A method and control arrangement for measuring a current flowing through a battery unit

Abstract

The present invention relates to a method for estimating the resistance of a shunt resistor (502) in an electric energy storage system (500) comprising an electric energy storage (501). A current (I) flows through the electric energy storage (501) and the shunt resistor (502) when powering a load. The method comprises: providing a measurement current (I meas) through a reference resistor (R ref), the measurement current (I meas) flowing also through the shunt resistor (502); measuring a voltage drop over the shunt resistor (502) resulting from the measurement current (I meas); measuring a voltage drop over the reference resistor (R ref) resulting from the measurement current (I meas); and estimating a resistance (R shunt) of the shunt resistor (502) utilizing the measured voltage drop over the shunt resistor (502), the measured voltage drop over the reference resistor (R ref) and the resistance of the reference resistor (R ref).

Description

[0001]A METHOD AND CONTROL ARRANGEMENT FOR MEASURING A CURRENT FLOWING THROUGH A BATTERY UNIT Technical field Aspects of the invention relate to a method and a control arrangement for measuring a current flowing through a battery unit. Further aspects of the invention relate to a computer program, a computer-readable medium and a vehicle comprising a control arrangement. Background Electric energy may be stored in electric energy storages of various kinds to thereby allow the stored energy to be used upon request. Such electric energy storages commonly comprise, for example, electric battery cells for storing the energy. Where the electric battery cell can be seen as a container chemically storing energy. The electric battery cells may come in various forms and shapes. For example, electric battery cells may, e.g., have a cylindrical shape. Electric battery cells may alternatively have a prismatic shape. The battery cells may be connected in series and / or in parallel, into an electric battery unit, which may be called an electric battery pack, in order to attain a desired total voltage and energy capacity. A conventional electric battery pack, or a plurality of battery packs, may form the complete enclosure or entity that delivers electric power to a product or equipment, for example an electric vehicle, such as a battery electric vehicle or a hybrid electric vehicle. In general, an electric energy storage in the form of a conventional electric battery pack includes or contains electric battery cells, a control or management system, which may be called a battery management system (BMS) and may, for example, be implemented partly as software, and often also a cooling and / or heating system. The electric battery cells of an electric battery pack may be arranged in two or more modules, where a conventional module may comprise a frame holding a plurality of electric battery cells, e.g., of a cylindrical or prismatic design, and a conventional electric battery pack may be assembled by interconnecting the modules. Each battery module may comprise suitable electronics for monitoring the battery cells of the module. The battery cells of a battery module may be interconnected using electrical conductors such as busbars or cables to provide for current distribution between the battery cells, and such busbars / cables may also be used to connect the battery cells to connector poles of the battery module, and also to interconnect battery modules, and finally also between the modules and the connectors of the battery pack. It is in general a requirement for proper operation of the product or equipment, such as battery electric vehicle or hybrid electric vehicle, being powered by the battery pack that a reliable current measurement of the current flowing through the electric energy storage can be determined in order to ensure that the behaviour of the product, equipment or vehicle corresponds to an expected behaviour. Summary It is an object of the invention to provide a method for estimating the resistance of a shunt resistor being utilized when measuring a current through a battery unit. According to a first aspect of the invention, the aforementioned and further objects are achieved through a method, e.g., performed by a control arrangement, for estimating the resistance of a shunt resistor in an electric energy storage system, the electric energy storage system comprising: an electric energy storage; a shunt resistor; wherein, when powering a load connected to the electrical energy storage, a current flows through the electric energy storage and the shunt resistor; the method comprising: providing, utilizing measurement current providing means, a measurement current flowing through a reference resistor, the measurement current flowing also through the shunt resistor, wherein the reference resistor and the means for providing the measurement current are connected to the shunt resistor as a circuit having a floating ground in relation to the ground of the electric energy storage system; measuring a voltage drop over the shunt resistor resulting from the measurement current; measuring a voltage drop over the reference resistor resulting from the measurement current; and estimating a resistance of the shunt resistor utilizing the measured voltage drop over the shunt resistor, the measured voltage drop over the reference resistor and the resistance of the reference resistor. As was mentioned above, it is in general pertinent that the current flowing through an electric energy storage can be accurately determined to ensure proper operation of the load, such as, e.g., one or more electrical machines of a battery electric vehicle, since otherwise the devices such as electrical machines being powered by the electric energy storage may not operate as desired, and, e.g., provide more or less power than expected. The current is oftentimes measured by measuring a voltage over a resistor, whereby the current may then be determined from the voltage and the resistance of the resistor. In particular, a shunt resistor is oftentimes used in this regard. However, when using shunt resistors, it is very important that the resistance of the shunt resistor does not change over time, since this would have an impact on measurement results as time progress. This is because it is assumed that the resistance of the shunt resistor is a particular resistance that does not change. Therefore, such shunt resistors are manufactured from materials that are affected by aging to only a very little extent, so that thereby the resistance of the shunt resistor is reliable over time. The shunt resistor must also be capable of withstanding high currents, since the current that flows through the electric energy storage also flows through the shunt resistor. The shunt resistor must also have a very low resistance to avoid losses and development of heat. These properties that the shunt resistor consequently must exhibit renders the shunt resistor expensive. Still, shunt resistors exhibit drawbacks. According to aspects of the invention it is provided a solution that may reduce the impact of such drawbacks. In particular according to the invention, it is provided a method that estimates the resistance of a shunt resistor in an electric energy storage system. This estimation may then be utilized to estimate the current flowing through the electric energy system. Similar to prior art solutions, a shunt resistor is utilized, where, when the electric energy storage is used to power a load connected to the electrical energy storage, a current flows through the electric energy storage, and also through the shunt resistor. However, according to the invention, it is provided, utilizing measurement current providing means, a measurement current that also flows through the shunt resistor, and also through a reference resistor. Furthermore, a voltage drop over the shunt resistor resulting from the measurement current is measured, and so is a voltage drop over the reference resistor that also result from the measurement current. A resistance of the shunt resistor is then estimated utilizing the measured