TECHNICAL FIELD

[0001] The present disclosure is directed at methods, systems, and techniques for estimating instantaneous state-of-charge of a lithium ion battery.

BACKGROUND

[0002] Lithium ion batteries enjoy several advantages over batteries that use more established battery chemistries, such as lead acid and nickel metal hydride batteries. For example, lithium ion batteries have relatively high energy and power densities, which permit a lithium ion battery of a certain capacity to be smaller than its lead acid or nickel metal hydride counterpart. However, lithium ion batteries also suffer from some disadvantages when compared to those more established battery chemistries. For example, lithium ion batteries should not be overcharged or undercharged as improper charging can result in sub-optimal power output, shorten battery lifespan, and damage the batteries' cells. Research and development is ongoing into methods, systems, and techniques for ameliorating the disadvantages associated with lithium ion batteries.

SUMMARY

[0003] According to a first aspect, there is provided a method for estimating instantaneous state-of-charge (SOC) of a lithium ion battery. The method comprises determining, at a first time, a first voltage measured across terminals of the lithium ion battery and a first current flowing through the lithium ion battery by electrically coupling the battery to a first load; determining, at a second time after the first time, a second voltage measured across the terminals of the lithium ion battery and a second current flowing through the lithium ion battery by electrically coupling the battery to a second load having a different impedance than the first load; estimating, from the first and second voltages and currents, an instantaneous internal resistance of the battery; estimating an open circuit voltage (OCV) of the battery using the instantaneous internal resistance and a state of charge voltage (SOC voltage) across the terminals of the lithium ion battery and state of charge current (SOC current) flowing through the lithium ion battery; and estimating the SOC of the battery by referencing the OCV against a mapping of open circuit voltages versus states-of-charge calibrated specifically for the battery. The mapping is accurate for the entire lifetime of the battery without recalibration and the lithium ion battery has a chemistry selected from the group consisting of lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), and lithium titanate (LTO). [0004] The second voltage may be used as the SOC voltage and the second current may be used as the SOC current. Alternatively, at a third time after the second time and prior to estimating the OCV, the SOC voltage and the SOC current may be determined by electrically coupling the battery to a third load having a different impedance than the second load. [0005] Change in charge of the battery between the second and third times may be less than or equal to 10% of total capacity of the battery, or more particularly 5% of the total capacity of the battery, or more particularly 1% of the total capacity of the battery.

[0006] The first, second, and SOC voltages and currents may be determined while the lithium ion battery is discharging or charging.

[0007] Change in charge of the battery between the first and second times may be less than or equal to 10% of total capacity of the battery, or more particularly less than or equal to 5% of total capacity of the battery, or more particularly less than or equal to 1% of total capacity of the battery. [0008] According to another aspect, there is provided a system for estimating instantaneous state-of-charge (SOC) of a lithium ion battery having a chemistry selected from the group consisting of lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), and lithium titanate (LTO). The system comprises a voltmeter, an ammeter, a first relay, and a first load electrically coupled in series, wherein the battery is to be electrically coupled across the voltmeter; a circuit branch comprising a second relay and a second load electrically coupled in series, wherein the circuit branch is electrically coupled in parallel across the first relay and the first load and wherein the second load has a different impedance than the first load; a controller communicatively coupled to the voltmeter, the ammeter, the first relay, and the second relay, the controller configured to perform a method comprising at a first time, closing the first relay and opening the second relay and measuring as a first voltage a voltage measured by the voltmeter and as a first current a current measured by the ammeter; at a second time after the first time, opening the first relay and closing the second relay and measuring as a second voltage the voltage measured by the voltmeter and as a second current the current measured by the ammeter; estimating, from the first and second voltages and currents, an instantaneous internal resistance of the battery; estimating an open circuit voltage (OCV) of the battery using the instantaneous internal resistance and a state of charge voltage (SOC voltage) across the terminals of the lithium ion battery and state of charge current (SOC current) flowing through the lithium ion battery; and estimating the SOC of the battery by referencing the OCV against a mapping of open circuit voltages versus states-of-charge calibrated specifically for the battery, wherein the mapping is accurate for the entire lifetime of the battery without recalibration.

