A METHOD FOR ESTIMATING A BATTERY STATE

Abstract

A method for estimating a battery state of an energy storage system (200) of a vehicle (201) using at least a dual estimator (301), wherein the energy storage system comprises a battery unit (202), and wherein the dual estimator comprises:—a first estimator configured to estimate said battery state using a prediction model, the first estimator having first tuning parameters, —a second estimator configured to track changes around nominal values of a set of battery parameters, the second estimator having second tuning parameters, e a feedback loop being provided between the first and second estimators, the method comprising:—obtaining operational data of the energy storage system, —feeding the operational data to the estimators, —using the first estimator, estimating the battery state, —using g the second estimator, estimating changes around the nominal values of the set of battery parameters, and at least one of:—adaptively adjusting the first and/or the second tuning parameters to account for fast timescale variations due to varying operating conditions, and—adaptively adjusting the nominal values of the set of battery parameters to account for slow timescale variations arising due to ageing.

Claims

1. A method for estimating a battery state of an energy storage system of a vehicle using at least a dual estimator, wherein the energy storage system comprises at least one battery unit, and wherein the dual estimator comprises: a first estimator comprising a first non-linear observer, the first estimator being configured to estimate at least one battery state of the energy storage system using a prediction model, the first estimator having first tuning parameters, a second estimator comprising at least one second non-linear observer, the second estimator being configured to track changes around nominal values of a set of battery parameters of the energy storage system, the second estimator having second tuning parameters, a first feedback loop being provided between the first and second estimators so that output from the first estimator is used as input to the second estimator and vice versa, the method comprising the steps of: obtaining operational data of the energy storage system by collecting measurement data relating to operating conditions of the energy storage system, feeding the obtained operational data to the first and the second estimators, respectively, using the first estimator, estimating the at least one battery state based on at least the obtained operational data and output from the second estimator, using the second estimator, estimating changes around the nominal values of the set of battery parameters based on at least the obtained operational data and output from the first estimator, wherein the method further comprises the steps of: adaptively adjusting at least one of the first and the second tuning parameters to account for fast timescale variations of the at least one battery state and/or the set of battery parameters arising due to varying operating conditions of the energy storage system, and adaptively adjusting the nominal values of the set of battery parameters to account for slow timescale variations arising due to ageing of the at least one battery unit.

2. The method according to claim 1, wherein the step of adaptively adjusting at least one of the first and the second tuning parameters is carried out based on at least the obtained operational data.

3. The method according to claim 1, wherein the second estimator comprises a plurality of said second non-linear observers wherein estimating changes around the nominal values of the set of battery parameters comprises using each one of said second non-linear observers to estimate the change around the nominal value of at least one battery parameter from said set of battery parameters.

4. The method according to claim 3, wherein at least one of said second non-linear observers is used to estimate the changes around the nominal values of at least two of said battery parameters varying at the same timescale.

5. The method according to claim 1, wherein the step of estimating changes around the nominal values of the set of battery parameters comprises: determining whether the obtained operational data fulfil a defined sensitivity criterion with respect to a sensitivity of the obtained operational data to each of the battery parameters to be estimated, for each battery parameter for which the sensitivity criterion is fulfilled, estimating a value of said battery parameter based on the obtained operational data and on output from the first estimator, for each battery parameter for which the sensitivity criterion is not fulfilled, setting a value of said battery parameter to its present value.

6. The method according to claim 1, wherein the step of adaptively adjusting the nominal values of the set of battery parameters to account for slow timescale variations comprises using a third estimator comprising a third non-linear observer, wherein a second feedback loop is provided between the dual estimator and the third estimator, so that output from the dual estimator is used as input to the third estimator and vice versa.

7. The method according to claim 6, wherein the step of adaptively adjusting the nominal values of the set of battery parameters to account for slow timescale variations comprises the steps of: storing the obtained operational data from measurements made during at least a defined time horizon to provide a historical data set, providing output from the dual estimator to the third estimator, using the third estimator, estimating the nominal value of at least one of the battery parameters based on the historical data set and on the output from the dual estimator, outputting the estimated nominal value of the at least one battery parameter from the third estimator to the dual estimator.

8. The method according to claim 7, further comprising downsampling of said output from the dual estimator, wherein said nominal value is estimated based on the downsampled output.

9. The method according to claim 6, wherein the estimation of said nominal value using the third estimator is carried out at a lower frequency than the estimations using the dual estimator, such as at a pre-determined lower frequency or after a pre-determined time interval or upon fulfilment of a defined condition.

