WO2017148496A1

WO2017148496A1 - A method and system for controlling a current being fed to a battery pack

Info

Publication number: WO2017148496A1
Application number: PCT/EP2016/052609
Authority: WO
Inventors: Lars Eriksson Normand
Original assignee: Volvo Truck Corp
Current assignee: Volvo Truck Corp
Priority date: 2016-03-01
Filing date: 2016-03-01
Publication date: 2017-09-08
Anticipated expiration: 2018-09-01
Also published as: US20190089167A1; EP3424123B1; CN108702004B; EP3424123A1; US11437827B2; CN108702004A

Abstract

The invention relates to a method for controlling a current being fed to a battery pack (4) comprising a plurality of battery cells (4a, 4b, 4c) connected in series and providing an output battery voltage (V_B). The method comprises the steps of: feeding a relatively low current (i₁) through a resistor (13) being arranged in series with said battery pack (4); and measuring the voltage across said resistor (13), thereby obtaining a value of said current (i₁). The invention also relates to a system for controlling a current being fed to a battery pack (4).

Description

A method and system for controlling a current being fed to a battery pack

TECHNICAL FIELD

The invention relates to a method for controlling a current being fed to a battery pack comprising a plurality of battery cells connected in series and providing an output battery voltage.

The invention also relates to a system for controlling a current being fed to a battery pack comprising a plurality of battery cells connected in series and providing an output battery voltage.

The invention is applicable to vehicles which are operated by means of at least an electric machine. In this disclosure, the invention will be described with respect to a vehicle in the form of a bus. However, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as heavy-duty vehicles, trucks, cars, trams and construction equipment. The invention is not limited to being used in connection with battery packs for vehicles, but can also be used for battery packs being used, for example, in solar cell arrangements and so-called "smart grid" electric utility networks for managing electricity demand in society. BACKGROUND

In the field of vehicles, there is a steady development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines. In particular, electrically operated vehicles have emerged as a promising alternative.

According to today^'s technology, a vehicle can be operated by means of an electric machine solely or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as for example carbon monoxide and oxides of nitrogen. The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, such as battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a battery pack with a plurality of rechargeable battery cells which, together with control circuits, form a system which is configured for providing electric power to an electric machine in a vehicle.

A vehicle which is operated by means of an electric machine is normally supplied with power from a rechargeable electrical energy storage system, i.e. a battery pack with rechargeable battery cells which can be charged by means of an external electric power supply. This is carried out after the energy storage system and the external power supply have been electrically connected by means of suitable connector elements. In the automotive field, an energy storage system normally comprises a battery pack with a large number of battery cells. A battery pack may for example be of the lithium-ion type. In the event that a 600 V lithium-ion battery pack is used, for example approximately 200 battery cells connected in series will then be needed to achieve a desired voltage in order to operate the vehicle. The available range for driving the vehicle depends on certain parameters such as the state of charge (SOC) of the battery pack.

The state of charge is an important parameter to use in order to prevent batteries from being operated during under- or over-charging situations, and to manage the energy in electric vehicles. Furthermore, it is known that batteries degrade over time and that the expected driving range and the fuel savings of a vehicle cannot be upheld towards the end of the lifetime of a battery, due to decreasing performance of the battery. Also, the decreasing performance will affect the magnitude of the power which can be received and supplied by the battery. It is previously known that requirements for optimum battery properties can be reached by means of a process referred to as battery cell equalization, or cell balancing. The reason for this is that the voltages of different battery cells in a battery pack will differ between cells during the course of time. This lack of balance between cells may result in degraded battery properties. There is consequently a need to equalize the state of charge for the battery cells. In this manner, it will be possible to obtain optimum properties of the battery pack.

According to known technology, there exist many different methods for cell balancing. One such known method is to discharge one or more battery cells having a cell voltage or state of charge (SOC) which differs considerably from the remaining battery cells, through a resistor which is coupled in parallel with each battery cell.

The patent document US 2014/306666 teaches a system for battery balancing, in particular for implementing balancing between batteries which are connected in parallel by controlling a power relay assembly. Also, the balancing process can be implemented by means of a control of a suitable value of a pre-charge resistor.

