Battery sensor calibration

Abstract

A battery sensor calibration system and method for calibrating a battery management system (BMS) for measuring a state of charge (SoC) of a battery of a vehicle or equipment comprising a lead-acid battery. Initially, the battery is fully charged and a maximum voltage reading is measured at full charge. This value is used to estimate a maximum voltage, OCV max , of the battery at SoC max .. The battery is partially discharged to a first intermediate SoC of the battery SoC x1 , where SoC max > Soc x1 > SoC min , wherein Soc min is an SoC at full discharge of the battery. A first intermediate voltage, OCV x1 of the battery at SoC X1 is measured and a voltage at full discharge, OCV min , of the battery at SoC min is estimated based on the measured values of OCV max at SoC max and OCV x1 at SoC x1 .

Description

[0001] Description

[0002] BATTERY SENSOR CALIBRATION

[0003] Technical Field

[0004] The present disclosure relates to calibrating one or more battery sensors used for calculating a state of charge of a battery, the battery sensor(s) being part of a Battery Management System (BMS). The present disclosure is suited for electric vehicles, vehicles with lead acid batteries and other equipment such as generators (e.g. Commercial and Industrial Generator Sets).

[0005]

[0006] Vehicles comprise batteries that are required to be charged for operation of the vehicle. The battery also provides a power source for any other electrical equipment, also called ancillary electrical devices, that the vehicle has such as water heaters, air conditioning, lights, radio, etc.

[0007] There are a number of ways to estimate a State of Charge (SoC) of a battery from battery parameters obtained via signals from battery sensors. The most common method used to calculate SoC requires measurement of parameters including voltage at full charge (Umax), voltage at full discharge (Umin), and battery capacity (C20). The relationship between these measurable parameters is described for example in Coleman, Martin, Chi Kwan Lee, Chunbo Zhu, and William Gerard Hurley. " State-of-charge determination from EMF voltage estimation: Using impedance, terminal voltage, and current for lead-acid and lithium-ion batteries." IEEE Transactions on industrial electronics 54, no. 5 (2007): 2550-2557. Whilst capacity measurements are typically readily available, voltage information is often not available to an end user or if provided by a manufacturer may not be accurate for a specific battery.

[0008] Due to significant battery-to-battery variability and variation, for example due to age of the battery, measuring SoC based on the above parameters can be inaccurately calculated. Accordingly, there remains a need for accurately measuring parameters including voltage at full charge Umax) and voltage at full discharge (Umin) of the battery to determine the SoC of a battery accurately.

[0009]

[0010] According to a first aspect, there is provided a method of calibrating a BMS for measuring an SoC of a battery of a vehicle or equipment comprising a rechargeable lead-acid battery. The method comprises fully charging the battery, such that the SoC is SoCmax. A voltage at full charge, OCVmax, of the battery at SoCmaxis measured. The method further comprises partially discharging the battery to a first intermediate SoC of the battery SoCxl, where SoCmax> SoCx1> SoCmin, wherein SoCminis at full discharge of the battery. A first intermediate voltage, OCVX1of the battery at SoCxl, is measured. An estimate of a voltage at full discharge, OCVmin, of the battery at SoCminis extrapolated from OCVmaxat SoCmaxand OCVX1at SoCxl.

[0011] The calibrating method described herein is integral to the vehicle, carried out for example by the vehicle alternator or charging system of the vehicle.

[0012] According to a second aspect, there is provided a vehicle or equipment comprising a battery, a battery management system comprising one or more sensors and a controller configured to perform the method according to the first aspect. The vehicle may be an electric vehicle or other vehicle comprising a lead-acid battery. The equipment may a generator or genset comprising a rechargeable lead acid battery.

[0013] These and other aspects and features of the present disclosure will be more readily understood after reading the following description in conjunction with the accompanying drawings.

[0014] Brief

[0015]

[0016] of the

[0017]

[0018] A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures.