voltage drop over the shunt resistor, the measured voltage drop over the reference resistor and the resistance of the reference resistor. As was mentioned above, even though shunt resistors are manufactured with an aim to maintain the same resistance over time, resistors of this kind may still exhibit drawbacks. For example, the shunt resistor may be subjected to unfavourable operating conditions, e.g., over-currents or excessive heat, that may change the properties of the shunt resistor, with the result that the measurement of the current flowing through the electric energy store suffers in accuracy. According to the invention, such problems with the resistance of the shunt resistors changing over time can be avoided or at least alleviated through the use of a reference resistor in order to estimate the resistance of the shunt resistor to thereby account for changes over time. The reference resistor is only subjected to the measurement current, and thereby to a considerably lower current than the general current flowing through the electric energy store. The reference resistor may thereby have a considerably higher resistance than the shunt resistor, with a further advantage that such resistors are easier to manufacture, cheaper to manufacture, and in particular can be manufactured with much higher accuracy regarding changes in resistance over time. Thereby the reference resistor can be used to estimate the resistance of the shunt resistor so that changes in the less accurate shunt resistor can be accounted for, e.g., when measuring the current flowing through the electric energy store. The invention also has the further advantage that since the resistance of the shunt resistor is estimated, tolerance requirements regarding the shunt resistor can be reduced so that thereby considerably cheaper shunt resistors may be used while still maintaining an overall accuracy that is higher than the accuracy that is obtained through the prior use of the more expensive shunt resistor. According to aspects of the invention, the method further comprises to measure the voltage drop over the shunt resistor resulting from the measurement current when the current through the electric energy storage is zero. In this way the resistance of the shunt resistor can be estimated without need for compensating the measurements for influence on measurements caused by the current flowing through the electric energy storage, although such compensation is carried out according to aspects of the invention. According to aspects of the invention, the method further comprises to control the magnitude of the measurement current such that the voltage drop over the reference resistor exceeds a predetermined voltage. In this way the voltage drop over the reference resistor can be controlled to a magnitude where measurements of the voltage can be performed with high accuracy without need for, or at least a reduced need for, e.g., amplifiers as may be the case for measurement of the voltage over the shunt resistor. According to aspects of the invention, the means for providing the measurement current is a current source, and the method comprises to utilize the current source to provide the measurement current flowing through the reference resistor. This provides for a straight-forward way of providing a current of a desired magnitude in the measurement circuit. According to aspects of the invention, the method further comprises to utilize a shunt resistor having a non-zero resistance being below 1 mΩ, and a reference resistor having a resistance being at least 10 times, or at least 100 times higher than the resistance of the shunt resistor. Hence a shunt resistor having a very small resistance can be used in order to impact the overall system performance as little as possible, whereas the reference resistor can be considerably larger, and thereby facilitate use of a resistor having a very high accuracy and very low tendency to drift in resistance as time progress. According to aspects of the invention, the method comprises to compensate the estimation of the resistance of the shunt resistor for influence of thermoelectric voltages, wherein the influence of thermoelectric voltages is determined by: measuring a first voltage drop over the shunt resistor for a measurement current flowing in a first direction through the shunt resistor, and measuring a second voltage drop over the shunt resistor for the measurement current flowing through the shunt resistor in the opposite direction, wherein the voltage drops are measured when the current through the electric energy storage is zero, calculating an influence of the thermoelectric voltages utilizing the first and second voltage drop, and compensating the voltage drop of the shunt resistor used when estimating the resistance of the shunt resistor for with the voltage drop caused by thermoelectric voltages. Thermoelectrical voltages arise as a consequence of temperature differences that may prevail between different materials being connected together. The temperature differences may arise due to resistance that arises at the contact point when the materials are connected together. Such resistances depend on the particular materials that are being joined, and also of the quality of the connection of the materials. These temperature differences may give rise to a voltage, which is independent from the current flowing through the connection. Such temperature induced voltages can hence be compensated for, to thereby increase measurement accuracy, where the thermoelectric influence can be determined by performing the measurements for a current that goes in both directions through the shunt resistor. It is also possible to perform the same compensation for the reference resistor, but in general the influence may be considerably higher on the voltage over the shunt resistor, since this voltage may be very much lower than the voltage over the reference resistor. According to aspects of the invention, one or more switches, for example, semiconductor switches or mechanical switches can be used to switch the direction of the measurement current through the shunt resistor to thereby perform the above measurements for the compensation. According to aspects of the invention, the method further comprises to estimate the resistance of the shunt resistor regularly, continuously and / or at predetermined intervals. In this way an updated value may always be available. According to aspects of the invention, a plurality of estimations of the resistance of the shunt resistor are carried out, and a filtered resistance is utilized as a measure of the resistance of the shunt resistor, where the filtered resistance of the shunt resistor takes the plurality of estimated resistances of the shunt resistor into account. According to aspects of the invention, it is provided a method for measuring a current in an electric energy storage system comprising an electric energy storage and a shunt resistor, and where, when powering a load connected to the electrical energy storage, a current flows through the electric energy storage and the shunt resistor. This method comprises determining a measure of the current flowing through the electric energy storage utilizing a voltage drop over the shunt resistor and the resistance of the shunt resistor, where the resistance of the shunt resistor is estimated according to any of the aspects described above. In this way, the current flowing through the electric energy storage can be accurately determined, since it can be ensured that an accurate value of the resistance of the shunt resistor is always available, where the resistance of the shunt resistor may be regularly, continuously and / or at predetermined intervals updated through estimation as described above. According to aspects of the invention, the estimated resistance of the shunt resistor is compensated with a temperature compensation factor to compensate the estimation for differences in temperature between the time of estimation of the resistance of the shunt resistor, and the time of determining the measure of the current flowing through the electric energy storage. That is, even though the resistance