METHOD FOR CONTROLLING ELECTRICAL CONNECTION OF BATTERY PACKS

Abstract

A method for controlling electrical connection of a plurality of battery packs of an electric energy storage system of a vehicle to a load during operation of the vehicle, the plurality of battery packs being configured to be selectively connected in parallel to the load. Based on at least an operational mode of the electric energy storage system and on the voltage of each one of the battery packs, and by allowing simultaneous connection of at least two battery packs to the load, proposing a connection sequence for electrically connecting at least a subset of the plurality of battery packs to the load. Prior to connecting at least the subset of battery packs to the load, and based on at least an internal resistance of each one of the battery packs within at least the subset of battery packs, determining whether the proposed connection sequence fulfils a predetermined connection condition.

Claims

1. A method for controlling electrical connection of a plurality of battery packs of an electric energy storage system to a load, the plurality of battery packs being configured to be selectively connected in parallel to the load, the method comprising the steps of: obtaining operational data relating to present operating conditions of the electric energy storage system, wherein the operational data include at least a voltage of each one of the plurality of battery packs; based on at least an operational mode of the electric energy storage system and on the voltage of each one of the battery packs, and by allowing simultaneous connection of at least two battery packs to the load, proposing a connection sequence for electrically connecting at least a subset of the plurality of battery packs to the load; electrically connecting at least the subset of battery packs to the load in accordance with the proposed connection sequence; wherein the method further comprises the step of: prior to connecting at least the subset of battery packs to the load, and based on at least an internal resistance of each one of the battery packs within at least the subset of battery packs, determining whether the proposed connection sequence fulfils a predetermined connection condition, wherein the step of connecting at least the subset of battery packs to the load is only performed if the connection condition is considered to be fulfilled.

2. The method according to claim 1, wherein, if the connection condition is not considered to be fulfilled, at least the steps of proposing a connection sequence and determining whether the proposed connection sequence fulfils the predetermined connection condition are repeated until the connection condition is considered to be fulfilled.

3. The method according to claim 1, wherein, if the connection condition is not considered to be fulfilled for any proposed connection sequence involving simultaneous connection of at least two battery packs, the method comprises: subject to a predefinable power constraint, connecting a single battery pack of the plurality of battery packs to the load; optionally, determining an earliest point in time at which the steps of proposing a connection sequence and determining whether the proposed connection sequence fulfils the predetermined connection condition will be repeated after connection of the single battery pack.

4. The method according to claim 1, wherein the predetermined connection condition comprises at least a predetermined circulation current connection condition relating to a circulation current expected to flow between the battery packs upon electrical connection of said subset of battery packs, and optionally a predetermined state-of-power connection condition relating to a total state-of-power of the electric energy storage system after electrical connection of said subset of battery packs in accordance with the proposed connection sequence, and/or optionally a predetermined state-of-energy connection condition relating to a total state-of-energy of the electric energy storage system after electrical connection of said subset of battery packs in accordance with the proposed connection sequence.

5. The method according to claim 4, wherein the step of determining whether the proposed connection sequence fulfils the predetermined connection condition comprises at least: based on at least the internal resistance and an open circuit voltage of each one of the battery packs, predicting a magnitude of the circulation current expected to flow between the battery packs upon electrical connection of said subset of battery packs in accordance with the proposed connection sequence, determining whether the predicted magnitude of the circulation current is within a predetermined allowable range.

6. The method according to claim 4, wherein the predetermined connection condition comprises the predetermined state-of-power connection condition, and wherein the step of determining whether the proposed connection sequence fulfils the predetermined connection condition comprises: determining a state-of-power, SoP, of each one of the battery packs within the subset of battery packs, determining whether a difference in state-of-power between the battery packs within the subset is below a predetermined state-of-power difference threshold.

7. The method according to claim 1, wherein the step of proposing a connection sequence comprises: identifying the subset of battery packs to connect, and proposing a point in time at which each battery pack within the subset should be connected, such as simultaneously with and/or subsequently to the other battery packs within the subset.

8. The method according to claim 1, further comprising: setting a prioritization strategy for electrical connection of the battery packs to the load; wherein the set prioritization strategy is taken into account in the step of proposing said connection sequence.

9. The method according to claim 8, wherein the set prioritization strategy is one of a first prioritization strategy, a second prioritization strategy, and a third prioritization strategy, wherein: using the first prioritization strategy, the connection sequence is proposed so as to maximize the state-of-power of the electric energy storage system, using the second prioritization strategy, the connection sequence is proposed so as to minimize the total number of connection instants at which at least one battery pack will be connected to the load, and using the third prioritization strategy, the connection sequence is proposed so as to maximize the state-of-energy of the electric energy storage system.

10. The method according to claim 8, wherein the predetermined connection condition is set in dependence on the selected prioritization strategy.

11. The method according to claim 1, further comprising: repeating the method steps until all battery packs of the electric energy storage system are connected.

12. The method according to claim 1, further comprising: after connection of at least the subset of battery packs to the load, determining whether a predetermined disconnection condition is fulfilled for disconnection of at least one battery pack within the subset of battery packs, only if the predetermined disconnection condition is fulfilled, disconnecting the at least one battery pack from the load.

