplary embodiment 104b of block 104 of the method 100) has returned to the reference electrode 20. If the charge stop condition is not met, the exemplary embodiment 108a returns to block 1102 to continue charging the reference electrode 20. If the charge stop condition is met, the exemplary embodiment 108a terminates, and the method 100 continues as described above.

[0073] In Fig. 12 illustrates a flowchart of an exemplary embodiment 112a of block 112 of method 100 (i.e., a method for discharging reference electrode 20). The exemplary embodiment 112a begins at block 1202. At block 1202, the controller 40 discharges the reference electrode 20. In an exemplary embodiment, to discharge the reference electrode 20, the controller 40 uses the charging circuit 50 of the interface circuit 42 to connect the negative terminal 22b (i.e., the anode 18) to the reference terminal 22c (i.e., the reference electrode 20), thereby allowing current to flow out of the reference electrode 20. In an exemplary embodiment, the controller 40 concurrently uses the measuring circuit 48 to measure a magnitude of the current flowing out of the reference electrode 20 over time to calculate the charge drained from the reference electrode 20.The control unit 40 also uses the measurement circuit 48 to measure the anode reference voltage while the reference electrode 20 is discharging. After block 1202, the exemplary embodiment 112a proceeds to block 1204.

[0074] In block 1204, the control unit 40 evaluates a discharge stop condition. In a first exemplary embodiment, the discharge stop condition is met when the first-order reference electrode voltage slope (as determined using the method described with reference to the first exemplary embodiment 508a of block 508 of the first exemplary embodiment 104a of block 104 of the method 100) is greater than or equal to the predetermined high first-order reference electrode voltage slope threshold 78b and the first-order reference electrode voltage slope is increasing. In other words, the discharge stop condition is met when the reference electrode SOC reaches the predetermined high SOC threshold.

[0075] In a second exemplary embodiment, the discharge stop condition is met when the second-order reference electrode voltage slope (as determined using the method described with reference to the second exemplary embodiment 508b of block 508 of the first exemplary embodiment 104a of block 104 of the method 100) is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold 88b. In other words, the discharge stop condition is met when the reference electrode SOC reaches the predetermined high SOC threshold. If the discharge stop condition is met, the exemplary embodiment 112a is completed, and the method 100 continues as described above.

[0076] With reference to Fig.13, an exemplary vehicle 90 is shown with the system 10, an electrical load 92, and a battery management system 94. The electrical load 92 and the battery management system 94 are in electrical communication with the system 10. In an exemplary embodiment, the electrical load 92 includes electrical and / or electromechanical components or systems of the vehicle 90 that require electrical power for operation. In one non-limiting example, the electrical load 92 includes the battery management system 94, a vehicle control unit, an infotainment system, one or more vehicle lights, a vehicle propulsion system including an electric motor, and / or the like. The battery cell 12 of the system 10 is configured to provide electrical power to the electrical load 92.

[0077] The battery management system 94 is used to monitor the battery cell 12, provide information about the condition of the battery cell 12 to external systems (e.g., the vehicle's control unit), and optimize the use of the battery cell 12 to extend the useful life of the battery cell 12 and protect the battery cell 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 cell 12.In an exemplary embodiment, the battery management system 94 performs measurements of the anode reference voltage and / or the cathode reference voltage (e.g., using the measurement circuit 48 of the interface circuit 42 of the reference electrode management system 14) and executes a mathematical model to estimate the SOC of the battery cell 12. The SOC of the battery cell 12 is used by other vehicle systems (e.g., the vehicle's control unit) to provide information about the vehicle's range and / or battery status to an occupant of the vehicle 90.

[0078] The system 10 and method 100 of the present description provide several advantages. Using the first exemplary embodiment 104a of block 104 of the method 100 (i.e., the first method for determining the reference electrode SOC), the reference electrode SOC may be determined independently of the SOC of the battery cell 12. Therefore, the reference electrode SOC may be monitored and frequently adjusted to prevent over-discharge of the reference electrode 20. Using the second exemplary embodiment 104b of block 104 of the method 100 (i.e., the second method for determining the reference electrode SOC), the reference electrode SOC may be determined independently of the SOC of the battery cell 12 and independently of an equilibrium state of the battery cell 12 (i.e., when the battery cell 12 is under any load).Therefore, the SOC value of the reference electrode can be frequently monitored and adjusted to prevent over-discharge of the reference electrode 20, even when the battery cell 12 is frequently loaded. In an exemplary embodiment, the first exemplary embodiment 104a and the second exemplary embodiment 104b are used in combination to strike a balance between accuracy and frequency of measuring the reference electrode SOC. By providing the reference electrode management system 14 attached to or otherwise integrated with the battery cell 12, the reference electrode management system 14 can regulate the reference electrode SOC even when the battery cell 12 is not installed in a host application (for example, when the battery cell 12 is being stored or transported), thereby extending the useful life of the reference electrode 20 and the battery cell 12.