of the shunt resistor may have recently been estimated, the characteristics of the shunt the resistance may still be such that the resistance may have a temperature dependency and hence change with changes in current resistor temperature. Therefore, in case the currently prevailing temperature differs from the temperature at which the estimation of the resistance was carried out, e.g., due to heating caused by high currents, this can be compensated for through the use of a temperature compensation factor, where this factor, e.g., may be obtained from the manufacturer of the shunt resistor. According to aspects of the invention, the method further comprises to neglect influence of thermoelectric voltages over the shunt resistor when measuring the current flowing through the electric energy storage. As was mentioned above, thermoelectric voltages may be accounted for when determining the resistance of the shunt resistor. However, even if such thermoelectric voltages are accounted for when determining the resistance of the shunt resistor, these thermoelectric voltages can be neglected when determining the current flowing through the electric energy storage. The reason for this is that since the current flowing through the electric energy storage may be very high, the voltage drop over the shunt resistor may also be considerably higher when measuring the current than when estimating the resistance of the shunt resistor. Thereby the influence of the thermoelectric voltages will be much smaller and may therefore be disregarded. Still, according to aspects of the invention such influence of thermoelectric voltages is accounted for. According to aspects of the invention, the electric energy storage system is an electric battery system comprising a plurality of electric battery cells and a plurality of electrical conductors interconnecting the electric battery cells, where the current being measured flows through the plurality of battery cells. Consequently, the invention is applicable, e.g., for conventional batteries used in electric vehicles. According to a further aspect, the invention relates to a control arrangement for determining the resistance of a shunt resistance in an electric battery system. It will be appreciated that all the embodiments described for the method aspects of the invention are applicable also to control arrangement aspects of the invention. Thus, all the aspects described for methods according to the invention may be performed by the control arrangement, which may also be a control device, i.e., a device. The control arrangement and its embodiments have advantages corresponding to the advantages discussed above with regard to the various aspects of methods according to the invention. Further advantageous aspects of the method and the control arrangement according to the present invention and further advantages with the aspects of the invention emerge from the detailed description. Brief Description of the Drawings Aspects of the invention will now be illustrated, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, where similar references are used for similar parts, in which: Fig.1 illustrates an exemplary battery electric vehicle in which aspects of the invention may be utilized; Fig.2 illustrates an exemplary battery module of a battery unit for which the resistance of a shunt resistor may be determined according to aspects of the invention; Fig.3 illustrates a battery unit comprising a plurality of battery modules according to Fig.2; Fig.4 illustrates an exemplary electric energy storage system comprising means for determining the current flowing through an electric energy storage according to the prior art; Fig.5 an exemplary electric energy storage system comprising means for estimating the resistance of a shunt resistor according to aspects of the invention; Fig.6 illustrates an exemplary method according to aspects of the invention; Fig.7 illustrates a further exemplary method according to aspects of the invention; Fig.8 schematically illustrates a control arrangement according to aspects of the invention. Detailed Description With reference to Fig.1, an embodiment of a vehicle 100 according to aspects of the invention is schematically illustrated. The vehicle 100 is illustrated as a tractor vehicle. However, for other embodiments, the vehicle 100 may, for example, be of any other kind of heavy vehicle, such as a bus or a truck. The vehicle may also, e.g., be a passenger car. The vehicle may also be of other types of vehicles. Although not illustrated in Fig.1, the vehicle 100 may be equipped with a trailer. The vehicle 100 comprises a powertrain configured as an electric vehicle EV, for example a hybrid electric vehicle, HEV, or a battery electric vehicle, BEV. The vehicle 100 may further, as illustrated, be a wheeled vehicle, i.e., a vehicle 100 having wheels 102. Only the wheels 102 on the left-hand side of the vehicle 100 are visible in Fig.1. It is to be understood that the vehicle 100 may have fewer or more wheels than what is shown in Fig.1. The powertrain comprises at least one electrical machine 101 configured to apply a propulsive power and / or a braking power to one or more of the wheels 102 of the vehicle 100. The at least one electrical machine 101 may be arranged essentially anywhere, for as long as power is provided to one or more of the wheels 102 of the vehicle 100. Various applicable examples exist in the art. The vehicle 100 is configured to comprise a plurality of electric battery units 103, 104, 105, such as battery stacks, and may also comprise further nondisclosed battery units. The current through each battery unit may, individually or collectively, be determined according to aspects of the invention. The battery units may form part of a battery arrangement 106 of the vehicle 100. The components of the powertrain of the vehicle 100, as well as other components in the vehicle may be controlled by a vehicle control system forming part of a vehicle electrical system via a control arrangement 120. The control arrangement 120 may be distributed on several control units configured to control different parts of the vehicle 100. The control arrangement 120 may, e.g., include a control unit for controlling the applying of a propulsive power and / or regenerative brake power of the electrical machine 101. The control arrangement may also comprise a control unit for diagnosing battery units, i.e., a control unit arranged for performing the method steps of the disclosed invention as is explained in the following. Such a control unit may, e.g., form part of a battery management system (BMS) that may be responsible for various other functions involving the battery. The control arrangement 120 will be schematically described in further detail in conjunction with Fig.8. The vehicle 100 may further include one or more sensors providing sensor data to the vehicle control system. For example, according to the invention, e.g., at least voltage sensing means are configured to measure a voltage over a shunt resistor and over a reference resistor. As is realized, the vehicle 100 may comprise a large number of control units and sensors for controlling various part of the vehicle, as is known in the art. Fig.1, only illustrates units / devices / entities of the vehicle that are required for understanding the present invention. Fig.2 schematically illustrates an exemplary battery module 200, according to embodiments of the invention. The battery module 200 may form a battery unit, or a plurality of battery modules may form a battery unit such as a battery stack. The battery module 200 comprises a plurality of battery cells, n according to the present example, denoted 201_1, 201_2, 201_3,…, 201_n. The battery cells 201_1, …, 201_n are interconnected by means of electrical connectors, such as busbars or cables 202. According to the illustrated example, the battery cells each comprises a cell fuse 204 in order to, for example, provide for a short-circuit and / or overvoltage protection. According to embodiments of the invention, no such cell fuses 204 are present. The battery module 200 further comprises external connectors 205, 206 to be