Maintaining state of charge of a battery of an electric vehicle

Abstract

A method and apparatus for maintaining a state of charge of a battery of an electric vehicle comprises charging the battery of the electric vehicle to a specified state of charge, resting the battery

Description

TECHNICAL FIELD The present disclosure relates to maintaining a state of charge of an electric vehicle. In particular, maintaining the state of charge of the battery level whilst powering one or more ancillary electrical devices. The present disclosure is suited for automotives and work machines including those in agricultural, construction, mining, and resource fields, for example. BACKGROUND Electric 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. During charging of electric vehicle machines there is a requirement while the charger is connected to maintain a target battery State of Charge, SOC. The SOC is a measurement of the amount of energy available in a battery at a specific point in time expressed as a percentage. 100% SOC is not representative of actual battery capacity and as a result, the battery allows charging to continue past the 100% SOC value. Allowing machine ancillary loads to operate at the same time as maintaining the desired SOC can be challenging, particularly where the ancillary loads are high voltage systems which demand high power consumption. It is therefore desirable to maintain a SOC while a charger is connected and whilst powering ancillary devices. Prior art ways to conserve battery power whilst charging ancillary devices comprise, for example, the electrical loads (of the ancillary devices) are energized by the battery charger before the storage battery starts being charged by the charger while the charger is being connected to the storage battery and also after the charging of the storage battery is finished and until the charger is deenergized. Therefore, the electrical load is energized by the charger without allowing a charging current to flow to or a discharge current to flow from the storage battery. However, switching the battery or the ancillary devices off might not work in all situations and may involve complex algorithms to achieve. Accordingly, there remains a need for maintaining an SOC of a battery of an electric vehicle while allowing ancillary loads to operate. SUMMARY According to a first aspect, there is provided a method of maintaining a state of charge of a battery of an electric vehicle. The method comprises charging the battery of the electric vehicle to a specified state of charge, resting the battery at the specified state of charge for a resting time period, determining a resting voltage, Vr, of the battery, setting an onboard charger to a constant voltage charging mode having a constant voltage, Vc, based on the resting voltage, Vr, and maintaining the specified state of charge of the battery whilst powering one or more ancillary electrical devices of the electric vehicle. According to a second aspect, there is provided an electric vehicle comprising an onboard charger, a battery, one or more ancillary electrical devices and a controller configured to perform the method according to the first aspect. 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. BRIEF DESCRIPTION OF DRAWINGS 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. Figure 1 illustrates a flow diagram of a method of maintaining a state of charge of a battery of an electric vehicle according to an embodiment of the present disclosure; Figure 2 illustrates an example of an electric vehicle in the form of a large mining truck, or haul truck according to an embodiment of the disclosure; Figure 3 illustrates a diagram of the internal electrical components of the electric vehicle illustrated in Figure 2; Figures 4A to 4C show graphical plots of the state of charge and current usage according to an embodiment of the disclosure. DETAILED DESCRIPTION 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. As used herein, the words “exemplary” and “example” mean “serving as an example, instance, or illustration.” Any implementation described herein as exemplary, or 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. The present disclosure provides a method of maintaining a battery level of an electric vehicle while allowing ancillary loads to operate. The solution provided herein charges the battery to a specified State of Charge (SOC), rests the battery once it has reached the desired SOC, and then resumes charging using a constant voltage mode at a voltage limit based on a resting voltage, Vr, of the battery. This allows for ancillary electrical devices of the electric vehicle to be used whilst maintaining the SOC of the battery whilst a charger is connected to the electric vehicle. Whilst the present disclosure applies to any charging scenario, it may be particularly beneficial for longer (e.g. overnight) charging of the electric vehicle where operators are not actively switching loads on and off. According to an example charge sequence, the battery is charged up to a target SOC and held at the SOC. Ancillary electrical devices may turn on after target SOC has been reached, for example to warm / cool a cab of the electric vehicle as part of preconditioning requirements set by an operator. In one example, the operator sets the desired SOC target and desired cab temperature before leaving the electric vehicle to charge overnight, then returns to the electric vehicle the next morning which has been charged up and warmed according to their chosen preferences. This can be achieved without the electric vehicle having to choose between the battery or the ancillary electrical devices drawing the power from the onboard chargers and avoids using complex algorithms to maintain the SOC whilst powering the ancillary electrical devices. Figure 1 illustrates a flow diagram of a method 100 of maintaining a state of charge of a battery of an electric vehicle according to an embodiment of the present disclosure. The method may be performed by a charge controller of the electric vehicle, for example an electronic control unit (ECU) or a battery management system (BMS). According to a first step 110, the method comprises charging the battery of the electric vehicle to a specified SOC. The SOC is specified usually between 40-100% and chosen by an operator at the time of charging. In practice, an operator may choose an SOC not above 80% to