[0009] The second voltage may be used as the SOC voltage and the second current may be used as the SOC current. Alternatively, prior to estimating the OCV, at a third time after the second time the SOC voltage and the SOC current may be determined by opening the first and second relays and measuring the SOC voltage and SOC current while the battery is electrically coupled to a primary load. [0010] Change in charge of the battery between the second and third times may be less than or equal to 10% of total capacity of the battery, or more particularly 5% of total capacity of the battery, or more particularly 1% of total capacity of the battery.

[0011] The first, second, and SOC voltages and currents may be determined while the lithium ion battery is charging or discharging.

[0012] Change in charge of the battery between the first and second times may be less than or equal to 10% of total capacity of the battery, or more particularly 5% of total capacity of the battery, or more particularly 1% of total capacity of the battery.

[0013] According to another aspect, there is provided a non-transitory computer readable medium having encoded thereon program code to cause a controller to perform a method for estimating instantaneous state-of-charge (SOC) of a lithium ion battery. The method comprises determining, at a first time, a first voltage measured across terminals of the lithium ion battery and a first current flowing through the lithium ion battery by electrically coupling the battery to a first load; determining, at a second time after the first time, a second voltage measured across the terminals of the lithium ion battery and a second current flowing through the lithium ion battery by electrically coupling the battery to a second load having a different impedance than the first load; estimating, from the first and second voltages and currents, an instantaneous internal resistance of the battery; estimating an open circuit voltage (OCV) of the battery using the instantaneous internal resistance and a state of charge voltage (SOC voltage) across the terminals of the lithium ion battery and state of charge current (SOC current) flowing through the lithium ion battery; and estimating the SOC of the battery by referencing the OCV against a mapping of open circuit voltages versus states-of-charge calibrated specifically for the battery. The mapping is accurate for the entire lifetime of the battery without recalibration and the lithium ion battery has a chemistry selected from the group consisting of lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), and lithium titanate (LTO). [0014] The second voltage may be used as the SOC voltage and the second current may be used as the SOC current. Alternatively, at a third time after the second time and prior to estimating the OCV, the SOC voltage and the SOC current may be determined by electrically coupling the battery to a third load having a different impedance than the second load.

[0015] Change in charge of the battery between the second and third times may be less than or equal to 10% of total capacity of the battery, or more particularly 5% of the total capacity of the battery, or more particularly 1% of the total capacity of the battery. [0016] The first, second, and SOC voltages and currents may be determined while the lithium ion battery is discharging or charging.

[0017] Change in charge of the battery between the first and second times may be less than or equal to 10% of total capacity of the battery, or more particularly less than or equal to 5% of total capacity of the battery, or more particularly less than or equal to 1% of total capacity of the battery.

[0018] This summary does not necessarily describe the entire scope of all aspects.

Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS [0019] In the accompanying drawings, which illustrate one or more example embodiments:

[0020] FIG. 1 is a circuit that can be used to perform one embodiment of a method for estimating instantaneous state-of-charge of a lithium ion battery;

[0021] FIG. 2 is a system for estimating instantaneous state-of-charge of a lithium ion battery, according to another embodiment; [0022] FIG. 3 shows graphs of voltage and current during discharge of an example lithium ion battery; and

[0023] FIG. 4 shows a method for estimating instantaneous state-of-charge of a lithium ion battery, according to another embodiment. DETAILED DESCRIPTION

[0024] Directional terms such as "top", "bottom", "upwards", "downwards",

"vertically", and "laterally" are used in the following description for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any article is to be positioned during use, or to be mounted in an assembly or relative to an environment. Additionally, the term "couple" and variants of it such as "coupled", "couples", and "coupling" as used in this description are intended to include indirect and direct connections unless otherwise indicated. For example, if a first device is coupled to a second device, that coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, if the first device is communicatively coupled to the second device, communication may be through a direct connection or through an indirect connection via other devices and connections.