Claims

[1] A method (100) for regulating a state of charge of a reference electrode (20) in a battery cell (12), the method (100) comprising: Determining a reference electrode state of charge, SOC, of ​​the reference electrode (20), wherein the battery cell (12) comprises an anode (18), a cathode (16) and the reference electrode (20) and wherein the reference electrode (20) is arranged between the anode (18) and the cathode (16); Comparing the reference electrode SOC with at least one predetermined low SOC threshold; and Adjusting 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. [2] The method (100) of claim 1, wherein determining the reference electrode SOC further comprises: Charging the reference electrode (20) and measuring an amount of charge supplied to the reference electrode (20); Measuring an anode reference voltage during charging of the reference electrode (20); and Determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode (20). [3] The method (100) of claim 2, wherein determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode (20) further comprises: Calculating a first-order reference electrode voltage slope, wherein the first-order reference electrode voltage slope is a first derivative of the anode reference voltage during charging of the reference electrode (20) with respect to the amount of charge supplied to the reference electrode (20); and Determining the reference electrode SOC based at least in part on the first-order reference electrode voltage slope. [4] The method (100) of claim 3, wherein determining the reference electrode SOC based at least in part on the first-order reference electrode voltage slope further comprises: Determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the first-order reference electrode voltage slope is greater than or equal to a predetermined low first-order reference electrode voltage slope threshold and that the first-order reference electrode voltage slope is decreasing; and Determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the first order reference electrode voltage slope is greater than or equal to the predetermined high first order reference electrode voltage slope threshold and that the first order reference electrode voltage slope is increasing. [5] The method (100) of claim 4, wherein adjusting the reference electrode SOC further comprises: Charging the reference electrode (20) in response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, wherein the reference electrode (20) is charged until the first order reference electrode voltage slope is greater than or equal to the predetermined high first order reference electrode voltage slope threshold and the first order reference electrode voltage slope increases. [6] The method (100) of claim 2, wherein determining the reference electrode SOC based at least in part on the anode reference voltage and the amount of charge supplied to the reference electrode (20) further comprises: Calculating a second-order reference electrode voltage slope, the second-order reference electrode voltage slope being a second derivative of the anode reference voltage, during charging of the reference electrode with respect to the amount of charge supplied to the reference electrode (20); and Determining the reference electrode SOC based at least in part on the second order reference electrode voltage slope. [7] The method (100) of claim 6, wherein determining the reference electrode SOC based at least in part on the second-order reference electrode voltage slope further comprises: Determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold in response to determining that the second-order reference electrode voltage slope is less than or equal to a predetermined low second-order reference electrode voltage slope threshold; and Determining that the reference electrode SOC is greater than or equal to the predetermined high SOC threshold in response to determining that the second order reference electrode voltage slope is greater than or equal to a predetermined high second order reference electrode voltage slope threshold. [8] The method (100) of claim 7, wherein adjusting the reference electrode SOC further comprises: Charging the reference electrode (20) in response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, wherein the reference electrode (20) is charged until the second-order reference electrode voltage slope is greater than or equal to the predetermined high second-order reference electrode voltage slope threshold. [9] The method (100) of claim 1, wherein determining the reference electrode SOC further comprises: Tracking the time elapsed since a previous charging 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 (20); and Calculating the reference electrode SOC based at least in part on the amount of charge lost. [10] The method (100) of claim 9, wherein adjusting the reference electrode SOC further comprises: Charging the reference electrode (20) in response to determining that the reference electrode SOC is less than or equal to the predetermined low SOC threshold, wherein the reference electrode (20) is charged until the lost amount of charge is returned to the reference electrode (20).

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