[0019] Figure 1 illustrates a flow diagram of a method of calibrating a battery according to an embodiment of the present disclosure;

[0020] Figure 2 illustrates a graphical representation of the linear relationship between SoC and OCV of a battery according to an embodiment of the present disclosure;Figure 3 illustrates a flow diagram of a method for calibrating a battery sensor for measuring SoC of a battery by calculating Umaxand Uminfor example in response to a user trigger;

[0021] Figure 4 illustrates a graphical representation of an implementation of the present disclosure according to a first discharge method having fixed current discharge;

[0022] Figure 5 illustrates an example of an electric vehicle in the form of a large mining truck, or haul truck.

[0023] Detailed Description

[0024] The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. Any implementation described herein as an example is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description.

[0025] The present disclosure addresses a problem of calibrating battery sensors for electric vehicles when they are in the field (i.e. not in a laboratory or other controlled setting). For example for batteries in electric vehicles although the scope is not so limited and can be applied to any lead-acid battery in a vehicle such as a starter battery or the like or other equipment comprising rechargeable lead acid batteries such as generators and gensets. A genset is a portable power supply source consisting of an engine and a generator. There is significant battery-to-battery variability and variation due to age that affects battery parameters. The solution provided herein provides a method of calibration for any battery, specific to the battery that is installed in a given vehicle or equipment and helps to provide an accurate reading that can be measured in the field, and not under strict testing conditions, using a machine integrated system and data measured by the machine integrated system,. The machine alternator, or other power source like the lithium-ion pack in a Battery Electric vehicle (BEV), and controlled loads to a sensor which is part of the Battery Management System (BMS) are used to calibrate Umaxand Uminin the field using on-machine data in a user-initiated calibration mode. The calibrating method described herein is integral to the vehicle or equipment, carriedout for example by the alternator or charging system, rather than by a portable tester or other device which is not an integral part of the vehicle or equipment.

[0026] Advantageously, the present disclosure provides a solution for calibration when a battery is changed, for example, or for use of non-standard or third-party batteries where data may not be readily available.

[0027] Battery state of charge (SoC) and Open Circuit Voltage (OCV) measurements are used to measure the voltage at full charge of the battery, Umax, and at an intermediate SOC of the battery. The voltage at full discharge of the battery, Umin, can be estimated using a linear equation based on the Nernst equation instead of draining the battery to make a measurement. In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction (half-cell or full cell reaction) from the standard electrode potential, absolute temperature, the number of electrons involved in the redox reaction, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation respectively.

[0028] Electric work vehicles, such as electric excavators, electric mining trucks, electric articulated trucks, electric wheel-loaders and the like are often used at worksites where the electric vehicles may be used by different operators and / or be supplied with different battery packs. For example, the worksite operator may rent batteries or have subscription agreement for a third-party to supply batteries for the work machines at the worksite. This variation in batteries being used in a machine means that an effective battery management system that can be recalibrated in the field on the electric work machine is highly desirable.

[0029] Figure 1 illustrates a flow diagram of a method of calibrating a battery that is part of the machine system and installed on an electric vehicle, other vehicle or equipment. In particular, the method illustrated is for calibrating one or more battery sensors of a BMS for measuring a State of Charge (SoC) of a battery. SoC describes the battery's current charge level as a percentage of its capacity. It is important for determining how much energy is left in the battery, for example to calculate when the battery will need to be recharged. Capacity is a measure of the entire energy potential of a battery and it is usually expressed in ampere-hours (Ah). It provides information on how much charge the battery can deliver at a particular discharge rate.The method may be performed by a controller, for example by an electronic control unit (ECU) or the BMS which controls and monitors the battery. One or more battery sensors are coupled to the battery, as part of the BMS, and are configured to take measurements therefrom. In one example, the battery sensor is a shunt-resistor-based battery sensor, although other alternatives are available that are suitable for making measurements according to the present disclosure. Sensors include those suitable for measuring charge, voltage and optionally temperature of the battery. In one example, the battery sensor is a shunt-resistor-based battery sensor, although other alternatives are available that are suitable for making measurements according to the present disclosure.