connected, either to further battery modules of a battery pack and / or to the connection points of the battery pack, where the connections of the connectors to the battery cells may also be realized through the use of electrical connectors such as busbars or cables. According to the illustrated example, the battery module 200 further comprises a cell management controller (CMC) 210. The cell management controller 210 may, inter alia, be utilized to monitor the battery cells of the battery module. For example, the cell management controller CMC 210 may be configured to individually measure the cell voltage over each individual battery cell of the battery module. The CMC 210 therefore comprises connections 211 to each pole of each battery cell 201_1, …, 201_n of the battery module 200. Fig.3 illustrates an exemplary battery unit 300 comprising a plurality of battery modules, e.g., according to the embodiment illustrated in fig.2. In particular, the battery unit of Fig.3 comprises three battery modules 301-303, but it is to be understood that the number of battery modules forming the battery unit 300 may be any suitable number of battery modules. The battery modules may also be physically packed together to reduce unnecessary use of busbars / cables. Furthermore, the vehicle electrical system may be configured to electrically connect the battery unit 300, either alone or as forming part of a larger battery arrangement of the vehicle comprising a plurality of battery units, to the one or more electrical machines of the vehicle to provide power for propelling and braking the vehicle. The figure illustrates connection points 304, 305 for such purposes. As is realized, the same current I will flow through all of the battery modules 301-303 since they are series connected, and thereby also through all of the battery cells. The voltage U over the battery unit 300 will also be the aggregated voltage of all battery cells of the battery modules 301- 303. As was mentioned, it is very important that the vehicle control system is aware of the current I that currently is flowing through the battery modules in order to be able to correctly control the vehicle, e.g., to ensure that the one or more electrical machines provide power that results in an expected acceleration and deceleration. It is therefore a requirement that the current can be accurately measured. This can be performed through the use of a shunt resistor having a very low resistance, and which also is designed to exhibit a resistance that does not change over time. Such resistors, however, as will be further discussed below, are expensive, and may also exhibit other drawbacks. According to the invention, it is provided a method for estimating the resistance of a shunt resistor being utilized when measuring a current flowing through a battery unit such as battery unit 300 of Fig.3. This provides for additional possibilities when it comes to the selection of a resistor for use when determining a current flowing through an electric energy storage system such as a battery unit according to Fig.3. Fig.4 illustrates an example of a prior art solution for measuring the current of a battery unit such as a battery pack, and hence, e.g., through a battery unit according to Fig.3. The battery unit is schematically illustrated by 401. As can be seen in the figure, a shunt resistor 402 is utilized to measure the current I flowing through the battery unit 401. The shunt resistor 402 is connected in series with the battery unit 401, and hence the current I flowing through the battery unit 401 also flows through the shunt resistor 402. The current I is used to power a load 404, such as one or more electrical machines, and is measured by measuring the voltage ^^^^௨^௧over the shunt resistor 402, and through knowledge of the resistance of the shunt resistor 402, a measurement of the current I can be obtained. The voltage ^^^^௨^௧is measured using suitable voltage measuring means 403, where such voltage measuring means are known in the art, and where the connections to the shunt resistor 402 are schematically illustrated by a dashed line. In case more than one battery unit is present, similar means for measuring the current may be provided for each battery unit. The current flowing I through the shunt resistor, and hence the current that flows through the battery unit, may be very large, e.g., in the order of 100-1500 A, but also higher or lower than the exemplified current interval. Given the high currents, the resistance of the shunt resistor 402, as was mentioned, needs to be very small, and therefore also the voltage over the shunt resistor will be comparatively small. This also provides challenges regarding the voltage measurement. Although the invention is not aimed specifically at the actual voltage measurement, methods for such measurement is briefly addressed below. As was also mentioned above, a problem with measuring the current I flowing through the battery unit 401 according to the illustrated solution is that the shunt resistor is an expensive component that in general is manufactured from particularly selected materials that suffer from an aging effect only to a very little extent. For example, the shunt resistor may be manufactured, e.g., from alloy manganin, which is a trademark name for an alloy typically consisting of 84.2% copper, 12.1% manganese, and 3.7% nickel. Such shunt resistors, however, are highly expensive, and resistors exhibiting the desired properties may be only available from few manufacturers. Still, despite the high cost, the properties of such shunt resistors may not be that perfect in reality. In particular, the resistance of the shunt resistor 402 may change over time, although the material being selected in the manufacturing process may reduce such effects. For example, the shunt resistor 402 may be subjected to over-currents, which in turn may result in heat, which in turn may change properties of the shunt resistor, such as resistance, which hence may drift over time, also for expensive shunt resistors of this kind. This may render measurements of the current flowing through the battery unit less exact as time progress. According to aspects of the invention, it is provided a solution that allows for measuring the current flowing through the shunt resistor in a manner where shunt resistors having less tolerance may be used, while at the same time providing the same or higher accuracies in the measurements. This is provided for by a method for estimating the resistance of the shunt resistor using a reference resistor, where the reference resistor is used to improve the precision of the shunt resistor. There exist reference resistors having considerably higher accuracies than shunt resistors of the above kind, i.e., shunt resistors that are capable of having high currents flowing through the resistor. The problem with reference resistors, however, is that they cannot withstand high currents, and hence not be subjected to the current levels that prevail, e.g., in electric vehicles. According to aspects of the invention, however, such a reference resistor is used, but in combination with a shunt resistor, and in a manner that allows for use of a shunt resistors having lower accuracy or tolerance than shunt resistors that are currently being used. Fig.5 illustrates an exemplary measurement setup according to aspects of the invention, where the measurement setup may form an integrated part of the electric energy storage system 500. Similar to Fig.4, the figure illustrates the electric energy storage 501 through which the current I to be measured flows in order to power a load 504 (not forming part of the electric energy storage system), such as one or more electrical machines for propelling the vehicle. Also similar to Fig.4, the solution illustrated in Fig.5 comprises a shunt resistor 502 being utilized in the determination of the current I. Furthermore, the figure also illustrates a reference resistor 505 and a current source 507, where the current source 507 is configured to generate a reference current, also denoted measurement current, ^^^^^^, which will circulate the shunt resistor 502, the reference resistor 