[0025] A lithium ion battery comprises one or more lithium ion cells; when the battery comprises multiple cells, they are electrically coupled together in one or both of parallel and series. Additionally, a lithium ion battery may comprise any one of a variety of different battery chemistries; example chemistries are lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), lithium titanate (LTO), and lithium cobalt oxide (LCO).

[0026] One problem encountered when using a lithium ion battery is determining the state of charge (SOC) of the battery at any given time, where SOC is expressed as a percentage of total charge. Typically, determining the SOC of a battery comprises obtaining the open circuit voltage (OCV) of the battery; however, obtaining the OCV of the battery is impeded by the battery's internal resistance. An unknown or poorly estimated internal resistance hinders the accuracy of the OCV value, and consequently the accuracy of the SOC.

[0027] The embodiments described herein are directed at methods, systems, and techniques for estimating instantaneous state-of-charge of a lithium ion battery. The internal resistance of the battery is first estimated by measuring first and second voltages and currents corresponding to different first and second loads, respectively, to the battery at two different times. From these voltage and current measurements, the battery's OCV can be estimated and compared against a mapping of open circuit voltages (OCVs) and states-of-charge (SOCs) calibrated specifically for the battery. The lithium ion battery has a chemistry selected from the group consisting of LMO, LFP, NMC, NCA, and LTO; this permits the mapping to be accurate for the entire lifetime of the battery without requiring recalibration. As used herein, the "lifetime" of a battery refers to the number of charge- discharge cycles that the battery can undergo until its capacity drops to 80% of its originally rated capacity. Typical battery lifetimes vary from about 500 cycles to about 10,000 cycles; in one example embodiment involving a battery comprising one or more SLPB 100216216H NMC cells from Kokam™ Co., Ltd., battery lifetime is about 2,000 cycles.

[0028] Referring now to FIG. 1, there is shown a circuit 100 that can be used to perform one embodiment of a method for estimating instantaneous SOC of a lithium ion battery 101. In FIG. 1, the battery 101 comprises a single lithium ion cell, modeled as voltage source 102 in series with a variable internal resistance 104 electrically coupled in series between a pair of battery terminals 110. The internal resistance 104 may vary, for example, with charge or discharge rate, temperature, coulombic efficiency, cycle life, and calendar life of the battery. Electrically coupled in series with the battery 101 are a switch 106 and a first load 108a. When the switch is closed, current flows in series through the battery 101, the switch 106, and the first load 108a. When the switch is open, no current flows through the battery 101. [0029] The cell that comprises the battery 101 in the depicted example embodiment is an SLPB 100216216H NMC cell from Kokam™ Co., Ltd. Before the cell is shipped to a customer, the cell's manufacturer generates through testing a mapping of OCVs versus SOCs calibrated specifically for that cell. An example mapping follows as Table 1 :

Table 1: OCV vs. SOC for SLPB 100216216H NMC Cell

[0030] In Table 1, the various charge and discharge rates refer to the charge and discharge rates the battery 101 experiences immediately prior to the 20 minute rest time that precedes the OCV measurement. It has been experimentally verified that the mapping shown in Table 1 is accurate for the entire lifetime of the battery 101 without requiring recalibration when the cell chemistry is LMO, LFP, NMC, NCA, or LTO. Lead acid batteries, nickel metal hydride batteries, and LCO batteries do not share this characteristic and consequently require periodic recalibration during their lifetimes in order to consistently be able to rely on OCV readings when determining SOC.

[0031] Referring now to FIG. 4, there is shown an example method 400 for estimating instantaneous state-of-charge of the lithium ion battery 101, according to another embodiment. The following describes FIG. 4 in conjunction with the circuit 100 of FIG. 1. [0032] The method 400 begins at block 402 and proceeds to block 404. At block

404, a user determines, at a first time, a first voltage measured across the battery's 101 terminals 110 and a first current flowing through the battery 101. The user does this by closing the switch 106 and using a voltmeter 208 (not shown in FIG. 1, but shown in FIG. 2) to measure the voltage drop across the first load 108a; this voltage drop is the first voltage. Dividing the first voltage by the first load's 108a impedance provides the first current. The user then proceeds to block 406 and determines, at a second time after the first time (and, in one example embodiment, immediately after), a second voltage measured across the battery's 101 terminals 110 and a second current flowing through the battery 101. This is done by opening the switch 106 and replacing the first load 108a with a second load 108b (not shown in FIG. 1, but shown in FIG. 2) having a different impedance than the first load 108b. The user then closes the switch 106 and measures the voltage drop across the second load 108b; this voltage drop is the second voltage. Dividing the second voltage by the second load's 108b impedance provides the second current.

