A METHOD FOR CONTROLLING ELECTRICAL CONNECTION OF BATTERY PACKS

Abstract

A method for controlling electrical connection of at least two battery packs (202, 203) of an energy storage System (200) of a vehicle (201) to a common load during operation of the vehicle, each one of the battery packs being connectable to the load via at least one respective switching device, the method comprising: —receiving measurement data relating to current operating conditions of the energy storage System, —based on at least the measurement data, estimating at least one battery state of each one of the battery packs, wherein said at least one battery state is at least one of an open circuit voltage and a state of charge, —based on the estimated at least one battery state of each one of the battery packs, controlling electrical connection of each battery pack to the load via the at least one respective switching device.

Claims

1. A method for controlling electrical connection of at least two battery packs of an energy storage system of a vehicle in parallel to a common load during operation of the vehicle, each one of the at least two battery packs being connectable to the load via at least one respective switching device, the method comprising: receiving measurement data relating to current operating conditions of the energy storage system, based on at least the measurement data, estimating at least one battery state of each one of the at least two battery packs, wherein said at least one battery state comprises at least a state of charge, based on at least the estimated state of charge and a measured temperature of the at least two battery packs, predicting a magnitude of a circulation current expected to flow between the at least two battery packs upon electrical connection of said at least two battery packs, based on the magnitude of the predicted circulation current, controlling electrical connection of each battery pack to the load via the at least one respective switching device, wherein the at least two battery packs are electrically connected only if the magnitude of the predicted circulation current is below a predefinable allowable circulation current magnitude.

2. The method according to claim 1, wherein controlling electrical connection of the at least two battery packs comprises: based on said estimated at least one battery state of each one of the at least two battery packs, determining if a predetermined connection condition is fulfilled, wherein the at least two battery packs are only electrically connected if the connection condition is considered to be fulfilled.

3. The method according to claim 2, wherein the connection condition is considered to be fulfilled if, for connection of a first and a second battery pack of the at least two battery packs, the at least one battery state of the first battery pack does not differ by more than a predefinable threshold from the at least one battery state of the second battery pack.

4. The method according to claim 3, wherein a value of the predefinable threshold is selected in dependence on a state of charge operating point of at least one of said battery packs that is already connected to the load.

5. (canceled)

6. (canceled)

7. The method according to claim 1, wherein the magnitude of the circulation current is predicted using a multi-battery prediction model.

8. (canceled)

9. The method according to claim 1, wherein at least a first battery pack of said at least two battery packs is already connected to the load, and wherein at least a second battery pack of said at least two battery packs is disconnected from the load, wherein controlling electrical connection of the at least two battery packs comprises: based on the estimated at least one battery state of at least the first and the second battery packs, determining whether the second battery pack may be connected to the load while the first battery pack remains connected to the load.

10. The method according to claim 1, further comprising: selecting a control strategy for controlling electrical connection of the at least two battery packs based on at least one of: operating conditions of the energy storage system, a usage scenario of the energy storage system, uncertainties in the estimation of the at least one battery state, and an uncertainty in a multi-battery prediction model used to predict a circulation current expected to flow between the at least two battery, packs upon electrical connection of the at least two battery packs, wherein controlling electrical connection of the at least two battery packs is further performed based on the selected control strategy.

11. A computer program comprising program code means for performing the method according to claim 1 when said computer program is run on a computer.

12. A computer readable medium carrying a computer program comprising program code means for performing the method according to claim 1 when said program code means is run on a computer.

13. A control unit for controlling electrical connection of at least two battery packs of an energy storage system of a vehicle to a common load during operation of the vehicle, the control unit being configured to perform the method according to claim 1.

14. An energy storage system of a vehicle, the energy storage system comprising: at least two battery pack combinations connected in parallel, each battery pack combination comprising a battery pack and at least one switching device connected in series, each battery pack being connectable to a load via the at least one switching device, at least one control unit configured to: based on measurement data relating to current operating conditions of the energy storage system, estimate at least one battery state of each one of the battery packs, wherein said at least one battery state comprises at least a state of charge, based on at least the estimated state of charge and a measured temperature of the at least two battery packs, predict a magnitude of a circulation current expected to flow between the at least two battery packs upon electrical connection of said at least two battery packs, based on the magnitude of the predicted circulation current, control electrical connection of each battery pack to the load via the at least one respective switching device, wherein the at least one control unit is configured to electrically connect the battery packs only if the magnitude of the predicted circulation current is below a predefinable allowable circulation current magnitude.

15. A vehicle such as a hybrid vehicle of a fully electrified vehicle, comprising an energy storage system according to claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0048] In the drawings:

[0049] FIG. 1 shows a vehicle in which a method according to embodiments of the invention may be implemented,

[0050] FIG. 2 shows parts of an energy storage system according to an embodiment of the invention, and

[0051] FIG. 3 is a flow chart illustrating a method according to an embodiment of the invention.