[0030] Measurements taken by the battery sensor(s) include the Open Circuit Voltage (OCV) which is a parameter of a battery cell and measured in Volts. It should be noted that voltage of the battery ~ OCV of the battery at low discharge currents (< 0.25 A) or the voltage of the battery when the battery is disconnected from any load. This is described for example at Perez, Richard. " Lead-acid battery state of charge vs. voltage." Home power 3Q (1993): p. 66-69. If the discharge currents are high (> 0.25 A), approximations of OCV can be calculated as a function of voltage and current, which are both measurable, and modelled battery impedance. Known models like power-law relationships between loaded current and OCV can be used to estimate the OCV for different discharged values of the battery in Ah.

[0031] The OCV of a battery cell is the potential difference between the positive and negative terminals when no current flows and the cell is at rest. An OCV curve is dependent on the chemistry of the cell and describes the charge and discharge of the battery, which can be non-linear. Hysteresis in charge vs discharge curves can result in errors of SoC if the BMS uses cell voltage to estimate SoC. However, the present calibration method described herein helps to calibrate the sensors for more accurate measurements. The BMS also comprises a thermocouple for measuring a temperature of the battery.

[0032] According to a first step 110, the method comprises fully charging the battery, such that the SoC is at a maximum SoC, SoCmaxIn one example, the battery is charged continuously by an alternator of the vehicle at Ualternator(~28 V) until the charging current indicates full charge, for example when no more charge is accepted and the charging current measures under a threshold amount, forexample 0.1 A. The SoC is specified as being at 100% when fully charged. The measurement of maximum SoC, SoCmax, anchors the calibration to a known and reliable measurement; i.e. when the battery is fully charged, it is known that the SoC is 100% to a degree of accuracy.

[0033] When a battery such as a lead-acid battery is first connected to a constant voltage power supply, the initial charging current is high because the battery is in a discharged state and the voltage difference between the battery and the power source is large. This current will gradually decrease as the battery voltage rises and the internal resistance increases. As the battery charges, the chemical reactions inside the battery generate an opposing voltage, and the battery’s SoC increases. As the SoC rises, the charging current decreases because the voltage difference between the charger and the battery narrows. The charging current will continue to drop until the battery reaches near full charge. Lead-acid batteries are considered fully charged when the charging current drops to a value that is typically 3-5% of the battery’s ampere-hour (Ah) rating. For example, for a 100 Ah battery, full charge would be reached when the charging current drops to 3-5 A. This is the point at which the battery is no longer able to accept a significant amount of charge and the voltage stabilizes at the charging voltage (often around 2.25-2.30 V per cell for lead-acid batteries). SoCmax(full charge) is defined by when the charging current drops below the target threshold (typically 3-5% of the rated Ah). This threshold helps avoid overcharging, which can damage the battery by causing excessive heat and gassing, leading to reduced lifespan.

[0034] According to a second step 120, the method further comprises determining a voltage at full charge, OCVmax, comprising an Open Circuit Voltage (OCV) of the battery at full charge, SoCmax. The voltage at full charge, OCVmax, is determined by measuring the voltage of the battery at full charge and estimating that the OCV reading corresponds to the maximum battery voltage, Umax.

[0035] The battery can be rested for a resting period to allow it to settle to an equilibrium state, after which a more accurate measurement of the OCVmax can be determined. It is usually required to rest for a period of between one and six hours, for example three hours. The measurement of OCVmax is made at the end of a resting period.Battery temperature can also be used in estimating values such as OCVmax, so that the voltage can be temperature corrected (to room temperature, typically measured at 25 degrees Celsius). This is performed by measuring a temperature of the battery at the time the voltages OCVmaxand OCVXare measured and temperature correcting the measurements of the voltages OCVmaxand OCVXbased on the measured temperature. Parameters are temperature-corrected (typically 3 mV / °C) to room temperature with thermocouple data from BMS and any other sources.