[0033] From the first and second voltages and currents, the user proceeds to block

408 and determines an instantaneous value of the internal resistance 104 (hereinafter interchangeably referred to as the "instantaneous internal resistance" 104) using Equation 1 :

¼ - V-,

^instantaneous _t _ _t ^J

'2 '1 where Rinstantaneous is the instantaneous internal resistance 104, Vi is the first voltage, V ₂ is the second voltage, Ii is the first current, and I ₂ is the second current.

[0034] The shorter the time between the first and second measurements, the more accurate the value of the instantaneous internal resistance 104 determined using Equation 1. The accuracy of the instantaneous internal resistance 104 can be estimated as a function of the percentage change in capacity of the battery 101 between measurements. For example, if the first voltage and current measurements are taken when the battery 101 is fully charged and the battery 101 is then discharged at a rate of 2C, the battery 101 will discharge completely in half an hour. Waiting 18 seconds between the first and second measurements while the battery 101 is discharging represents a 1% change in charge relative to total capacity of the battery between measurements and consequently a 1% error in the accuracy of the instantaneous internal resistance 104; that is, accuracy of the instantaneous internal resistance 104 varies proportional with change in charge as a proportion of total capacity of the battery 101. Accuracy may vary with different embodiments; for example, in one embodiment change in charge relative to total capacity of the battery 101 between the first and second times is less than or equal to 10%; in another embodiment, less than or equal to 5%; and in another embodiment, less than or equal to 1%. The instantaneous internal resistance 104 is periodically redetermined; in various example embodiments, it may be redetermined at least daily, at least hourly, or at least every minute, depending on battery usage, type, and on desired accuracy. [0035] Once the user determines the instantaneous internal resistance 104, the user proceeds to block 410 and uses the instantaneous internal resistance 104 to estimate the battery's 101 OCV at any current level. The user can do this by opening the switch 106 and replacing the second load 108b with a third load (not depicted). The user then closes the switch 106 and measures the voltage drop across the third load at a third time after the second time; this voltage drop is a third voltage. Dividing the third voltage by the third load's impedance provides a third current. The user may determine the OCV using the third voltage and third current by applying Equation 2:

OCV = V ₃ + (7 ₃ X Rinstantaneous) (2) where V ₃ is the third voltage and I ₃ is the third current.

[0036] The more quickly the instantaneous internal resistance 104 is used in Equation 2 after it is estimated by Equation 1, the more accurate the OCV estimate is. This is because as time passes, the factors that influence the value of the instantaneous resistance 104 change, resulting in changes in the internal resistance 104 not captured at the time Equation 1 was applied. For relatively high accuracy, the user accordingly applies Equation 2 immediately after determining the instantaneous resistance 106. Accuracy in the OCV estimate decreases roughly in accordance with the accuracy of the instantaneous internal resistance 104, as described above in respect of Equation 1. Accuracy may vary with different embodiments; for example, in one embodiment change in charge relative to total capacity of the battery 101 between the second and third times is less than or equal to 10%; in another embodiment, less than or equal to 5%; and in another embodiment, less than or equal to 1%. [0037] After estimating the OCV at block 410, the user proceeds to block 412 and estimates the SOC of the battery 101 by referencing the OCV against a mapping of OCVs versus SOCs calibrated specifically for the battery 101, such as the mapping of Table 1. While Table 1 lists different OCVs for different discharge rates, in alternative embodiments (not depicted) the mapping used to determine SOC may specify OCVs at only one discharge rate. As mentioned above, the mapping of Table 1 is accurate for the entire lifetime of the battery 101 without requiring recalibration when the cell chemistry is LMO, LFP, NMC, NCA, or LTO. After estimating the SOC, the user proceeds to block 414 and the method 400 ends.