505 and the current source 507. This measurement circuit forms a separate circuit in which the main current I will never circulate. The measurement circuit comprising the reference resistor 505 is further illustrated as being grounded at a local ground point 508, but this ground point is not connected to actual ground, but is a floating ground and only forms a reference. The measurement circuit comprising the reference resistor 505 is hence not grounded to the ground of the vehicle, and in particular not to the ground of the electric energy system. Since the measurement circuit is free-floating, the current being generated in this system will also be the current that returns to the system, and hence the measurement circuit will be completely independent from the current flowing through the electric energy storage. In this way, it can be ensured that undesired currents, in particular excess currents, will never float in the circuit of the reference resistor 505, but the current in the measurement circuit may instead be determined by the current source 507. This also means that the large current I flowing through the shunt resistor 502 will not enter the measurement circuit comprising the reference resistor 505. The measurement circuit hence form a small local free-floating (in terms of electric potential) system, where the overall voltage difference in the measurement circuit may be kept at, e.g., only, 5-10V, where this can be controlled by, e.g., use of the constant source, or generator, 507, that generates a predetermined current of a desired magnitude. Even if the main current is high, the voltage upstream the shunt resistor in relation to the local ground 508 will still be very small, due to the very small resistance of the shunt resistor. The figure also schematically illustrates measurement circuits 506 for measuring the voltage over the reference resistor 505, and measurement circuits 503 for measuring the voltage over the shunt resistor 502, the use of which will be explained below. As was mentioned, the main current I may be very high, with the inherent requirement that the shunt resistor has a very low resistance to avoid energy loss and excess heat. The free-floating reference circuit comprising the reference resistor, however, is subjected to a considerably smaller current, i.e., the measurement current, ^^^^^^, which allows use of a reference resistor having a considerably higher resistance than the shunt resistor, and which also only needs to be capable of being subjected to considerably smaller currents. For example, according to aspects of the invention, a reference resistor being capable of being subjected to a measurement current, ^^^^^^, of, e.g., 1 ampere, or only a few amperes, is used in combination with a shunt resistor being capable of being subjected to the current I that is used to power the vehicle, and which, as was mentioned, may be in the order of hundreds of amperes. The resistance ^^^^௨^௧of the shunt resistor 502 may typically be in the order of 5-100 micro ohms, and may be mounted on a busbar in order to as much as possible improve conductivity in the connection points, and conversely as little as possible affect the overall operation of the system, since the shunt resistor is essentially only utilized for the purpose of measuring the current I. The reference resistor 505, on the other hand, may have a resistance ^^^^^, e.g., in the order of 0.5 ohms and hence be, e.g., a thousand times, or more, greater than the resistance ^^^^௨^௧of the shunt resistor 502. Given this considerably larger resistance, it is also considerably easier to manufacture a resistor exhibiting a better stability, accuracy, and capabilities of maintaining the resistance over time than is possible with a resistor having an extremely low resistance as is necessarily the case with the shunt resistor being used in the measuring of very large currents. It is further to be noted that the shunt resistor 502 is forming part of a high voltage system, e.g., in the order of 400-1000V as is common, e.g., in electric vehicles today, and a voltage that may even increase further in times to come. Still, the measurement circuit according to the invention may form a low voltage circuit, since the voltage drop over the shunt resistor 502 will be small also for high currents due to the very small resistance, and the remaining portions of the measurement system including the reference resistor 505 will be subject to only a small measurement current, where the components of the measurement system as explained may be designed to have a floating ground being dependent on the currently prevailing potential of the shunt resistor to thereby prevent excessive currents flowing through the reference resistor. The use of a measurement circuit comprising the reference resistor 505 according to aspects of the invention allows the use of a shunt resistor with less requirements regarding the tolerances of the shunt resistor, and thereby also use of a less expensive shunt resistor while still maintaining, or even increasing, measurement accuracy in comparison to prior art solutions. For example, exact calibration of the shunt value during manufacturing / assembly can be avoided according to the invention. Fig.6 illustrates an exemplary method for estimating the resistance of the shunt resistor 502. The method starts in step 601, where it is determined whether the resistance of the shunt resistor is to be estimated. This may be configured to be carried out, e.g., each time the vehicle is started. The estimation may also be configured to be carried out, e.g., at regular intervals such as every day, every week, every month and / or at any other suitable interval. The estimation may also be configured to be carried out in case, e.g., an over-current has been detected. The estimation may also be configured to be carried out, e.g., when it is determined that no current is flowing through the shunt resistor 502, as may be the case when the vehicle is standing still and no systems are consuming power from the energy storage system. Consequently, the resistance may be estimated regularly, continuously and / or at predetermined intervals. When it is determined that the resistance of the shunt resistor is to be estimated, the method continues to step 602 where a measurement current ^^^^^^is controlled to flow through the measurement circuit and hence through the reference resistor 505, where this may be carried out through the use of the current source 507 which then causes the current ^^^^^^to flow through the shunt resistor 502 and the reference resistor 505. Consequently, a defined and thereby known measurement current ^^^^^^is controlled to flow through the measurement circuit. The current ^^^^^^may be determined, e.g., in dependence of a desired voltage drop, such as a desired minimum voltage drop, over the reference resistor 505. In step 603 a voltage drop over the shunt resistor 502 caused by the measurement current ^^^^^^is measured. This is carried out through the use of measurement circuits 520 illustrated in Fig.5. This can be carried out according to prior art solutions for measuring the voltage over the shunt resistor 502. It is to be noted in this regard, that since the ^^^^^ / ^^^^௨^௧ratio may be in the order of 1000, or even 10000, the ^^^^௨^௧value may be measured with a gain amplification of about 100 or higher. For example, an analogue-to-digital- converter, ADC, with a built-in GIA (Galvanically Isolated amplifier) amplifier GIA with a gain of, e.g., 1x to 128x can be used in combination with a low gain chopper pre-amplifier of about 10 to 30 times. The ADCs’ gain and pre-amplifier gain values may be selected to minimize the gain error from gain errors on different gain values in the ADC. A part of the precision in the measurement may then be controlled by the gain linearity and temperature dependence. Other methods for measuring the voltage over the shunt resistor may also be utilized. In step 604, correspondingly, a voltage drop over the reference resistor 505 caused by the measurement current ^^^^^^is measured. The voltage drop over the reference resistor 505 may be measured using any suitable