[0036] According to a third step 130, the battery is partially discharged. A first intermediate voltage, OCVx, of the battery at a first intermediate SoC of the battery, SoCxl, is measured. The first intermediate SoC of the battery, SoCxl, is less than full charge, SoCmax, but greater than full discharge, SoCmin. It will be appreciated that the first intermediate SoC of the battery, SoCxl,can be at any value less than 100%. However, it is preferable to take a first measurement of the first intermediate SoC of the battery, SoCxl, at a value equal to or above around 50% SoC at least because this reading can be taken earlier compared to measurements which require the battery to be drained beyond 50%. It may also be preferred to take a measurement at below 90% SoC. Whilst it may be more reliable to take regular readings at small increments of the SoC as the battery discharges, sufficient accuracy can be achieved by taking a reading at larger increments, for example at 90% and below. One reading of the first intermediate SoC of the battery, SoCxl, is suitable to perform the calibration method described herein and so it is desirable to take a measurement that provides a balance between waiting for the battery to be discharged and accuracy of the results. Methods using one intermediate value only to estimate Uminbenefit from being within the range of between 10% to 90%, with preferable values between 60% to 80%. It will be appreciated that any value can be used to make the calculations but that greater accuracy can be achieved using a value which is at least 10% SoC less than full charge.

[0037] Optionally, a second intermediate SoC of the battery, SoCx2, is measured which is different to the first intermediate SoC, SoCxl. A second voltage reading, OCVx2, is measured a SoCx2. This can help to improve the estimation ofSoCmin. Further values at further intermediate SoC of the battery can be taken without limitation.

[0038] In one embodiment, for example as illustrated in Figures 2 and 4, a first intermediate SoC of the battery, SoCxl, is measured at SoC = 80% and a second intermediate SoC of the battery, SoCx2, is measured at SoC = 60%. It will be appreciated that this is an example only, and that other values of SoC are available.

[0039] Temperature correction based on a measured temperature of the battery at the first, second, and further measured intermediate voltages can be performed by measuring the temperature of the battery at the time the reading is taken by the one or more sensors and temperature correcting the values accordingly.

[0040] The battery itself can be discharged in different ways between the readings. Two examples of different techniques used to discharge the battery are discussed herein, other techniques may be available.

[0041] A first discharging option comprises discharging the battery in a controlled manner using fixed current discharge between SoCmaxand SoCx. In this embodiment, the controller can be used to discharge the battery according to known and fixed current and impedance load. The alternator forms part of the integrated system used in the calibration. Other loads can be optionally added to alter the discharge current rate. The battery is discharged to a desired SoC value and left to settle to equilibrium for a resting period before a voltage measurement of the battery is taken.

[0042] In combination with Coulomb counting, the SoC of the battery can be calculated. Coulomb counting (CC) is a book-keeping method where the charge transferred through the battery during full charge-discharge process is counted by monitoring the input and output current continuously through current sensors of the BMS. Thus, the transferred amount of ampere hours are tracked and consequently the remaining capacity is known when starting from a battery which is fully charged (i.e. at full capacity).

[0043] When the battery is discharged using fixed current discharge, Coulomb counting can be used to track battery discharge and stop battery discharge at a predetermined value of SoCxto measure the first intermediate voltage, OCVX. In other embodiments, Coulomb counting can be used to trackbattery discharge and stop battery discharge at a predetermined value of depth of discharge (DoD) of battery capacity to measure the first intermediate voltage, OCVX.

[0044] A second option for discharging the battery during the calibration is available, for example if controlled discharge is not suitable due to additional components added to the discharge load or time availability. In a first scenario, the battery is discharged at low current, for example at around < 0.25 A, for an extended period before measuring OCVX. This extended period can take place, for example, when an electric vehicle is parked over the weekend or for up to a few weeks without being used. The battery loses charge due to leakage, for example. A voltage reading of the battery in parked mode can be approximated as the OCV. Coulomb counting can be used in this example by measuring the charge output to give an approximation of SoC. Coulomb counting can be used to estimate the change in SoC as total Coulomb discharge divided by total battery capacity. Discharge of the battery between 5% and 10% of the battery is suitable to perform calibration using this discharge method. Different set ups will take different amounts of time in parked mode to reach a suitable discharge of the battery using the second discharge option.

[0045] In a second scenario, the battery is discharged at high current > 0.25 A, e.g. with loaded discharge. Voltage of the battery ~ OCV of the battery at low discharge currents (< 0.25 A). Direct measurement of the OCV via measuring the battery voltage is therefore not available at high current discharge but corrections such as battery impedance models or Puekert’s law can be used to correct for the effect of higher currents if low-current data is not available. An estimate of OCV is calculated as a function of measured values of voltage and current in combination with modelled battery impedance. Known models like power-law relationships between loaded current and OCV can be used to estimate the intermediate OCVXfor different discharged values of the battery in Ah.