[0038] Referring now to FIG. 3, there are shown example graphs of voltage across the battery 101 versus time ("voltage graph 302") and current through the battery 101 versus time ("current graph 304") during the battery's 101 discharge. The battery 101 is discharged at a rate of less than 3.0 C. During ti (1 s to 20 s) the switch 106 is open and the voltage measured across the battery's 101 terminals 110 is, by definition, the OCV of the battery 101. During t _ls the battery's 101 OCV is accordingly 3.722 V, which according to Table 1 corresponds to the battery 101 having an SOC of 50%. The voltages and currents measured during t ₂ (21 s to 40 s) and t ₃ (41 s to 60 s) are 3.712 V @ 10 A and 3.622 V @ 100 A, respectively. Applying Equation 1 with 3.712 V as V _ls 3.622 V as V ₂ , 10 A as Ii, and 100 A as I ₂ results in a finding that the instantaneous internal resistance 104 is 0.001 Ω. During t ₄ , the voltage and current are 3.672 V @ 50 A. Solving Equation 2 using this value of the instantaneous internal resistance, 3.672 V as V ₃ , and 50 A as I ₃ results in an OCV of 3.722 V. The mapping of Table 1 shows that this corresponds to a 50% SOC. The voltage and current during t ₅ (81 s to 100 s) when the switch 106 is open is also 3.722 V; as the voltage when the switch 106 is open is by definition the battery's 101 OCV, the 3.722 V voltage measured during t ₅ confirms the accuracy of the OCV determined using Equation 2 during time t ₄ .

[0039] Referring now to FIG. 2, there is shown a system 200 for estimating instantaneous SOC of the battery 101, according to another embodiment. The battery 101 in FIG. 2 comprises multiple NMC lithium ion cells electrically coupled together, with some cells electrically coupled together in series and those groups of series-coupled cells electrically coupled together in parallel. While the mapping of Table 1 applies to a single lithium ion cell, before shipping the battery 101 to the customer the battery 101 manufacturer or distributor may generate an analogous mapping for the battery 101, with that mapping also being reliable for the battery's 101 entire lifetime.

[0040] In FIG. 2, the battery 101 may be electrically coupled to its primary load

(not shown), such as an engine. The system 200 may be electrically coupled in parallel to the battery 101 to permit the battery's 101 SOC to be determined while the battery 101 is being used to power its primary load. This permits a customer to determine the battery's 101 SOC in real-time while using the battery 101.

[0041] The system 200 comprises a controller 212 configured to perform the method 400 of FIG. 4. For example, in one embodiment the controller 212 may comprise a processor communicatively coupled with a non-transitory computer readable medium such as a non-volatile random access memory, with the memory having encoded on it as program code for execution by the processor the method 400 of FIG. 4. The system 200 also comprises, and the controller 212 is communicatively coupled to each of, first and second relays 204a,b, an ammeter 206, and a voltmeter 208. [0042] The first relay 204a, the ammeter 206, and the first load 108a are electrically coupled to the battery 101 in series. The second relay 204b and the second load 108b are electrically coupled in series, and collectively comprise a circuit branch that is electrically coupled in parallel across the first relay 204a and the first load 108a. The voltmeter 208 is electrically coupled across the battery's 101 terminals 110. By closing the first relay 204a and opening the second relay 204b the controller 212 is able to electrically couple the battery 101 to the first load 108a but not the second load 108b, and by closing the second relay 204b and opening the first relay 204a the controller 212 is able to electrically couple the battery 101 to the second load 108b but not the first load 108a. This hardware arrangement permits the controller 212 to perform the method 400 of FIG. 4, as described below.

[0043] The controller 212 begins at block 402 and proceeds to block 404 where it determines, at the first time, a first voltage measured across the terminals 110 of the battery 101 and a first current flowing through the battery 101. The controller 212 does this by opening the second relay 204b, closing the first relay 204a, and using the voltage reading that the voltmeter 208 outputs as the first voltage and the current reading that the ammeter 206 outputs as the first current.