Even though there exist several methods and systems for battery cell balancing, there is still a need for improvements within this field, in particular for implementing cell balancing without an unnecessary loss of charge from the battery pack. In particular, it has been noted that during a cell balancing process, it is of high importance to be able to control a current which is fed to the battery pack in a very accurate manner. In this manner, the properties of the battery pack can be optimized.

SUMMARY

An object of the invention is to provide a method and system for controlling a current which is fed to a battery pack, which particularly but not exclusively can be used during cell balancing of a battery pack, and in which problems relating to unwanted cell voltage drop and reduced power delivery capacity for the battery pack can be overcome.

According to a first aspect, the object is achieved by a method for controlling a current being fed to a battery pack comprising a plurality of battery cells connected in series and providing an output battery voltage. Furthermore, the method comprises the steps of: feeding a relatively low current through a resistor being arranged in series with said battery pack; and measuring the voltage across said resistor, thereby obtaining a value of said current.

An advantage of the invention is that it can be used for controlling said current, in particular during cell balancing of a battery pack which is carried out in a manner in which a battery cell voltage drop can be reduced during the cell balancing. Also, the power delivery capacity for the battery pack can be optimized as a result of the battery cell balancing using a current control according to the invention. According to an embodiment, the current is fed during a phase when the charge and discharge ability of the battery pack is below a predetermined limit. This means that when the charge and discharge ability of the battery pack is sufficiently high, the accurate current control as mentioned above is no longer needed. Also, this means that a battery pack having a relatively low temperature, for example, can be fed with said current until it reaches a condition in which it can be charged in a regular manner.

Furthermore, and according to an embodiment, the above-mentioned relatively low current can be fed to said battery pack during a process of battery cell balancing. In this manner, the above-mentioned goal of optimizing the power delivery properties of the battery pack can be obtained.

According to an embodiment, the magnitude of the current is controlled so as to compensate for a loss of electric charge in said battery cells during said cell balancing. This results in an efficient cell balancing process in which the loss of charge can be minimized.

According to a further embodiment, said current is fed through a resistor forming part of the battery pack. The resistor may be adapted for limiting an inrush current when said battery pack is connected to said charging unit. By feeding the current through the resistor, an accurate control of the magnitude of the current can be obtained.

According to a second aspect, the above-mentioned object can be obtained by means of a system for system for controlling a current being fed to a battery pack comprising a plurality of battery cells connected in series and providing an output battery voltage.

Furthermore, said system comprises a control unit which is adapted for feeding a relatively low current through a resistor, which is arranged in series with said battery pack, and for measuring the voltage across said resistor, thereby obtaining a value of said current. Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

Fig. 1 shows a simplified perspective view of a vehicle in which the present invention can be implemented.

Fig. 2 is a schematic view of an arrangement for balancing a battery pack according to an embodiment of the invention.

Fig. 3 is a flow chart showing the manner in which the invention is implemented.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Different aspects of the present disclosure will be described more fully hereinafter with reference to the enclosed drawings. The method and system disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth below.

The invention will now be described with reference to an embodiment and the enclosed drawings. With initial reference to Fig. 1 , there is shown a simplified perspective view of a vehicle in the form of a bus 1 which according to the embodiment is of the electric type and is equipped with an electric machine 2 which can be used for operating the bus 1 . This is shown schematically in Fig. 1 by means of a rear axle 3 which is connected to the electric machine 2.

The bus 1 carries an electric energy storage system in the form of a battery pack 4 which in turn comprises a plurality of battery cells (not shown in detail in Fig. 1 ). As will be described in greater detail below, the battery cells are connected in series to provide an output DC voltage. Suitably, the battery cells are of lithium iron phosphate (LiFeP04) type, but other types may also be used.

The battery pack 4 is also connected to an electronic control unit 5 which is arranged for measuring one or more predetermined parameters which are indicative of the state of operation of the battery pack 4. For example, the control unit 5 can be configured for measuring the voltage of the battery pack 4 and its battery cells, or one or more alternative parameters such as the battery current or the temperature of each battery cell. Such parameters can be used for controlling the condition and operation of the battery pack 4. The battery pack 4 will be described in greater detail below with reference to Fig. 2.

According to the embodiment, the battery pack 4 is arranged on the roof of the bus 1 , as indicated in Fig. 1 , but other arrangements of the battery pack 4 are also possible within the scope of the invention.