10. The method according to claim 6, wherein the third estimator uses an optimization based estimation scheme, such as a moving horizon estimation model.

11. The method according to claim 1, wherein obtaining operational data of the energy storage system further comprises pre-filtering of the collected measurement data to remove high-frequency noise.

12. The method according to claim 1, wherein at least one of the non-linear observers is a Kalman filter, or a recursive least square method based observer, or a particle filter, or a sliding-mode observer.

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

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

15. A control unit configured to perform the method according to claim 1.

16. A battery management system for an energy storage system comprising the control unit according to claim 15.

17. A vehicle, such as a hybrid vehicle or a fully electrified vehicle, comprising an energy storage system and a control unit according to claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0056] In the drawings:

[0057] FIG. 1 shows a vehicle in which a method according to the invention may be implemented,

[0058] FIG. 2 is a flow-chart illustrating a method according to an embodiment of the invention,

[0059] FIG. 3 is a block diagram illustrating an embodiment of the invention,

[0060] FIG. 4 is another block diagram illustrating steps of a method according to an embodiment of the invention,

[0061] FIG. 5 is another block diagram illustrating steps of a method according to an embodiment of the invention.

[0062] The drawings are schematic and not necessarily drawn to scale.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0063] 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, 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.

[0064] FIG. 1 shows a simplified perspective view of an all-electric vehicle in the form of a bus 201, which according to an embodiment is equipped with an electric machine (not shown) for operating the bus.

[0065] The bus 201 carries an electric energy storage system (ESS) 200 comprising a battery pack 202, the 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 include a plurality of battery packs.

[0066] 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.

[0067] 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.

[0068] A method for estimating a battery state of an ESS such as the one illustrated above according to an embodiment of the invention is schematically illustrated in FIG. 2. The steps are carried out repeatedly, such as at a certain time interval, but may not necessarily be carried out in the order shown in FIG. 2. The method steps may in this embodiment be carried out in the ESS control unit 208 or in the BMU 204. Reference is also made to FIG. 3, which is a block model illustrating an implementation of a method according to an embodiment of the invention.

[0069] In a first step 101, operational data of the energy storage system are obtained by collecting measurement data relating to operating conditions of the energy storage system, i.e. temperature T, voltage V and current level I of the battery pack 202 of the ESS 200.

[0070] In a second step 102, the obtained operational data are fed to a first estimator 302 and a second estimator 303, respectively, forming part of a dual estimator 301. The first estimator 302 comprises a first non-linear observer, such as a Kalman filter or a variant thereof, configured to estimate the at least one battery state x of the ESS using a prediction model, the first estimator 302 having first tuning parameters. The second estimator 303 comprises at least one second non-linear observer, such as a Kalman filter or a variant thereof, and is configured to track relatively fast changes around nominal values of a set of battery parameters θ of the ESS, said changes arising due to changes in operating conditions of the ESS. The second estimator 303 has second tuning parameters and may e.g. assume a random walk model for dynamics of the battery parameters.

[0071] A first feedback loop is provided between the first and second estimators so that output from the first estimator 302 is used as input to the second estimator 303 and vice versa.

[0072] In a third step 103, using the first estimator 302, the at least one battery state is estimated based on at least the obtained operational data and output from the second estimator 303 produced in a fourth step 104. The battery state may e.g. be a state of charge (SoC) and/or a state of internal voltage losses, or states relating to those.

[0073] In the fourth step 104, using the second estimator 303, changes around the nominal values of the set of battery parameters are estimated based on at least the obtained operational data and output from the first estimator produced in the third step 103. The set of battery parameters may herein be the parameters of the prediction model used by the first estimator. In case an equivalent circuit model is used, the battery unit may be modelled as an ohmic resistor having an internal resistance R.sub.0, and a series of n resistor-capacitor (RC) sub-circuits having resistances R.sub.i and capacitances C.sub.i, wherein i=1, . . . , n, n being an integer number, e.g. n=2. Time constants of the RC branches may be expressed as τ.sub.i=R.sub.i(t)C.sub.i(t). The set of battery parameters are in this case described by a time dependent vector θ(t)=[R.sub.0(t), R.sub.i(t), C.sub.i(t), τ.sub.i(t)] and the battery state SoC is described by a time dependent vector x(t)=[V.sub.i(t), z(t)], V.sub.i(t) being the voltage drop across each RC branch, i.e. internal voltage losses, and z(t)=η/Q*I(t), where η is the battery's coulombic efficiency, Q is battery's total capacity, and I(t) is the current supplied to the battery. The time dependent parameter vector θ(t) may also include Q(t) and OCV(t).