[0046] A fourth step 140 comprises extrapolating from OCVmaxat SoCmaxand OCVX1at SoCxl, to estimate a voltage at full discharge, OCVmin, of the battery at SoCmin. Extrapolation can be visualised, for example as described further in relation to Figure 2 below, by considering a relationship between voltage and SoC of the battery by plotting a graph of measured SoC against measured OCV.A relationship between the data points Umaxand UX1can be calculated and used to extrapolate Uminat SoC at voltage at full discharge, OCVmin, of the battery.

[0047] A rate of change of battery energy discharge with respect to battery voltage is based on the Nernst equation and can be estimated using the following equation:

[0048] max Umin)

[0049]

[0050] Capacity

[0051] Where Umaxis battery voltage at full charge, Uminis battery voltage at full discharge, and capacity is the full energy capacity of the battery. The capacity of the battery is typically a trusted value received for example from a manufacturer of the battery. However, the voltages of the battery at full charge and full discharge, whilst they may be provided by the manufacturer, are less reliably known.

[0052] Tests have shown that a linear relationship between the measured and plotted values of Umax, Ux, Ux2,..., Uminas described above is a good approximation suitable for calculating Uminby extrapolation according to the above method. The relationship is based on the thermodynamics of how lead acid batteries work as described for example in Coleman, Martin, Chi Kwan Lee, Chunbo Zhu, and William Gerard Hurley: " State-of-charge determination from EMF voltage estimation: Using impedance, terminal voltage, and current for lead-acid and lithium-ion batteries." IEEE Transactions on industrial electronics 54, no.

[0053] 5 (2007): 2550-2557. This relationship can be utilised to determine a value of Uminwithout having to completely drain the battery. This is advantageous at least because draining the battery to full discharge can be harmful to the battery. The present disclosure therefore provides a method which calculates the voltage at full discharge of the battery, OCVmin, which is required to accurately measure the SoC, without causing damage to the battery.

[0054] In some embodiments, values of [OCV] max at [SoC] max and [OCV] min at [SoC] min are determined from the calibration methods described herein and a value of a capacity of the battery.

[0055] Figure 2 illustrates a graphical representation of the linear relationship between SoC and OCV of a battery. The values in Figure 2 are of anexample battery used for illustrating the concept only. It will be appreciated that batteries vary considerably, and that Figure 2 is merely demonstrative in this regard.

[0056] SoC (%) is plotted along the x axis and OCV (V) is plotted along the y axis. A first data point is plotted of the voltage at full charge, OCVmax, against the SoC at full charge, SoCmax, as a first value, Umax. A second data point of the first intermediate voltage, OCVX1, is plotted against the first intermediate SoC, SoCxl, as a second value, Uxi. Optionally, the method may further comprise plotting one or more further data points of the one or more further intermediate voltages, OCVx2,..., 0CVxN, against the one or more further intermediate SoC, SoCx2,..., SoCxN, as one or more further values Ux2,... UxN. Additional measurements of the battery voltage (OCV) are taken at one or more further times, where the SoC of the battery is less than full charge but greater than full discharge. Illustrated in Figure 2, values of the OCV at two different SoC are taken; at SoC = 80% and SoC = 60%. These have been plotted as data points UX1and Ux2respectively in Figure 2.

[0057] A measurement of the battery voltage, OCVmax, is taken by a sensor coupled to the battery when the battery is fully charged (i.e. at SoCmax) and rested to equilibrium. This voltage is provided as an estimate of the voltage at full charge Umaxin Volts (V).

[0058] In some embodiments, the estimate of the voltage at full charge Umaxand the other measured voltages Ux1and Ux2(and any further readings) are temperature corrected. This is achieved by measuring a temperature of the battery at the time the battery voltage, OCVmax, is taken and temperature-corrected to room temperature at 25°C (typically at 3 mV / °C).