Other components of the propulsion system of the bus 1 , which are not shown here in detail, are also connected to the control unit 5. Even though the invention is described with reference to a vehicle in the form of a bus, it can be used for virtually any type of vehicle which is operated by means of at least an electric machine and which has an energy storage system comprising a battery pack with a number of battery cells.

During operation of the bus 1 , the energy storage system 4 will deliver the required power to the electric machine 2, which in turn is driving the rear axle 3. The manner in which an electric machine can be used for operating a vehicle is generally previously known and for this reason, it is not described in detail here.

The bus 1 is equipped with a first electric connector element 6, suitably in the form of a pantograph which is mounted on the roof of the bus 1 . The first connector element 6 is arranged for being connected to a second electric connector element 7 in the form of an overhead electrical conductor wire which is configured for conducting a charging current having a certain voltage. The overhead conductor 7 forms part of an external power supply 8, suitably in the form of an AC grid system. In this manner, the battery pack 4 can be supplied with an electrical current by means of the connection between the overhead wire 7 and the pantograph 6. More precisely, the electric current is fed to an on-board charging unit 9 which is connected to the battery pack 4 for charging thereof. The charging unit 9 is also connected to the control unit 5.

According to the embodiment, the pantograph 6 and the overhead wire 7 are arranged so that charging of the battery pack 4 takes place while the bus 1 is standing still, i.e. either at a charging station at a bus terminal or at a bus stop or a similar position. Alternatively, charging of the battery pack 4 can be carried out during operation of the bus 1 .

According to the embodiment shown in Fig. 1 , the vehicle 1 is arranged to be operated by means of the electric machine 2 only. According to a further embodiment (which is not shown in the drawings), the vehicle may be a hybrid vehicle, for example a so-called plug- in hybrid vehicle which is equipped with an internal combustion engine and an electric machine which are connected to each other via a clutch. Both the internal combustion engine and the electrical machine can then be used alternately or in parallel to operate the vehicle. As mentioned, the arrangement for charging can also be of the plug-in type comprising a charging cable connecting the battery in the vehicle to a charging station.

In the shown example, a process for charging the battery pack 4 can be initiated when the bus approaches the overhead wire 7 so that the pantograph 6 and the wire 7 can come into contact with each other. This means that charging of the battery pack 4 is initiated upon connection of the battery pack 4 to the external power supply 8. Regarding the second connector element 7, it can be noted that it can be arranged as an overhead wire, as shown in Fig. 1 . Alternatively, the invention can be implemented with a second connector element in the form of a current conducting power rail which is arranged along the road surface. Such an arrangement is configured to cooperate with one or more current collectors which are movable and lowered towards the ground, and which may be configured to be connected with said current conducting power rail during operation of the vehicle. The invention can also be implemented by means of other types of conductors, for example current wires arranged along the side of a vehicle and corresponding with suitable current collectors on the vehicle.

An embodiment of the invention will now be described in greater detail with reference to Fig. 2, which is a schematic figure showing the battery pack 4, the control unit 5, the onboard charging unit 9 and certain other associated components of the vehicle 1 . According to an embodiment, the battery pack 4 comprises a plurality of battery cells, symbolically represented by three battery cells 4a, 4b, 4c, which are connected in series and which provide an output battery voltage (V_B). The battery pack 4 contains a large number of battery cells, suitably in the magnitude of 200 cells, although the specific number may vary. According to the embodiment, the battery cells 4a, 4b, 4c are of the lithium iron phosphate (LiFeP0₄) type, although the principles of the invention are equally applicable to other types of battery cells. Also, although the embodiment comprises one battery pack, it should be noted that the invention is applicable in cases where several battery packs are combined.

As mentioned above with reference to Fig. 1 , the battery pack 4 is connected to an electric machine (not shown in Fig. 2) and is configured for operating said electric machine, which in turn operates the vehicle in question. Furthermore, the battery pack 4 is connected to the on-board charging unit 9 so as to allow charging of the battery pack 4 when the charging unit 9 is connected to the external power supply 8. The external power supply 8 is typically configured for supplying a 400 V AC three-phase voltage. Also, the charging unit 9 is connected to the battery pack 4 via a positive main contactor 10 and a negative main contactor 1 1 . The charging unit 9 typically supplies 600 V DC to the battery pack 4.