conserve battery lifetime. Standard ways of charging comprising using an onboard charger, which is connected to an external power source, and charging using a mixture of constant current and constant voltage modes is used to charge the battery to the specified SOC. Ancillary electrical devices may be used during the first step 110, which may prolong the time it takes the battery to reach the specified SOC. During this initial charging, ancillary devices are powered by the onboard charger. According to a second step 120, the method comprises resting the battery at the SOC for a resting time period. The battery is rested to allow it to settle, after which a more accurate measurement of the SOC can be determined. To determine an accurate SOC of the battery, it is usually required to rest for a period of between 4 to 15 hours. However, embodiments of the present disclosure allow for a resting period which is much shorter than this. In some examples, the resting time period may be less than 10 minutes, optionally wherein the resting period is below 5 minutes. A minimum resting time period may also be set, for example for a period of 2 minutes. According to a third step 130, the method further comprises determining a resting voltage, Vr, of the battery. This allows a more accurate determination of the SOC to be calculated. The present disclosure provides a way of estimating a resting voltage, Vr, after a period which is defined by a few minutes rather than a few hours. In some embodiments, when the battery is rested according to this shortened timescale, a calibrated offset can be applied to improve the accuracy of the measured resting voltage, Vr. In some examples, the determination of Vr can be made at the end of the resting time period. However, there may be scenarios where desired resting time is unachievable in which case Vr may be determined using an estimate. Methods for estimating Vr can include using a lookup table accessible to the controller of the electric vehicle. Battery temperature can also be used in estimating Vr. According to a fourth step 140, the method further comprises setting an onboard charger to a constant voltage charging mode having a constant voltage, Vc, based on the resting voltage, Vr. By setting the constant voltage, Vc, based on the resting voltage, Vr, the SOC of the battery can be maintained. A battery management system measures and controls battery parameters and communicates with a controller to set battery charging current and voltage limits. In some examples, the onboard charger can be set to constant voltage mode with a constant voltage output, Vc, according to the following equation: VC = VR± offset Where Vc is the constant voltage output, Vr is the resting voltage of the battery at the specified SOC, and the offset is a calibrated offset. The calibrated offset accounts for variations in accuracy of the measurement of the SOC measured after the resting time period. A value of the calibrated offset may be between 0.1 V to 1 V. In some examples the calibrated offset is based on an ambient temperature of the environment local to and surrounding the electric vehicle. In some examples, the calibrated offset is based on the measured ambient temperature. For example, in colder climates, the battery may settle relatively quickly and the calibrated offset may accordingly be smaller than a calibrated offset for an electric vehicle in a hotter ambient environment. Alternatively, the calibrated offset can be based on battery temperature, for example on one or more predetermined threshold battery temperatures. A temperature sensor in communication with the controller measures the battery temperature at the SOC to determine an estimate Vr. If the ambient temperature and / or the battery temperature is below a predetermined threshold temperature, a first calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger. However, if the ambient temperature and / or the battery temperature is above the predetermined threshold temperature, a second calibrated offset is used to calculate the constant voltage, VC, output by the onboard charger. In some examples, there may be a first predetermined threshold temperature. However, in other examples, there may be a plurality of predetermined threshold temperatures, for example two threshold temperatures which determine what the calibrated offset should be. In some examples, the calibrated offset can be based on a combination of ambient temperature and battery temperature. The electric vehicle comprises one or more temperature sensors for measuring ambient and battery temperatures of the electric vehicle and the surrounding environment. The value of the calibrated offset could be affected by the resting time period, for example where longer resting time of the battery results in a smaller calibrated offset because the SOC is more accurate compared to a shorter resting time of the battery. The calibrated offset may also be affected by a number of ancillary electrical devices that are in use during the initial charging period where more connected ancillary electrical devices in use during the initial charging period results in a higher accuracy of the resting voltage of the battery at the SOC and accordingly a smaller calibrated offset. The output current limit of the onboard charger when charging the electric vehicle in constant voltage mode is calculated by a battery limit plus a maximum ancillary load. The battery limit depends on the battery at the time of charging and is a property of the battery itself. The maximum ancillary load is the maximum power consumption calculated according to a maximum output if all the ancillary electrical devices of the electric vehicle were switched on at once. According to a fifth step 150, the method further comprises maintaining a state of charge of the battery whilst powering one or more ancillary electrical devices of the electric vehicle. Ancillary electrical devices include any device which is part of the vehicle and which is powered by electricity. Such as water heaters, air conditioning, lights, radio, etc. Ancillary electrical devices that are powered whilst the external charger is still connected to the electric vehicle can be powered by either the battery or the onboard charger, depending on which one of the two has a higher output voltage. As an ancillary device is turned on, a transient demand for power occurs because of load variation