[0059] When plotted on the graph of SoC (%) along the x-axis and OCV (V) along the y-axis, a linear relationship between the data points Umax, Uxl, and Ux2can be seen. By extrapolating these data points using the linear relationship, an intercept of the graph at SoC = 0% can be calculated. At the intercept, a further data point, Umin, can be plotted, which determines a value of a voltage at full discharge, OCVmin, of the battery at full discharge, SoCmin.In alternative embodiments not illustrated in Figure 2, current discharged from the battery determined via Coulomb counting measured in ampere hours (Ah) is plotted along the x axis instead of SoC.

[0060] Figure 3 illustrates a flow diagram of a method for calibrating a battery sensor for measuring a state of charge (SoC) of a battery by calculating Umaxand Uminfor example in response to a trigger. In one example, the calibration is triggered in response to a user request. Alternatively, calibration can be automatic, for example after replacing a battery.

[0061] According to first step 302 a BMS is installed on the battery. The BMS comprises various sensors for measuring parameters of the battery.

[0062] According to a second step 304, values of Umin, Umax, and capacity are input to the BMS. These values may be provided by a manufacturer in some examples or may be values determined by a previous calibration exercise. If no values are provided or available, default values can be input for example to be able to initialise the calibration method.

[0063] According to a third step 306 the alternator is switched on to charge the battery until it is fully charged, for example when a “battery full” condition is met. Additionally or alternatively, full charge may be indicated by the charging current dropping below a threshold value. The alternator charges batteries in Constant Current Constant Voltage (CCCV) charging. CCCV is characterised by high initial current when the voltage is low with decreasing voltage as the voltage gradually increases. A typical voltage used to Charge the batteries using an alternator is 28 Volts.

[0064] At step 308 if the charge current is below a threshold current, Ith, the charge is switched off. If the charge current is above the threshold current, Ith, the battery is continued to be charged until the charge current is less than the threshold current, Ith. In one example, the threshold current, Ith, is 0.1 A.

[0065] At step 310, when the charge is switched off, the battery is allowed to settle for a resting period with low current and / or no load applied until it reaches equilibrium. Typical equilibrium resting times are from one to six hours, though other times may be used. Equilibrium describes a state when the battery voltage has settled and is not fluctuating such that the value of OCV measured is representative of the actual voltage of the battery at the determined SoC. The resting period depends on the internal characteristics of each battery and ismodelled using resistance and capacitance values. A time constant is determined which is indicative of an approximate time for the voltage value to reach the OCV. The exact equation is described by Equation (5): Moo, C. S., K. S. Ng, Y. P. Chen, and Y. C. Hsieh; " State-of-charge estimation with open-circuit-voltage for lead-acid batteries." In 2007 Power Conversion Conference-Nagoya, pp. 758-762. IEEE, 2007.

[0066] At step 312 the voltage of the battery, OCVmax, is measured and Umaxis updated to the measured value. Accordingly, Umaxis calibrated.

[0067] Optionally, steps 306 to 312 can be repeated a number N times so that an average value of Umaxcan be determined. Optional parameter estimation methods can also be used to improve estimate accuracy, for example Bayesian estimation. Bayesian estimation improves smoothness and accuracy.

[0068] The value of Uminis then calibrated using the calibrated value of Umax. At step 314 the battery is discharged. According to the first option described above, the battery is discharged with a fixed load resistor until a target current is attained. In one example, the target current is 5 A. Coulomb counting is used to track the battery capacity by measuring the energy in ampere hours that is discharged from the battery. Alternatively, the battery is discharged according to the second option described above by a low current for an extended period.

[0069] At step 316 voltage readings are taken which measure the voltage of the battery. If the battery is being discharged using the fixed load discharge, the discharge is stopped at a fixed value of battery capacity, the loads are removed and the battery is left to settle for a resting period before the measurements are taken. If readings of the SoC are available, measurements of the voltage can be taken at fixed values such as 80% and 60% SoC. If SoC information is not available, Depth of Discharge (DoD) can be used instead. DoD measures a maximum fraction of a battery’s capacity (in Ah) which is removed from the charged battery on a regular basis. Coulomb counting can be used to track battery discharge and used to stop battery discharge at a predetermined value of depth of discharge (DoD) of battery capacity to measure OCVX. Fixed values of DoD at 20% and 40% correspond to SoC at 80% and 60% respectively. One or more measurements can be taken according the present disclosure. The method requires only one measurement to be taken but two or more measurements canbe taken to improve accuracy. The measurements at 80% and 60% SoC are an example only and different values can be used to take measurements.