[0052] The drawings are schematic and not necessarily drawn to scale.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0053] In the present detailed description, various embodiments of the method according to the present invention are mainly described with reference to an all-electric bus 201 as shown in FIG. 1, comprising a propulsion system in the form of battery powered electric motors. However, it should be noted that various embodiments of the described invention are equally applicable for a wide range of hybrid and electric vehicles.

[0054] The bus 201 carries an electric energy storage system (ESS) 200 comprising a first battery pack 202 and a second battery pack 203, each battery pack comprising a plurality of battery cells. The battery cells are connected in series to provide an output DC voltage having a desired voltage level. Suitably, the battery cells are of lithium-ion type, but other types may also be used. The number of battery cells per battery pack may be in the range of 50 to 500 cells. It is to be noted that the ESS may also include more than two battery packs.

[0055] A sensor unit (not shown) may be arranged for collecting measurement data relating to operating conditions of the ESS, i.e. measuring temperature, voltage and current level of the associated battery pack 202. Measurement data from each sensor unit is transmitted to an associated battery management unit (BMU) 204, which is configured for managing the individual battery pack 202 during operation of the bus 201. The BMU 204 can also be configured for determining parameters indicating and controlling the condition or capacity of the battery pack 202, such as the state of charge (SOC), the state of health (SOH), the state of power (SOP) and the state of energy (SOE) of the battery pack 202. Herein, only one BMU 204 is shown, but as mentioned above, each battery pack preferably has its own associated BMU.

[0056] The BMU 204 is connected to and configured to communicate with an ESS control unit 208, which controls the ESS. The ESS control unit 208 may include a microprocessor, a microcontroller, a programmable digital signal processor or another programmable device. Thus, the ESS control unit 208 comprises electronic circuits and connections (not shown) as well as processing circuitry (not shown) such that the ESS control unit 208 can communicate with different parts of the bus 201 or with different control units of the bus 201. The ESS control unit 208 may comprise modules in either hardware or software, or partially in hardware or software, and communicate using known transmission buses such a CAN-bus and/or wireless communication capabilities. The processing circuitry may be a general purpose processor or a specific processor. The ESS control unit comprises a non-transitory memory for storing computer program code and data. Thus, the skilled person realizes that the ESS control unit may be embodied by many different constructions. This is also applicable to the BMU 204.

[0057] Turning now to FIG. 2, a schematic diagram of parts of the ESS 200 is shown. Each of the battery packs 202, 203 of the ESS 200 form part of a respective battery pack combination 210, 211, connected in parallel to each other. Each battery pack combination 210, 211 comprises, apart from the battery pack 202, 203, a respective switching device 205, 206 connected in series with the respective battery pack 202, 203. Such switching devices may also be referred to as battery disconnect units (BDU:s). The battery packs 202, 203 are connectable to a load 209, which may e.g. be an electric machine for propulsion of the vehicle 201, via the respective switching devices 205, 206. The switching devices 205, 206 are movable between an ON state in which the respective battery pack 202, 203 is connected to the load 209, and an OFF state in which the respective battery pack 202, 203 is not connected to the load 209. In order for the two battery packs 202, 203 to be electrically connected to each other, both respective switching devices 205, 206 need to be in the ON state. One or more control unit(s) (not shown) is/are configured to communicate with the respective battery packs 202, 203 and control switching of the switching devices 205, 206.

[0058] A method for controlling electrical connection of the two battery packs 202, 203 of the ESS 200 to the load 209 according to an embodiment of the invention is illustrated in the flow diagram in FIG. 3. It is to be noted that the method may of course also be used for connecting more than two battery packs. The method may be carried out by the control unit 208, but the method may also be carried out by another control unit, or different steps of the method may be carried out in different control units. For example, some steps may be carried out in the BMU:s 204.

[0059] In a first step 101, measurement data relating to current operating conditions of the energy storage system 200 are received from the sensor unit. The measurement data may include battery current I, terminal voltage V and battery temperature T of each battery pack. The measurement data may be filtered measurement data, from which noise has been removed.

[0060] In a second step 102, based on at least the received measurement data, at least one battery state of each one of the battery packs is estimated. The at least one battery state is at least one of an open circuit voltage (OCV) and a state of charge (SOC), i.e. one or both of the OCV and the SOC of the battery packs 202, 203 may be estimated. A plurality of different methods exist for estimating SOC and OCV on the basis of measurement data relating to operating conditions. For example, estimators such as non-linear estimators, e.g. some type of Kalman filters, and variants of recursive nonlinear observers, may be used. The SOC and OCV may also be estimated using optimization based estimation schemes, e.g. a moving horizon estimation, total least squares, recursive-least-squares, etc.