in the circuit. Transients can be satisfied initially by power from the battery, which is quicker to respond to these demands than the onboard charger. This may be particularly the case when the ancillary electrical devices coupled to the electric vehicle are high voltage systems, which may be more often used in larger vehicles such as work vehicles. Whilst this initially depletes the power from the battery, and since the onboard charger is set to a constant voltage charging mode having a constant voltage, Vc, the output from the onboard charger becomes greater than the output of the battery after a short amount of time between few seconds and a few minutes, and a current to power the ancillary electrical device begins to be drawn from the onboard charger instead of the battery. This change in power supply to the electrical ancillary device is determined on which of the onboard charger and the battery have a higher output voltage and occurs naturally without any intervention or control command from the battery management system. The battery itself can also draw power from the onboard charger to replace power used in powering on the ancillary electrical device. By setting the onboard charger to constant voltage mode at the constant voltage, Vc, calculated based on the resting voltage, Vr, complex control algorithms for maintaining the SOC can be avoided. Overcharging of the battery is also avoided. Current discharged from the battery when the ancillary electrical device is switched on causes the internal voltage of the battery to drop below that of the onboard charger. The SOC can be maintained at the specified level by the battery drawing a current from the onboard charger to charge it back up to the original SOC due to this drop in voltage. Operators of the electric vehicle can therefore use the ancillary electrical devices without reducing the machine runtime when the SOC that the operator specified has been reached. Neither the battery nor the ancillary device are switched off or disconnected using this method and the SOC of the battery can be maintained whilst an ancillary electrical device is powered. A further advantage of the present disclosure is that stable voltage supplied to the system can aid battery balancing, which aims to improve the available capacity of a battery pack with multiple cells by working to maintain an equivalent state of charge at each cell of the battery pack. Figure 2 illustrates an example of an electric vehicle 200 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 200 could be any kind of electric vehicle or electric work vehicle. The electric vehicle 200 comprises a frame 205, an electric motor 210 supported by the frame 205, and a drivetrain 215 being operatively driven by the electric motor 210. The electric motor 210 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. The drivetrain 215 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 200. The electric vehicle 200 further comprises a vehicle inlet 220 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 220 may embody a common connector and in other embodiments the vehicle inlet 220 may use a proprietary connection format unique to the type, make, or model of the electric vehicle 200. It should be understood that, while the vehicle inlet 220 is shown to be located on a lower section of the frame 205, this is for illustration only, and the vehicle inlet 220 may be located elsewhere on the vehicle 200 without limitation. Where the electric vehicle 200 is a work machine, the placement and orientation of the vehicle inlet 220 may differ significantly across different vehicle types, makes, and / or models, which may themselves range in size and shape. Not illustrated in Figure 2 are any ancillary electrical devices of the electric vehicle such as heating or air conditioning units. Sensors, such as temperature sensors, are also present but not shown. Figure 3 illustrates a diagram of the electrical components of the electric vehicle 200 illustrated in Figure 2 in a charging state 300. The components comprise an external power source 310, an onboard charger (OBC) 320, a battery system 330 comprising a battery and a battery management system (BMS), ancillary loads 340 from one or more ancillary electrical devices, and machine engine control unit (ECU) 350. A controller, which may be part of the BMS or the ECU, is also provided to control the method described above for maintaining an SOC of the electric vehicle 200. The external power source 310 may comprise an AC wall socket and is connectable to the onboard charger 320 during charging of the electric vehicle. The onboard charger 320 may comprise a plurality of chargers. The onboard charger 320 is connected to the ECU 350 by a two way datalink. The ECU 350 is connected to the battery system 330 via a datalink. The battery system 330 and the onboard charger 310 are both connected to provide power to the ancillary electrical devices by an HV bus. Other components that are not illustrated may include an electric powertrain (ePT) for powering the electric motor 210 and driving the vehicle, and sensors in communication with the onboard charger 320 and the battery 330 and BMS which can be used in controlling the above described method. The sensors may include, but are not limited to, a temperature sensor for measuring an ambient temperature of the environment local to the electric vehicle 200, for example. The power source 310 corresponds with the vehicle inlet 220 of Figure 2 and is configured to receive power from an external power source, such as an external electric vehicle charger that is plugged into the inlet 220. When the external electric vehicle charger is connected to the electric vehicle 200 in a charging mode, for example at a charging station, the external power source provides power to the onboard charger 320. The onboard charger 320 charges the battery 330 in the charging state 300, and can also provide power to the ancillary loads 340, by-passing the battery. Ancillary loads 340 comprise one or more ancillary electrical devices of the electric vehicle 200 such as heaters, air-conditioning, lights, radio, etc. During a charging state 300 of the electric vehicle 200 illustrated in Figure 3, the power source 310 is connected to the onboard charger 320. The battery 330 is charged to an SOC as specified by an operator of the electric vehicle 200. One or more of the ancillary loads 