[0044] The controller 212 then proceeds to block 406 and, at the second time, simultaneously opens the first relay 204a and closes the second relay 204b. The controller 212 uses the voltage reading that the voltmeter 208 outputs as the second voltage and the current reading that the ammeter 206 outputs as the second current. As the first and second loads 108a,b have different impedances, the first and second voltages and the first and second currents are different from each other.

[0045] Once the controller 212 has obtained the first and second voltages and currents, the controller 212 disconnects the battery 101 from the first and second loads 104a,b by opening the first and second relays 204a,b. This conserves energy that could be used by the battery's 101 primary load. [0046] The controller 212 then proceeds to block 408 and estimates the instantaneous resistance 104 of the battery 101 from the first and second voltages and currents by applying Equation 1. After estimating the instantaneous resistance 104, the controller 212 proceeds to block 410 and estimates an OCV of the battery 101 by applying Equation 2. When applying Equation 2 the controller 212 uses the instantaneous resistance 104 estimated at block 408, the voltage that the voltmeter 208 outputs as the SOC voltage (V ₃ ), and the current flowing through the battery as the SOC current (I ₃ ), which can be measured by an ammeter (not shown) electrically coupled in series between the battery 101 and its primary load (assuming all of the current flowing through the battery 101 passes through the primary load); the SOC voltage and SOC current are measured at the third time. The controller 212 then proceeds to block 412 where it estimates the battery's 101 SOC by referencing the OCV estimated at block 410 against a mapping of OCVs vs. SOCs calibrated specifically for the battery 101, which is analogous to the mapping of Table 1. After estimating the SOC, the controller 212 proceeds to block 414 where the method 400 ends. The controller 212 may periodically repeat the method 400 so as to refresh the SOC reading available to the customer.

[0047] While in the foregoing embodiments the OCVs following the battery's

101 having been discharged at the different discharge rates in Table 1 are used to determine SOC, Table 1 also lists OCVs measured after the battery 101 has been charged at different charge rates. Consequently, in an alternative embodiment (not depicted), the SOC of the battery 101 may be determined while it is being charged. While OCVs measured after charging at multiple charge rates are shown in Table 1, in alternative embodiments (not depicted) the mapping used to determine SOC may specify OCVs associated with only one charge rate. [0048] Additionally, in the embodiments above the SOC voltage and SOC current are measured when the battery 101 is used to power a load other than the first and second loads 108a,b (in FIG. 2 that load is the "third load", while in FIG. 4 that load is the "primary load"). In these embodiments, the SOC voltage and SOC current are measured at the third time, which is after the first and second times at which voltages and currents are measured to estimate the instantaneous internal resistance 104. However, in an alternative embodiment (not depicted), the load used while measuring the SOC voltage and SOC current and the second load 108b may be identical and the SOC voltage and SOC current may be identical to the second voltage and current used to estimate the instantaneous internal resistance 104.

[0049] The controller 212 used in the foregoing embodiments may be, for example, a processing unit (such as a processor, microprocessor, or programmable logic controller) communicatively coupled to a non-transitory computer readable medium having stored on it program code for execution by the processing unit, microcontroller (which comprises both a processing unit and a non-transitory computer readable medium), field programmable gate array (FPGA), or an application-specific integrated circuit (ASIC). Examples of computer readable media are non- transitory and include disc-based media such as CD-ROMs and DVDs, magnetic media such as hard drives and other forms of magnetic disk storage, semiconductor based media such as flash media, random access memory (including DRAM and SRAM), and read only memory.

[0050] It is contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification. [0051] For the sake of convenience, the example embodiments above are described as various interconnected functional blocks. This is not necessary, however, and there may be cases where these functional blocks are equivalently aggregated into a single logic device, program or operation with unclear boundaries. In any event, the functional blocks can be implemented by themselves, or in combination with other pieces of hardware or software.

[0052] While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modifications of and adjustments to the foregoing embodiments, not shown, are possible.