[0074] The third step 103 and the fourth step 104 are carried out repeatedly, such as at a predetermined frequency, and output from the first estimator 302 is fed to the second estimator 303 and vice versa.

[0075] In a fifth step 105, at least one of the first and the second tuning parameters is adaptively adjusted to account for fast timescale variations of the at least one battery state and/or the set of battery parameters, said fast timescale variations arising due to varying operating conditions of the energy storage system. The main tuning parameters using Kalman filters as the first and second non-linear observers are the covariance matrices, including the state process noise covariance Σ.sub.w and the measurement noise covariance Σ.sub.v, and the parameter process noise Σ.sub.r. Adaptively adjusting the tuning parameters may thus comprise adapting the covariance matrices at every time step of the method.

[0076] In a sixth step 106, the nominal values of the set of battery parameters, such as the nominal values Q*, R*, OCV* of the capacity Q, the resistance R and the open circuit voltage OCV, are adaptively adjusted to account for slow timescale variations arising due to ageing of the at least one battery unit, corresponding to an estimation of the battery SoH. This may comprise using a third estimator 307 comprising a third non-linear observer, wherein a second feedback loop is provided between the dual estimator 301 and the third estimator 307, so that output from the dual estimator 301 is used as input to the third estimator 307 and vice versa. The sixth step 106 may comprise the sub-steps of: [0077] storing the obtained operational data from measurements made during at least a defined time horizon to provide a historical data set, [0078] providing output from the dual estimator 301 to the third estimator 307, [0079] using the third estimator 307, estimating the nominal value of at least one of the battery parameters based on the historical data set and on the output from the dual estimator 307, [0080] outputting the estimated nominal value of the at least one battery parameter from the third estimator 307 to the dual estimator 301.

[0081] The sub-steps may not necessarily be carried out in this order.

[0082] The sixth step 106 may also comprise the sub-step of downsampling of said output from the dual estimator 301, wherein said nominal value is estimated based on the downsampled output. Downsampling may be achieved by averaging of x(t) and θ(t) over certain sub-intervals within a defined time horizon. These sub-intervals are defined by splitting a given time horizon into various segments.

[0083] The method may comprise either one of the fifth and sixth steps 105, 106, or preferably both of the fifth and sixth steps.

[0084] FIG. 3 is a block model illustrating an implementation of a method according to an embodiment of the invention. A dual estimator 301 as described above is provided, comprising a first estimator 302 configured to carry out the third step 103 and a second estimator 303, configured to carry out the fourth step 104. The dual estimator 301 further comprises an adaptation block 308 for carrying out the fifth step 105. In addition to the dual estimator 301, an ageing block, or SoH estimation block, in the form of a third estimator 307 is provided, configured to carry out the sixth step 106.

[0085] As measurement data relating to current I, voltage V and temperature T are collected, the data are fed to the dual estimator 301 and used by the first estimator 302 as well as by the second estimator 303. The first non-linear observer of the first estimator 302 is used to estimate values of the state vector x(t) base on the equivalent circuit model, while the second non-linear observer of the second estimator 303 is used to track fast changes around the nominal values Q*, R*, OCV* of the capacity Q, the resistance R and the open circuit voltage OCV using a random walk model. The state vector x(t) is continuously fed from the first estimator 302 to the second estimator 303, while the parameter vector θ(t) is fed from the second estimator 303 to the first estimator 302. The adaptation block 308 adaptively adjusts the tuning parameters of the first and second non-linear observers, respectively, based on the measurement data. This is typically carried out using stochastic analysis of measurement residuals, i.e. based on differences between the measurement data and predicted values of V and T as based on the equivalent circuit model.

[0086] The dual estimator 301 outputs data including the parameter vector θ(t) and the state vector x(t) to the third estimator 307 at a certain frequency or time interval. Generally, data is fed to the third estimator 307 at a much lower frequency than an updating frequency of the dual estimator 301 itself. The output from the dual estimator 301 may also be downsampled prior to feeding to the third estimator 307, or alternatively inside the third estimator 307. Furthermore, the measurement data relating to current I, voltage V and temperature T are also fed to the third estimator 307 and historical measurement data from measurements made during a defined time horizon are stored as a historical data set, i.e. a trajectory. The trajectory together with the parameter vector θ(t) and the state vector x(t) received from the dual estimator 301 are in the third estimator 307 used by an optimization based estimation scheme, such as a moving horizon estimation model, to estimate nominal values Q*, R*, OCV* of the capacity Q, the resistance R and the open circuit voltage OCV, respectively. The estimated nominal values are fed back to the dual estimator 301.