340 can also be powered during the charging state. The electric vehicle during charging remains stationary for the duration of the charging period. During a driving state (not illustrated), the external charger, which provides the power source 310, is disconnected. When the charger is disconnected, all the electric power to the electric vehicle 200 is provided by the battery 330. The electric powertrain which drives the motor and the ancillary loads 340 both draw power from the battery 330. Maximising the amount of power stored in the battery 330 by maintaining the SOC of the battery 330 during the charging state 300 helps to extend the length of time that the electric vehicle 200 can be driven for in the driving state once the external charger has been disconnected. The onboard charger 320 and the battery 330 remain powered (e.g. not disconnected from the circuit) throughout the charging period. A worked example and measurements are illustrated in Figures 4A to 4C, which show a graphical plot of the SOC and current usage according to an embodiment of the present disclosure. In each of the graphs 4A to 4C, the top graph plots a maximum charge voltage limit on the y-axis against time on the x-axis. The middle graph plots time along the x-axis against current output of the battery and the onboard charger along the y-axis. The bottom graph plots time along the x-axis against SOC along the y-axis. Each graph illustrates a different example scenario. The worked example of Figure 4A sets the SOC to 71% as illustrated therein. A resting time period of the battery was 2 minutes. A constant voltage, Vc, of 325.1V (as illustrated in the top plot of Figure 4A) is applied in a constant voltage charging mode to maintain 71% SOC. As illustrated, the SOC is maintained at 71% throughout the constant voltage charging mode. As described above, the constant voltage charging mode maintains the specified SOC better if the battery has had a chance to settle during the resting period. Current outputs of the battery, Cb, and onboard charger, Cobc, are illustrated. At time t = 6 minutes 45 seconds, an ancillary electrical device (a heater) is powered on. As can be seen from the graph at the time when the ancillary electrical device is turned on, the battery reacts first and provides an initial current output Cb= -10 A. The longer the ancillary electrical device is switched on for, the battery current output, Cb, tends to zero and the onboard charger current output, Cobc, increases until the onboard charger is providing the power to the ancillary electrical device. Figure 4B illustrates a plot of an ancillary electrical device being turned on and off again for a short period of time (around 4 minutes). Throughout the duration of Figure 4B, the onboard charger voltage output remained constant, as well as the current limit. The Vc was set to 315 V to maintain an SOC of 50%. As seen from the in the middle graph, when the ancillary electrical device is turned on, the battery and chargers react to the demand. The battery output tends towards 0 A after the ancillary electrical device is switched on. In this example, the target SOC was set to 50% and as can be seen from the bottom graph of Figure 4B, this was maintained to within 0.2% during the ancillary electrical device being used. It can be seen that the SOC dipped slightly when the ancillary electrical device was turned on and then tended back to the original value after the ancillary electrical device was turned off again as the onboard chargers replenish the lost battery power. Whilst the embodiments above have been described with reference to an electric vehicle, the technology could also be used in hybrid vehicles, dependent on the machine electronics of the specific hybrid vehicle. Figure 4C illustrates a plot of a resting time period, Tr, of the battery (10 minutes to 40 minutes) followed by an ancillary electrical device being turned on and off again for a short period of time (1 hour, 2.5 minutes to 1 hour, 7.5 minutes). The battery was charged using constant voltage charging, with a rest period of 30 minutes. During the rest period, as illustrated in the top plot, the battery voltage relaxes back towards the open-circuit voltage (OCV). After the resting time period is finished (at around 40 minutes) the onboard charger outputs are enabled with a constant voltage, Vc, of 349 V to maintain the target SOC of 97%. In the bottom plot, although no power is being transferred to the battery by the onboard charger during the resting time period, the SOC rises slightly. This is due to the battery recalculating the SOC during the rest period. When the ancillary electrical device is turned on and off, the SOC is maintained around the setpoint. Figure 4D illustrates a plot where a very short resting time period was provided to the battery and an ancillary electrical device was powered during charging (i.e. during initial charging period of step 110). A target SOC of 31% was reached and the onboard charger was set to constant voltage output mode with Vc at 304.7 V. At t = 5 minutes, the onboard charger float voltage to match a bus voltage, where the float voltage is the voltage at which the battery is maintained after being fully charged to maintain that capacity by compensating for self-discharge of the battery. The battery current output normalises to around 0 A at t = around 7 minutes. In this example, the onboard chargers take up the ancillary electrical device load requirement. INDUSTRIAL APPLICABILITY Electric vehicles can be left to charge overnight and configured by the operator to certain conditions in the morning. During these overnight charges, ancillary electrical devices can be timed to come on at a certain point to condition the cab ready for the operator to use the vehicle in the morning. It is undesirable for the operator to come to a warmed cab with a depleted battery in the morning. Minimising usage of energy stored in the battery until the electric vehicle has been disconnected from a charger is desirable to prolong the time the vehicle or work machine can be used until a next charge is required. A method and system for maintaining a state of charge of a battery of an electric vehicle whilst powering one or more ancillary electrical devices of the electric vehicle is disclosed herein. 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 5 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