measurement means, and it can in particular be noted that this voltage may be easier to measure with high accuracy since the voltage drop over the reference resistor may be, e.g., 1000 times or 10000 times higher than the voltage drop over the shunt resistor, and in particular be controlled to be of a magnitude where use of amplifiers can be avoided or at least reduced. It is to be noted in regard of the measurements of the voltage drop over the reference resistor 505 and the voltage drop over the shunt resistor 502 that the measurements may be configured to be carried out simultaneously. In this way, any fluctuations in the measurement current ^^^^^^can be accounted for, since the fluctuations will not impact the measurement result when measurements are carried out simultaneously. According to aspects of the invention a constant current source may be utilized, and in case the constant current source is capable of maintaining a steady current, the measurements may be carried out at different points in time. In step 605 the resistance of the shunt resistor 502 is estimated utilizing the measured voltage drop over the shunt resistor 502, the measured voltage drop over the reference resistor 505 and the resistance ^^^^^of the reference resistor. This can, for example, be carried out according to the following equations:^^^^^ ൌ ^^^^^^^^^^^ (1)^^^^௨^௧ ൌ ^^^^௨^௧^^^ ^ ^^^^^^^ (2) The resistance Rshuntmay, as mentioned, be configured to be estimated when the main current I = 0. The resistance of the shunt resistor may then be determined as: The estimated resistance of the shunt resistor may then be used, e.g., when determining the current I flowing through the electric energy storage. According to aspects of the invention, it is hence provided a method for estimating the resistance of the shunt resistor, where this estimation may be carried out at any suitable time. The resistance of the shunt resistor that has been estimated according to aspects of the invention may then be utilized, e.g. to estimate the current I flowing through the electric energy system. The method allows that shunt resistors for which tolerance requirements are less strict may be utilized, which may thereby reduce the overall cost of installation. Furthermore, even though a shunt resistor having a lower tolerance is used, the overall accuracy may be increased, since the estimation of the resistance of the shunt resistor may be arranged to be carried out at any time this is deemed necessary, so that there always will be an accurate estimation of the resistance of the shunt resistor even if the resistance of the shunt resistor may have deviated over time from the resistance that prevailed upon initial installation. With regard to the resistance of the reference resistor, this can, for example, be very precisely determined prior to installation, and then be stored in the control system of the vehicle to be used in the calculations, since this resistance is not expected to change over time. In general, both the resistance value, with a precision equal to or better than 0.1%, as well as worst case ageing can be obtained from the reference resistor supplier and then be stored in the control system for use according to the invention. It is further to be noted that although the resistance ^^^^௨^௧of the shunt resistor 502 has been exemplified as being determined when the main current I is zero above, it is also possible to determine the resistance ^^^^௨^௧of the shunt resistor 502 when the main current I is non-zero. This, however, may require that the measurement current ^^^^^^can be directed in both directions through the shunt resistor 502 for the same main current I. In this way it is possible to eliminate the voltage contribution emanating from the main current I to thereby provide a measurement of the voltage drop over the shunt resistor emanating only from the measurement current to allow determination of the resistance of the shunt resistor according to equation 4 above. Furthermore, with reference to the measurement current, this can be controlled, e.g. through appropriate control of the current source 507 to a current that results in at least a predetermined voltage drop over the reference resistor. In this way it can be assured that the voltage drop is high enough to provide for accurate measurement of the voltage without, e.g., need for amplification circuits that may be required for the measurement of the voltage over the shunt resistor as described above. According to aspects of the invention, it is also possible to use the voltage over the reference resistor to control the current that flows through the measurement circuit. Since the reference resistor is considered to have a resistance that is always constant, the voltage can be used to determine the current that flows through the measurement circuit. Furthermore, according to aspects of the invention, the measurement results can also be compensated for by compensating for thermoelectrical voltages that may occur in the measurement circuit. For example, thermoelectric voltages may arise in connections between different materials, such as, for example, between the material of the shunt resistor and, e.g., the busbar(s) to which the shunt resistor can be connected when being integrated in the system. In general, thermoelectrical voltages arise as a consequence of the temperature difference that prevail between the materials. Such temperature differences may arise due to the fact that there is oftentimes a contact resistance between the materials, i.e. a resistance that arises at the contact point when the materials are connected together. The contact resistance depends on the materials that are used and the quality of the connection of the materials. It is hence not the current flowing through the shunt resistor that gives rise to these voltages, but the temperature difference between the materials. It is to be noted that thermoelectric are voltages are independent from the current flowing through the shunt resistor. Such thermoelectrical voltages may be in the order of up to a plurality of microvolts per degree of temperature difference, and even if such voltages may be in the order of a single microvolt per degree of temperature difference, this may give rise to a measurement inaccuracy when estimating the resistance of the shunt resistor. These undesired voltages can be compensated for to increase the accuracy of the measurement results. This can be carried out by applying a current, such as the measurement current described above, through the shunt resistor in both directions, and compare the resulting voltages over the shunt resistor, i.e. in a manner similar to what has been described for measurements while a main current I is flowing. These voltages may then be determined using the following equations: The voltage drop ^^௧^^^^^over the shunt resistor that emanate from thermoelectric voltages may thereby be determined, so that the measured voltage drop over the shunt resistor can be compensated for the influence of this voltage to thereby increase the accuracy of the estimation of the resistance of the shunt resistor. Similar to above, the providing of the measurement current through the shunt resistor in both directions may be accomplished, e.g., through suitable use of switches, such as semiconductor switches or other suitable electrically operated switches, that in a straightforward manner may provide for such change of direction of the current. Furthermore, as was mentioned, the estimation of the resistance of the shunt resistor may be utilized when determining the current I flowing through the electric energy storage. An exemplary method 700 in this regard is illustrated in Fig.7. The method 700 starts in step 701, where it is determined whether the current I flowing through the electric energy storage is to be determined. This may be arranged to be carried out continuously, or whenever needed where this may be determined by the regular control of the vehicle. That is, the method may be utilized any time the control of the vehicle requires a measurement of the main current I, such as any time a load is utilizing the electric energy storage