[0070] Figure 4 illustrates a graph of an implementation of the present disclosure according to a first discharge method having fixed current discharge. The graph shows discharge current (A) against time (hours) of a battery that is fully charged at time = 0. A fixed discharge current of 5 A is used in this example.

[0071] A first intermediate voltage, 0CVx, at a first intermediate SoC of the battery, SoCxlis measured at time t = 30 hours when the battery has reached DoD = 20% (SoC = 80%). The discharging is paused for a period of six hours at 0 A discharge current. Discharging at a current of 5 A is resumed six hours later after the battery has reached equilibrium and the voltage has been measured.

[0072] A second measurement of an intermediate voltage OCVx2is being measured in the illustrated example. The second intermediate voltage, OCVx2, at a second intermediate SoC of the battery, SoCx2is measured at time t = 60 hours when the battery has reached DoD = 40% (SoC = 60%). The discharging is paused for a period of six hours at 0 A discharge current before the second voltage reading is taken.

[0073] It will be appreciated that the values of DoD = 20% and 40% are used for example only, and that other values could be used.

[0074] Returning to Figure 3, at step 318 the voltage measurements of OCV at the determined battery capacity (or capacities) are plotted on a graph against SoC and a best-fit line is calculated. The slope of the line is used to determine the intercept at full discharge of the battery and calculate Umin.

[0075] Steps 314 to 318 can be repeated a number N times so that an average value of Umincan be determined.

[0076] The calibrated values of Umaxand Uminare then updated in the system. These values can then be used in calculating the SoC of the battery in normal use of the electric vehicle with improved accuracy. Calibration can be performed again, for example if the readings become unreliable or if the battery is replaced with a new one.

[0077] Figure 5 illustrates an example of an electric vehicle 500 in the form of a large mining truck, or haul truck. It will be appreciated that this is an example only and that the electric vehicle 500 could be any kind of electric vehicle or electric work vehicle. The electric vehicle 500 comprises a frame 505, an electric motor510 supported by the frame 505, and a drivetrain 515 being operatively driven by the electric motor 510.

[0078] The electric motor 510 is powered by chemical energy stored in a rechargeable battery pack. No limitation is intended herein for a composition or topology of the rechargeable battery pack, which may be lead-acid, lithium-ion, nickel-metal hydride, etc. A battery pack may comprise a single power storage module, or a plurality of power storage modules. A battery pack may be referred to elsewhere as a battery.

[0079] The drivetrain 515 of the electric vehicle features the wheels and tires as shown, or it may engage the ground in a separate fashion, such as by employing crawler belts, tracks, treads, and the like, in order to propel the electric vehicle 500.

[0080] The electric vehicle 500 further comprises a vehicle inlet 520 or charging port that is operatively and electrically connected to the rechargeable battery pack and designed to receive electrical power from an external source. In some embodiments, the vehicle inlet 520 may embody a common connector and in other embodiments the vehicle inlet 520 may use a proprietary connection format unique to the type, make, or model of the electric vehicle 500. It should be understood that, while the vehicle inlet 520 is shown to be located on a lower section of the frame 505, this is for illustration only, and the vehicle inlet 520 may be located elsewhere on the vehicle 500 without limitation. Where the electric vehicle 500 is a work machine, the placement and orientation of the vehicle inlet 520 may differ significantly across different vehicle types, makes, and / or models, which may themselves range in size and shape.

[0081] The electrical components of the electric vehicle 500 illustrated in Figure 5 may comprise an external power source, an onboard charger (OBC), a battery system comprising a battery and a battery management system (BMS), ancillary loads from one or more ancillary electrical devices, machine engine control unit (ECU), and one or more sensors. A controller, which may be part of the BMS or the ECU, is also provided to control the method described above. The external power source 310 may comprise an AC wall socket and is connectable to the onboard charger during charging of the electric vehicle. The onboard charger may comprise a plurality of chargers. Other components may include an electric powertrain (ePT) for powering the electric motor and driving the vehicle,and sensors in communication with the onboard charger and the battery and BMS which can be used in controlling the above described method and measuring quantities required.