According to the embodiment, a pre-charge contactor 12 is connected in parallel with the positive main contactor 10. Also, a pre-charge resistor 13 is connected in series with the pre-charge contactor 12. All the contactors 10, 1 1 , 12 are connected to the control unit 5 and can be controlled by the control unit 5 in a manner which will be described in more detail below.

As shown in Fig. 2, the control unit 5 is connected to the charging unit 9 and to the battery pack 4, and is arranged for initiating a balancing process for the battery pack 4 depending on the distribution of energy between the battery cells 4a, 4b, 4c. This will be described below.

In order to carry out a cell balancing process, the control unit 5 is configured for monitoring the status of the battery pack 7a, i.e. to monitor the status of each battery cell 4a, 4b, 4c. For this reason, the control unit 5 may be configured with sensor units (not shown) for measuring the voltage of each battery cell and for transmitting information related to measured voltage values to the control unit 5.

Furthermore, the charging unit 9 is connected to a DC/DC converter 14, which is configured for supplying electric power to certain low voltage components, such as for example a second battery unit 15 for powering components such as air condition systems, heating devices and lighting units. A further example of a component to which the DC/DC converter 14 is connected is an electrical heater 16 which is configured for heating a liquid which flows in a liquid circuit 17. The liquid is pumped through the circuit 17 by means of a pump 18. The liquid circuit 17 is arranged close to the battery pack 4 so as to be used for a temperature control of said battery pack 4. According to the embodiment, the liquid circuit 17 also comprises a cooling package 19 such as a radiator or a heat exchanger. The liquid is transported in the liquid circuit 17 to the battery pack 4 and then back to the pump 18. In this manner, a temperature control of the battery pack 4 can be implemented.

Consequently, the on-board charging unit 9 supplies electric energy both in the form of a traction voltage which is supplied to the battery pack 4 and also a low voltage, via the DC/DC converter 14, to certain electric components of the vehicle such as the heater 16, the pump 18 and the low voltage battery 15.

As mentioned initially, this disclosure relates to a method and system for balancing the battery pack 4. It is previously known that the battery cells of a battery pack may differ over time as regards their state of charge (SOC). This is due to the fact that as the battery cells are charged and discharged, the electric energy is distributed in an uneven manner between the cells. This means that, from time to time, there will be a need for a cell balancing process wherein the differences regarding the state of charge for the battery cells are evened out.

A known way of balancing battery cells is to generate energy by conducting current through a resistor which is coupled in parallel to each battery cell. This can be done for a number of battery cells for which the state of charge is relatively high. To be able to carry out this cell balancing, the cell voltage needs to be kept at a relatively high energy level in the cell. This is due to the inherent properties of the measured cell voltage and the corresponding state of charge parameters, which have a clear detectable relation to each other only when the cell voltage is relatively high. Furthermore, it is known to use a sensor arrangement which is configured for measuring one or more parameters indicating the operation of the battery, for example in the form of a voltage sensor for each battery cell in order to measure the cell terminal voltage for each cell. Such a sensor arrangement could then be used for detecting various parameters related to the operation of the battery. This means that a cell balancing process can be initiated when the control unit 5 has detected that a given number of battery cells has a cell voltage and/or state of charge which differs from predetermined threshold values.

A problem which may occur during the above-mentioned cell balancing process is that the cell voltage for the battery cells in question may drop considerably during said procedure. This means also that there may be a loss of charge from the battery pack 4 during the cell balancing, i.e. the energy from the battery cells may be drained to a certain extent. This means that the cell voltage may drop so much that it will be outside the part of the voltage versus state of charge curve in which there is a clear and defined relationship between the voltage and state of charge.

For the above-mentioned reasons, and for solving the problem mentioned above, the invention is based on the concept that it comprises a process of controlling a relatively small current , as indicated in Fig. 2, which is supplied to the battery pack 4 during the balancing procedure. The current is supplied by means of the charging unit 9. This means that the loss of charge through the bleeder resistor of each battery cell in question can be limited. Also, the battery cell voltage drop will be limited during the cell balancing, which is an advantage.