and a main current I is flowing. In step 702 a voltage drop over the shunt resistor is determined, e.g., in a manner similar to what has been described above, and in step 703 the resistance ^^^^௨^௧of the shunt resistor 502 is determined, e.g., by retrieving a stored value of the resistance ^^^^௨^௧, where the resistance ^^^^௨^௧has been estimated according to what has been described above. The resistance ^^^^௨^௧, as described, can be updated regularly, continuously and / or at predetermined intervals to always provide a currently prevailing value. Thereby, an accurate measure of the current can be determined since the determination utilizes an updated value of the resistance of the shunt resistor. Furthermore, the estimated resistance ^^^^௨^௧of the shunt resistor 502 may also be compensated with a temperature compensation factor to compensate the estimation for differences in temperature between the time of estimation of the resistance ^^^^௨^௧of the shunt resistor 502 and time of determining the measure of the current flowing through the electric energy storage 501. As was mentioned above, the resistance of the shunt resistor 502 may be estimated, e.g., in situations when the current I flowing through the electric energy storage is zero. Furthermore, the estimation may be carried out, e.g., when the vehicle has been standing still for a period of time, with the result that vehicle components may have cooled off in relation to normal operating temperatures. In case the resistance of the shunt resistor is estimated in such a situation, and the vehicle subsequently is taken into use, various components, including the shunt resistor 502 may increase in temperature, e.g., due to the oftentimes high currents that may prevail in systems of this kind. Therefore, the estimated resistance of the shunt resistance 502 may need to be compensated for by a temperature compensation factor to account for such temperature differences. Such temperature compensation factors, however, may in general be obtained from the manufacturer of the shunt resistor. Furthermore, as was mentioned above, influence of thermoelectric voltages over the shunt resistor may be accounted for when estimating the resistance of the shunt resistor. However, when it comes to determining the current I flowing through the electric energy storage, influence of such thermoelectric voltages may be neglected. The reason for this is that in general the voltage drop over the shunt resistor will be considerably higher when determining the current I flowing through the electric energy storage than when estimating the resistance of the shunt resistor, since the resistance may be estimated when the main current is zero. The higher the voltage over the shunt resistor is, the less impact will the thermoelectric voltages have, since these are independent from the current as described above. According to an aspect of the invention, it is provided a control arrangement for carrying out the methods according to the invention. The control unit 120, e.g. a device or a control arrangement, according to the invention may be configured to perform all aspects that have been described with regard to methods. Hence the control arrangement 120 is provided with the above-described advantages for the various aspects of the invention. The invention also relates to a vehicle 100 including the control arrangement 120. Fig.8 illustrates a control arrangement 800 / 120, which may be utilized to carry out the invention. The control arrangement 800 / 120 comprises a computing unit 801, which can be constituted by essentially any suitable type of processor or microcomputer, e.g., a circuit for digital signal processing (Digital Signal Processor, DSP), or a circuit having a predetermined specific function (Application Specific Integrated Circuit, ASIC). The computing unit 801 is connected to a memory unit 802 arranged in the control arrangement 800 / 120, which memory unit provides the computing unit 801 with, e.g., the stored program code and / or the stored data which the computing unit 801 requires to be able to perform computations. The computing unit 801 is also arranged to store partial or final results of computations in the memory unit 802. In addition, the control arrangement 800 / 120 is provided with devices 811, 812, 813, 814 for receiving and transmitting input and output signals. These input and output signals can contain waveforms, impulses, or other attributes which, by the devices 811, 813 for the reception of input signals, can be detected as information and can be converted into signals which can be processed by the computing unit 801. These signals are then made available to the computing unit 801. The devices 812, 814 for the transmission of output signals are arranged to convert signals received from the computing unit 801 in order to create output signals by, e.g., modulating the signals, which can be transmitted to other parts of and / or systems in the vehicle 100. Each of the connections to the devices for receiving and transmitting input and output signals can be constituted by one or more of a cable; a data bus, such as a Controller Area Network CAN bus, a Media Orientated Systems Transport MOST bus, or some other bus configuration; or by a wireless connection. A person skilled in the art will appreciate that the above-stated computer can be constituted by the computing unit 801 and that the above- stated memory can be constituted by the memory unit 802. Control systems in modern vehicles commonly comprise communication bus systems consisting of one or more communication buses for linking a number of electronic control units, ECU's, or controllers, and various components located on the vehicle. Such a control system can comprise a large number of control units and the responsibility for a specific function can be divided amongst more than one control unit. Vehicles of the shown type thus often comprise significantly more control units than are shown in Fig.1, which is well known to the person skilled in the art within this technical field. The invention may be implemented by the above mentioned control unit 120, or any other suitable control unit, and also be distributed over more than one control unit, e.g., all control units participating in the control of the energy storage resources. The invention can also, however, be implemented wholly or partially in one or more other control units being present in the vehicle 100, or in one or more control unit dedicated to the invention. Here and in this document, units are often described as being arranged for performing steps of the method according to the invention. This also includes that the units are designed to and / or configured to perform these method steps. Control units and other units may be logically separated but physically implemented in the same unit or can be both logically and physically arranged together. These units may, e.g., correspond to groups of instructions, which can be in the form of programming code, that are input into, and are utilized by a processor / computing unit 801 when the units are active and / or are utilized for performing its method step, respectively. The person skilled in the art will appreciate that the aspects described herein for estimating the resistance of the shunt resistor, and determining the current flowing through the electric energy storage may also be implemented in a computer program, which, when it is executed in a computer, instructs the computer to execute the method. The computer program is usually constituted by a computer program product 803 stored on a non-transitory / non-volatile digital storage medium, in which the computer program is incorporated in the computer-readable medium of the computer program product. The computer-readable medium comprises a suitable memory, such as, e.g.: Read-Only Memory ROM, Programmable Read-Only Memory PROM, Erasable PROM EPROM, Flash memory, Electrically Erasable PROM EEPROM, a hard disk unit, etc. The invention is not limited to the above-described aspects. Instead, the invention relates to, and encompasses all different aspects being included within the scope of the independent claims.