[0087] FIG. 4 is a block model illustrating an embodiment of the second estimator 303. A pre-filtering block 309 is provided, to which the measurement data relating to I, V, and T are fed. In the pre-filtering block, the collected measurement data are filtered to remove high-frequency noise. Filtered operational data relating to I, V, and T, and also the estimated internal state vector x(t) from the first estimator 302, or elements thereof such as the internal voltage losses V.sub.i, are thereafter fed to a data classification and selection block 310, which will be explained in further detail below. As explained above, the second estimator comprises at least one second non-linear observer illustrated by a block 311. Data relating to the internal state as a function of time, x(t), are provided from the first estimator 302 to the second non-linear observer 311, wherein values of the parameter vector θ(t)=[R.sub.0(t), R.sub.i(t), C.sub.i(t), τ.sub.i(t)], or rather fast changes around nominal values of these, are estimated. The estimated values are output to a smoothing block 312, in which e.g. low pass filtering, moving average filtering, or Kalman filtering, is used to smooth the trajectories and thereby improve the quality of all parameter estimates, for example an improved estimated resistance R.sub.est(t).

[0088] FIG. 5 is a block model illustrating in further detail an embodiment of the second estimator 303, wherein the second estimator 303 comprises a plurality of non-linear second observers 304, 305, 306. In this case, the data classification and selection block 310 comprises a data sensitivity checking block 313, in which it is determined whether the obtained operational data from the pre-filtering block 309 and the estimated internal state vector x(t) obtained from the first estimator 302, or elements thereof such as the internal voltage losses V.sub.i, fulfil a defined sensitivity criterion with respect to a sensitivity of the obtained operational data and of the estimated internal state vector x(t), or elements thereof such as the internal voltage losses V.sub.i, to each of the battery parameters to be estimated. The sensitivities may for example be obtained online by computing partial derivatives (so-called Jacobians) of the operational data and the internal voltage losses V.sub.i with respect to each parameter. The sensitivity check may then, for example, be performed by testing the obtained sensitivities against pre-defined suitable thresholds.

[0089] The set of battery parameters are thereafter partitioned into n+1 data subsets, wherein n is the number of RC branches: a first subset 314 comprising data for which the sensitivity criterion with respect to the internal resistance R.sub.0 is fulfilled, a second subset 315 comprising data for which the sensitivity criterion with respect to the parameters of the first RC-branch is fulfilled, a third subset 316 for which the sensitivity criterion with respect to the parameters of the second RC-branch is fulfilled, and so on. Of course, the number of subsets may vary and different criteria may be used to partition data into subsets. For example, one or two different subsets may be used for parameters of each RC branch. In case of a model having two RC branches, as illustrated in FIG. 5, one subset may be used for the static resistance R.sub.0, one of the time dependent parameters R.sub.1 and τ.sub.1 of the first RC branch, and another one for the time dependent parameters R.sub.2 and τ.sub.2 of the first RC branch. Generally, it is useful to partition data such that data sensitive to battery parameters varying on different timescale are stored in different subsets.

[0090] For each battery parameter for which the sensitivity criterion is fulfilled, a value of said battery parameter is estimated based on the obtained operational data and on output from the first estimator 302, and based on the current nominal value of the battery parameter. The observer 304 is used to estimate the internal static resistance R.sub.0 using data from the first subset 314, the observer 305 is used to estimate the resistance R.sub.1 and the time constant τ.sub.1=R.sub.1(t)C.sub.1(t) of the first RC-branch using data from the second subset 315, and the observer 306 is used to estimate the resistance R.sub.2 and the time constant τ.sub.2=R.sub.2(t)C.sub.2(t) of the second RC-branch using data from the third subset 316. Smoothing of the respective parameter trajectories is carried out using smoothing blocks 317, 318, 319, obtaining estimated time dependent functions R.sub.0, est(t), R.sub.1, est(t), τ.sub.est(t), R.sub.2, est(t), τ.sub.2, est(t), forming part of the parameter vector θ(t).

[0091] The battery state, e.g. SoC or SoH as estimated by the method may be used to e.g. predict the amount of power that the ESS is able to deliver during the coming time period, such as during the coming 30 s, or to predict the remaining driving range of the ESS, or to predict when the ESS or battery unit(s) thereof need(s) to be replaced. The estimated battery state may e.g. be communicated to an electronic control unit of the vehicle, such as an engine control unit (ECU). The drivetrain of the vehicle may be controlled based on the mentioned predictions.

[0092] 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.

[0093] 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.