[0082] Although not illustrated, the present disclosure is also suitable for vehicles and equipment comprising rechargeable lead-acid batteries (such as gensets) as described above. Any examples described herein using electric vehicles can be suitably adapted for use in vehicles and equipment comprising rechargeable lead-acid batteries.

[0083] Industrial

[0084]

[0085] Measuring the State of Charge (SoC) of electric batteries is required to determine how much energy is left in the battery, for example to determine when the battery will need to be recharged. Calibrating the sensors is important for ensuring that the determined SoC is accurate. Battery-to-battery variability can cause measurements to be inaccurate thereby giving an incorrect indication of the charge left to an end user. A method and system for calibrating battery sensors to improve SoC calculations is disclosed herein. The calibration described herein can be performed in the field, rather than in a lab under strict testing conditions.

[0086] While the preceding text sets forth a detailed description of the embodiments of the present disclosure, it should be understood that the scope of protection is defined by the words of the appended claims. The detailed description is to be construed as exemplary only and does not describe every possible embodiment since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented which would still fall within the scope of the claims defining the scope of protection.

Claims

Claims1. A method of calibrating a battery management system (BMS) for measuring a state of charge (SoC) of a battery of a vehicle or equipment, the method comprising:fully charging the battery, such that the SoC is SoCmax, measuring a voltage at full charge, OCVmax, of the battery at SoCmax,partially discharging the battery to a first intermediate SoC of the battery SoCxl, where SoCmax> SoCx1> SoCmin, wherein SoCminis at full discharge of the battery;measuring a first intermediate voltage, OCVx1of the battery at SoCx1, andextrapolating from OCVmaxat SoCmaxand OCVX1at SoCxl, to estimate a voltage at full discharge, OCVmin, of the battery at SoCmiri.

2. The method of claim 1, further comprising measuring a temperature of the battery at the time OCVmaxand OCVX1are measured; and temperature correcting the measurements of OCVmaxand OCVX1based on the measured temperature.

3. The method of claim 1 or claim 2, further comprising measuring the voltage at full charge, OCVmax, after the battery has been left to settle for a resting period.

4. The method of any preceding claim, wherein SoCxl, is between 50% SoC and 90% SoC, optionally wherein SoCxlis between 60% SoC and 80% SoC.

5. The method of any preceding claim, wherein the battery is discharged between SoCmaxand SoCx1using fixed current discharge.

6. The method of claim 5, further comprising using Coulomb counting to track battery discharge; andstopping battery discharge at a predetermined value of SoCx1to measure OCVx1.

7. The method of claim 5, further comprising using Coulomb counting to track battery discharge; andstopping battery discharge at a predetermined value of depth of discharge (DoD) of battery capacity to measure OCVx.

8. The method of claim 6 or claim 7, further comprising allowing the battery to settle for a resting period after stopping battery discharge at the predetermined value of SoCx1.

9. The method of claim 8, further comprising measuring the first intermediate voltage, OCVx1, after the resting period.

10. The method of any of claims 1 to 4, wherein the battery is discharged between SoCmaxand SoCx1at a low current for an extended period.

11. The method of any preceding claim further comprising: at a second intermediate SoC of the battery SoCx2, which is different to the first intermediate SoC, SoCxl, measuring a second voltage reading, OCVx2; andextrapolating from OCVmaxa SoCmax, OCVX1a. SoCxl, and OCVx2at SoCx2, to estimate OCVminof the battery at SoCmin.

12. The method of any preceding claim, further comprising repeating the measurements to determine an average SoCmaxand an average SoCmin.

13. The method according to any preceding claim, wherein the calibration is user triggered.

14. A method of measuring a state of charge (SoC) of a battery of an electric vehicle comprising using the values of OCVmaxat SoCmaxand OCVminat SoCmindetermined according to any of claims 1 to 13 in combination with a value of a capacity of the battery.

15. A vehicle or equipment comprising:a battery;a battery management system comprising one or more sensors configured to measure a voltage of the battery and a state of charge of the battery, and a controller configured to perform the method of any one of the preceding claims 1 to 14.

Images