1. A method of maintaining a state of charge of a battery of an electric vehicle, the methodcomprising:charging the battery of the electric vehicle to a specified state of charge;resting the battery at the specified state of charge for a resting time period;determining a resting voltage, Vr, of the battery;setting an onboard charger to a constant voltage charging mode having a constant voltage, Vc, based on the resting voltage, Vr; andmaintaining the specified state of charge of the battery whilst powering one or more ancillary electrical devices of the electric vehicle.

2. The method of claim 1, wherein powering the one or more ancillary electrical devices is performed by either the onboard charger or the battery depending on which has a higher output voltage.

3. The method of claim 1 or claim 2, wherein the resting time period is less than 10 minutes.

4. The method of claim 3, wherein the resting time period is less than 5 minutes.

5. The method of any preceding claim, wherein the constant voltage, Vc, output by the onboard charger is calculated by the resting voltage, Vr, and a calibrated offset.

6. The method of claim 5, further comprising measuring one of: an ambient temperature of an external environment of the electric vehicle, and a battery temperature; andwherein the calibrated offset is based on the measured ambient temperature, the battery temperature or the ambient temperature and the battery temperature.

7. The method of claim 6, wherein if the ambient temperature is below a first threshold temperature, a first calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger.

8. The method of claim 7, wherein if the ambient temperature is above the first threshold temperature, a second calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger.

9. The method of claim 7 or claim 8, wherein if the ambient temperature is below the first threshold temperature and below a second threshold temperature, the first calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger;wherein if the ambient temperature is above the first threshold temperature and below the second threshold temperature, the second calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger; andwherein if the ambient temperature is above the first threshold temperature and above a second threshold temperature, a third calibrated offset is used to calculate the constant voltage, Vc, output by the onboard charger.

10. The method according to any preceding claim, wherein an output current of the onboard charger operating in the constant voltage mode is limited to a battery current limit plus a maximum power consumption of the one or more ancillary electrical devices.

11. The method of claim 10, wherein an output current from the onboard charger increases as the battery current tends to zero after one or more of the ancillary devices is powered on.

12. The method of any preceding claim, further comprising:turning on an ancillary device after the resting period, wherein initial power to the ancillary device is provided by the battery.

13. The method of claim 12, further comprising an output voltage of the battery dropping below the constant voltage output of the onboard charger, and wherein the power to the ancillary electrical device is provided by the onboard charger.5 14. The method of any preceding claim, wherein the onboard charger and the battery remainconnected to power the one or more ancillary electrical devices throughout a charging period.

15. An electric vehicle comprising:an onboard charger;10 a battery;one or more ancillary electrical devices;one or more temperatures sensors; anda controller configured to perform the method of any one of the preceding claims 1 to 14.1515

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