[0061] In a third step 103, based on the estimated at least one battery state of each one of the battery packs 202, 203, i.e. OCV and/or SOC, electrical connection of each battery pack 202, 203 to the load 209 via the respective switching devices 205, 206 is controlled. For example, a signal may be sent to the respective switching device 205, 206 that it is to be set to the ON state or to the OFF state, depending on the estimated OCV and/or SOC of the respective battery pack 202, 203.

[0062] The battery packs 202, 203 may e.g. be electrically connected only if a predetermined connection condition is considered to be fulfilled. The predetermined connection condition may for example be defined so that it is considered to be fulfilled if the estimated SOC and/or OCV of the first battery pack 202 does not differ by more than a predefined threshold from the estimated SOC and/or OCV of the second battery pack 203. If the first battery pack 202 is already connected to the load 209, the predefined threshold may be selected in dependence on a current SOC operating point of the first battery pack 202. Different thresholds may thereby be selected for different SOC operating points. A look-up table or map may be used, describing a relation between the battery state, e.g. the SOC, and a circulation current likely to arise between the battery packs 202, 203 upon connection of the second battery pack 203 to the load 209, for different SOC operating points.

[0063] In an exemplary embodiment, an optional fourth step 104 of predicting a magnitude of a circulation current expected to flow between the battery packs 202, 203 upon electrical connection of the battery packs is carried out. The prediction is based on at least the estimated SOC, as estimated in step 102, state of health (SOH), estimated based on state of resistance (SOR) and capacity (SOQ), and a measured temperature. A dynamic multi-battery prediction model may be used to predict the magnitude of the circulation current. The step 103 of controlling electrical connection of the battery packs 202, 203 may in this case be performed based on the predicted magnitude of the circulation current. For example, the connection condition may be set so that it is considered fulfilled only if the magnitude of the predicted circulation current is below a predefined allowable circulation current magnitude, i.e. a circulation current threshold.

[0064] A dynamic state-space model of a parallel multi-battery pack system, derived mainly using single battery models and exploiting parallel connection constraints, may be used as the multi-battery prediction model:

{dot over (x)}(t)=A.sub.I(t).Math.x(t)+B.sub.I(t).Math.I.sub.dem(t)

y(t)=C.sub.I(t).Math.x(t)+D.sub.I(t).Math.I.sub.dem(t)

[0065] Herein, full state of the complete ESS 200 is represented by x=[x.sub.1 . . . x.sub.n].sup.T, wherein a state of each constituent battery pack BP, of the ESS is represented by x.sub.i=[V.sub.1i V.sub.2i OCV.sub.i SOC].sup.T, T herein denoting vector transpose. The output of the system is represented by y=[I.sub.bi . . . I.sub.bn].sup.T, where I.sub.bi is the output current of each BP.sub.i. The control input of this state-space model is the total demanded input current I.sub.dem. The model matrices A.sub.I, B.sub.I, C.sub.I, and D.sub.I are nonlinear functions of system parameters (R.sub.01, R.sub.1i, R.sub.2i, C.sub.1i, C.sub.2i, Q.sub.bi, KR.sub.i−1,i, KR.sub.i) and system electro-thermal and ageing states (SOC.sub.i, T.sub.bi, SOQ.sub.i, SOR.sub.i).

[0066] To save computational power, the step 104 of predicting the circulation current may only need to be carried out if the estimation of the battery state(s) of the battery packs 202, 203, carried out in step 102, reveals that there is a non-negligible difference between the estimated values of corresponding battery states of the battery packs, e.g. if the estimated SOC of the first battery pack 202 differs significantly from the estimated SOC of the second battery pack 203.

[0067] It is also possible to perform an optional fifth step 105 of selecting a control strategy for controlling electrical connection of the battery packs 202, 203. The control strategy may be selected based on operating conditions of the ESS 200, a usage scenario of the ESS 200, uncertainties in the estimation of the at least one battery state, and an uncertainty in the multi-battery prediction model used to predict the circulation current expected to flow between the battery packs 202, 203 upon electrical connection thereof. The step 103 of controlling electrical connection of the battery packs 202, 203 may be performed based on the selected control strategy.

[0068] The control strategy may be to control the electrical connection of the battery packs 202, 203 based on SOC or OCV thresholding, or based on the magnitude of a predicted circulation current, or based on a combination of those. For example, the connection condition may be set in dependence on the control strategy.

[0069] Although the figures may show a sequence, the order of the steps may differ from what is depicted. Also two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various connection steps, processing steps, comparison steps and decision steps. Additionally, even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art.

[0070] The control functionality of the example embodiments may be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwire system. Embodiments within the scope of the present disclosure include program products comprising machine-readable medium for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions.

[0071] 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.