In summary, according to the embodiment, the control unit 5 is adapted to control the charging unit 9 to feed a relatively low current i₁ to the battery pack 4 during the cell balancing. The supply of this relatively low current i₁ is suitably terminated when the cell balancing process is terminated.

According to an embodiment, the current i₁ is controlled by feeding it through the resistor 13 during the cell balancing process. More precisely, since the magnitude of the resistance of the resistor 13 is known, the current i₁ can be calculated by measuring the voltage. The resistor 13 can be a so-called pre-charge resistor which forms part of the battery pack 4 and is configured for a pre-charge function in which an inrush current from the charging unit 9 can be limited when the battery pack 4 is connected to the charging unit 9. In this manner, the resistor 13 can be used to pre-charge a system capacitance, symbolically indicated by means of the letter C in Fig. 2. This is needed to equalize the voltage between the charging unit 9 and the battery pack 4 for a reduction of an inrush current when the contactors 10, 1 1 are closed. However, the invention is not limited to arrangements comprising a pre-charge function but can be used also without such a function. In such case, the current i₁ is fed through a resistor which is not involved in any pre-charge function. In case a pre-charge function is used, the relatively small current i₁ is fed through the pre-charger resistor 13 for a certain time period, after which the pre- charge resistor 13 is disconnected by opening the pre-charge contactor 12. Next, the positive main contactor 10 is closed.

The relatively small current i₁ is fed through the resistor 13 during a phase when the battery cells cannot receive any substantial amount of charge. Subsequently, when both the positive main contactor 10 and the negative main contactor 1 1 have been closed, the charging unit 9 can be used for normal charging of the battery pack 4.

The relatively low current can be controlled since the battery pack voltage V_B applied by the charging unit 9 is known. Also, the resistance of the resistor 13 is known. According to an embodiment, the resistance is of the magnitude 100 ohms. This means that the current through the resistor 13 can be accurately controlled during the cell balancing process by detecting the voltage drop over the resistor 13 and calculating the current. Alternatively, a current sensor (not shown) for detecting the battery current can be used in order to control the relatively low current H .

According to an embodiment, the relatively low current i₁ should preferably be

approximately 100 mA. In any case, it should not exceed 200-300 mA. The actual magnitude of this current is chosen so as to compensate for the amount of discharge in the battery cells in question so that there is generally no voltage drop in the battery cells. Also, the control of the current through the resistor 13 is used until the charge and discharge ability of the battery pack 4 have reached above a predetermined limit. At this stage, an accurate control of the current is no longer needed and the positive main contactor 10 can be closed. Optionally, the magnitude can also be controlled to prevent damage to the battery cells in a relatively cold battery pack. By feeding a current through the resistor 13, the battery current is limited to a low level which cannot cause damage. Finally, the current can be kept at a relatively low level during the cell balancing process even if voltage variations in the electric system occur, for example in a situation where a load is connected or disconnected.

Fig. 3 is a schematic flow chart illustrating the operation of the invention. Initially, the control unit 5 is configured for charging the battery pack 4 by having the positive main contactor 10 and the negative main contactor closed 1 1 (step 20 as shown in Fig. 3). The control unit 5 is also configured for detecting whether to initiate a cell balancing process (step 21 ). This is done by detecting whether there are battery cells having a voltage level or a state of charge (SOC) which deviates from certain predetermined threshold values. It is known that battery packs degrade over time, and by diagnosing certain battery parameters such as the cell terminal voltage, the cell capacity and the cell state of charge, an indication of the state of operation of the battery pack 4 can be obtained.

If a cell balancing process is to be initiated, the charging unit 9 feeds the above- mentioned relatively low current to the battery pack (step 22). According to an embodiment, the current is controlled so as to compensate for a loss of charge which occurs in the battery pack 4 during the cell balancing process. Preferably, the current is fed through the resistor 13, by closing the pre-charge contactor 12 and opening the positive main contactor 10. The current can be supplied to the battery pack by gradually increasing the current feed. The control unit 5 is arranged to continuously measure whether the cell balancing process is considered to be finished, which is when the voltage over the battery cells involved in the cell balancing have reached a predefined voltage range. At this stage, the balancing is considered to be completed (step 23). According to the embodiment, the feeding of the current i₁ is then also terminated. Next, the pre-charge resistor 13 is disconnected, i.e. the pre-charge contactor 12 is opened, and the positive main contactor 10 is again closed (step 24).