Claims

Claims 1. A method, performed by a control arrangement, for estimating the resistance of a shunt resistor (502) in an electric energy storage system (500), the electric energy storage system (500) comprising: an electric energy storage (501); a shunt resistor (502); wherein, when powering a load connected to the electrical energy storage (501), a current (^^) flows through the electric energy storage (501) and the shunt resistor (502); the method comprising: providing, utilizing measurement current providing means, a measurement current ^^^^^^^^ flowing through a reference resistorthe measurement current ^^^^^^^^ flowing also through the shunt resistor (502), wherein the reference resistor ^^^^^^^ and the means for providing the measurement current ^^^^^^^^ are connected to the shunt resistor (502) as a circuit having a floating ground in relation to the ground of the electric energy storage system (500); measuring a voltage drop over the shunt resistor (502) resulting from the measurement current ^^^^^^^^; measuring a voltage drop over the reference resistor ^^^^^^^ resulting from the measurement current ^^^^^^^^; and estimating a resistance ^^^^^௨^௧^ of the shunt resistor (502) utilizing the measured voltage drop over the shunt resistor (502), the measured voltage drop over the reference resistor ^^^^^^^ and the resistance of the reference resistor ^^^^^^^.

2. A method according to claim 1, further comprising: measuring the voltage drop over the shunt resistor (502) resulting from the measurement current ^^^^^^^^ when the current (^^) through the electric energy storage (501) is zero.

3. A method according to claim 1 or 2, further comprising: controlling the magnitude of the measurement current ^^^^^^^^ such that the voltage drop over the reference resistor ^^^^^^^ exceeds a predetermined voltage.

4. A method according to any one of the claims 1-3, wherein the measurement current providing means is a current source, the method comprising: utilizing the current source to provide the measurement current ^^^^^^^^ flowing through the reference resistor ^^^^^^^.

5. A method according to any one of the claims 1-4, further comprising: utilizing a shunt resistor (502) having a non-zero resistance being below 1 mΩ, and a reference resistor ^^^^^^^ having a resistance being at least 10 times, or at least 100 times higher than the resistance ^^^^^௨^௧^ of the shunt resistor (502).

6. A method according to any one of the claims 1-5, further comprising: compensating the estimation of the resistance ^^^^^௨^௧^ of the shunt resistor (502) for influence of thermoelectric voltages, wherein the influence of thermoelectric voltages is determined by: measuring a first voltage drop over the shunt resistor (502) for a measurement current flowing in a first direction through the shunt resistor (502), and measuring a second voltage drop over the shunt resistor (502) for the measurement current flowing through the shunt resistor (502) in the opposite direction, wherein the voltage drops are measured when the current through the electric energy storage (501) is zero, calculating an influence of the thermoelectric voltages utilizing the first and second voltage drop, and compensating the voltage drop of the shunt resistor (502) used when estimating the resistance ^^^^^௨^௧^ of the shunt resistor (502) for with thevoltage drop caused by thermoelectric voltages.

7. A method according to claim 6, further comprising: utilizing one or more switches to switch the direction of the measurement current through the shunt resistor (502).

8. A method for measuring a current in an electric energy storage system (500), the electric energy storage system (500) comprising: an electric energy storage (501); a shunt resistor (502); wherein, when powering a load connected to the electrical energy storage (501), a current (^^) flows through the electric energy storage (501) and the shunt resistor (502); the method comprising: determining a measure of the current flowing through the electric energy storage (501) utilizing a voltage drop over the shunt resistor (502)and the resistance ^^^^^௨^௧^ of the shunt resistor (502), wherein the resistance ^^^^^௨^௧^ of the shunt resistor (502) is estimated according to the method according to any one of the claims 1-7.

9. A method according to claim 8, further comprising: determining the resistance ^^^^^௨^௧^ of the shunt resistor (502) regularly, continuously and / or at predetermined intervals, and updating the resistance ^^^^^௨^௧^ of the shunt resistor (502) being utilized in the determining of the measure of the current flowing through the electric energy storage (501).

10. A method according to claim 8 or 9, further comprising: compensating the estimated resistance ^^^^^௨^௧^ of the shunt resistor (502) with a temperature compensation factor to compensate the estimation for differences in temperature between the time of estimation of the resistance ^^^^^௨^௧^ of the shunt resistor (502) and time of determining the measure of thecurrent flowing through the electric energy storage (501).

11. A method according to any one of the claims 8-10, further comprising: neglecting influence of thermoelectric voltages over the shunt resistor (502) when measuring the current flowing through the electric energy storage.

12. A method according to any one of the claims 1-11, wherein the electric energy storage system (500) is an electric battery system, the electric battery system comprising: a plurality of electric battery cells; a plurality of electrical conductors interconnecting the electric battery cells; wherein the current being measured flows through the plurality of battery cells.

13. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of the claims 1-12.

14. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of the claims 1-12.

15. A control arrangement for estimating the resistance ^^^^^௨^௧^ of a shunt resistor (502) in an electric energy storage system (500), the electric energy storage system (500) comprising: an electric energy storage (501); a shunt resistor (502); wherein, when powering a load connected to the electrical energy storage (501), a current (^^) flows through the electric energy storage (501) and the shunt resistor (502); the control arrangement, when estimating the resistance ^^^^^௨^௧^ of the shunt resistor (502):being configured to provide, utilizing measurement current providing means, a measurement current ^^^^^^^^ to flow through a reference resistor the measurement current ^^^^^^^^ flowing also through the shunt resistor (502), wherein the reference resistorand the means for providing the measurement current ^^^^^^^^ are configured to be connected to the shunt resistor (502) as a circuit having a floating ground in relation to the ground of the electric energy storage system (500); being configured to control measuring of a voltage drop over the shunt resistor (502) resulting from the measurement current^^^^^^^^; being configured to control measuring of a voltage drop over the reference resistor ^^^^^^^ resulting from the measurement current ^^^^^^^^; and being configured to estimate a resistance ^^^^^௨^௧^ of the shunt resistor ^502^ utilizing the measured voltage drop over the shunt resistor (502), the measured voltage drop over the reference resistor ^^^^^^^ and the resistance of the reference resistor ^^^^^^^.

16. A vehicle (100) comprising a control arrangement according to claim 15.

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