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

For example, even though this disclosure refers to a vehicle 1 in the form of a bus, the invention can generally be implemented in virtually any type of vehicle which is operated by means of at least an electric machine. Such vehicles include cars, buses, trams, transport vehicles, construction equipment and trains.

Claims

1 . A method for controlling a current being fed to a battery pack (4) comprising a plurality of battery cells (4a, 4b, 4c) connected in series and providing an output battery voltage (V_B), said method being characterized in that it comprises the steps of:

- feeding a relatively low current ( ) through a resistor (13) being arranged in series with said battery pack (4); and

- measuring the voltage across said resistor (13), thereby obtaining a value of said current

2. Method according to claim 1 , further comprising:

- feeding said current ( ) during a phase when the charge and discharge ability of said battery pack (4) is below a predetermined limit.

3. Method according to claim 1 or 2, further comprising:

- initiating a process of balancing said battery pack (4) depending on the distribution of energy between said battery cells (4a, 4b, 4c);

- feeding said relatively low current ( ) to said battery pack (4) during said balancing; and

- terminating the step of feeding said relatively low current ( ) when said balancing process is terminated.

4. Method according to claim 3, further comprising:

- initiating said process of balancing of battery pack (4) by transferring energy between battery cells.

5. Method according to claim 3 or 4, further comprising:

- controlling the magnitude of said current ( ) so as to be adapted to compensate for a loss of electric charge in said battery cells (4a, 4b, 4c) during said cell balancing.

6. Method according to any one of the preceding claims, further comprising:

- feeding said current ( ) through the resistor (13) for limiting an inrush current when said battery pack (4) is connected to a charging unit (9).

7. Method according to any one of the preceding claims, further comprising: - controlling said current ( ) to a magnitude which does not exceed approximately 200- 300 mA.

8. A system for controlling a current being fed to a battery pack (4) comprising a plurality 5 of battery cells (4a, 4b, 4c) connected in series and providing an output battery voltage

(V_B), said system being characterized in that it comprises a control unit (5) which is adapted for feeding a relatively low current ( ) through a resistor (13), which is arranged in series with said battery pack (4), and for measuring the voltage across said resistor (13), thereby obtaining a value of said current ( ).

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9. System according to claim 8, wherein the control unit (5) is configured for feeding said current ( ) during a time period when the charge and discharge ability of said battery pack (4) is below a predetermined limit.

15 10. System according to claim 8 or 9, wherein the control unit (5) is configured for

initiating a process of balancing said battery pack (4) depending on the distribution of energy between said battery cells (4a, 4b, 4c), for feeding said relatively low current ( ) to said battery pack (4) during said balancing and for terminating the step of feeding said relatively low current ( ) when said balancing process is terminated.

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1 1 . System according to claim 10, wherein each battery cell (4a, 4b, 4c) is associated with a resistor coupled in parallel and being arranged to be selectively connected by means of a controllable switch during said cell balancing process.

25 12. System according to claim 10 or 1 1 , wherein said control unit (5) is adapted for

controlling the magnitude of said current ( ) to compensate for a loss of electric charge in said battery cells (4a, 4b, 4c) during said cell balancing.

13. System according to any one of claims 8-12, further comprising a charging unit (9) for 30 charging said battery pack (4) and for feeding said relatively low current ( ) to said battery pack (4).

14. System according to claim 13, wherein said current ( ) is fed through said resistor (13) for limiting an inrush current when said battery pack (4) is connected to said charging unit

35 (9).

15. System according to claim 13 or 14, wherein said battery pack (4) further comprises a main contactor (10) and that said control unit (5) is configured for closing the main contactor (10) and disconnecting said resistor (13) after feeding said current ( ).

5

16. A vehicle (1 ) comprising a system according to any one of claims 8-15.

17. A computer program comprising program code means for performing the steps of any of claims 1 -7 when said program is run on a computer.

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18. A computer readable medium carrying a computer program comprising program code means for performing the steps of any of claims 1 -7 when said program product is run on a computer.

15 19. A control unit (5) for controlling a balancing process in a battery pack (4) and being configured to perform the steps of the method according to any of claims 1 -7.

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