13. A control unit of an electric energy storage system comprising at least two battery packs configured to be selectively electrically connected in parallel, wherein the control unit is configured to execute the steps of the method according to claim 1.

14. A computer program comprising instructions to cause a computer to execute the steps of the method according to claim 1.

15. A computer readable medium having stored thereon the computer program according to claim 14.

16. An electric energy storage system comprising at least two battery packs configured to be selectively electrically connected in parallel and a control unit according to claim 13.

17. A vehicle comprising an electric energy storage system according to claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0071] In the drawings:

[0072] FIG. 1 shows a vehicle in which a method according to the invention may be implemented,

[0073] FIG. 2 is an example embodiment of an electric energy storage system according to the invention;

[0074] FIG. 3 is a flow-chart illustrating a method according to an embodiment of the invention,

[0075] FIG. 4 is a block diagram illustrating a method according to an embodiment the invention,

[0076] FIG. 5 is a diagram illustrating connection sequences proposed in accordance with an example embodiment, and

[0077] FIG. 6 is another diagram illustrating connection sequences proposed in accordance with another example embodiment.

[0078] The drawings are schematic and not necessarily drawn to scale.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0079] In the present detailed description, 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.

[0080] FIG. 1 shows a simplified perspective view of an all-electric vehicle in the form of a bus 100, which according to an embodiment is equipped with an electric propulsion unit 4 for propulsion of the bus. Of course, other loads may be provided in addition to or instead of the electric propulsion unit 4, for example auxiliary systems requiring electric power, and/or an on-board charger, and/or a power take-off.

[0081] The bus 100 carries an electric energy storage system (ESS) 1 comprising a plurality of parallel-connected battery packs 2, each battery pack 2 comprising a plurality of battery cells (not shown). 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, or up to many thousands of cells in the case of small format cells. It is to be noted that each battery pack may include a plurality of battery modules, in turn comprising a plurality of battery cells in the form of e.g. battery cell strings.

[0082] Sensor units (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 battery cells. Measurement data from each sensor unit is transmitted to an associated ESS control unit 3, which is configured for managing the ESS 1 during operation of the bus 100 and which is in this case also used for controlling connection of individual battery packs 2 of the ESS 1 to the load 4. The ESS control unit 3 can also be configured for determining parameters indicating and controlling the condition or capacity of the ESS 1, such as the state-of-charge (SoC), the state-of-health (SoH), the state-of-power (SoP), the state-of-capacity (SoQ), the state-of-resistance (SoR) and the state-of-energy (SoE) of the ESS 1, including each battery pack 2. A single control unit 3 is shown, which may be e.g., a so-called Domain Control Unit, DCU, configured to implement complete control functionality on all levels of the ESS. However, it is to be understood that the ESS may instead be provided with multiple control units. For example, the ESS may be provided with battery management units, BMUs (not shown), for managing individual battery units, such as battery packs 2 and/or battery modules, of the ESS 1. The BMU of each battery unit then receives and processes measurement data corresponding to its associated battery unit and also estimates state-of-capacity SoQ(i), SoR(i), SoH(i), and SoC(i). Each BMU then sends this data to the ESS control unit. It is possible to have either a dedicated ESS Master Control Unit or to select one of the BMUs and let it function as an ESS master control unit in addition to its battery unit level functionality. The control unit for controlling connection of individual battery packs 2 of the ESS 1 to the load 4 may also be a separate control unit.

[0083] The ESS control unit 3 may include a microprocessor, a microcontroller, a programmable digital signal processor or another programmable device. Thus, the ESS control unit 3 comprises electronic circuits and connections (not shown) as well as processing circuitry (not shown) such that the ESS control unit 3 can communicate with different parts of the bus 100 or with different control units of the bus 100. The ESS control unit 3 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 3 comprises a non-transitory memory for storing computer program code and data. Thus, the skilled person realizes that the ESS control unit 3 may be embodied by many different constructions. This is also applicable to other control units of the ESS 1.

[0084] FIG. 2 is a circuit diagram schematically illustrating an exemplary ESS 1 including four battery packs 2a, 2b, 2c, 2d. The battery packs 2a, 2b, 2c, 2d are electrically connected in parallel with an electric motor drive EMD, including a DC-link capacitor 5, via a traction voltage bus. Each battery pack 2a, 2b, 2c, 2d comprises a battery cell stack 7 and a battery disconnect unit (BDU) 8 for selectively connecting and disconnecting the battery cell stack 7 to/from the traction voltage bus in response to control signals from the ESS control unit 3. The BDU 8 also provides a so-called pre-charge process through a pre-charge resistor and contactor before connecting a main positive contactor of the battery pack. As shown herein, the BDU 8 typically comprises three contactors, of which two are main positive and negative contactors whereas the third one is the pre-charge contactor connected with the pre-charge resistor. The pre-charge process is needed to avoid any large current spike (in-rush current) that may happen due to a big voltage difference between the DC-link capacitor and the battery pack 2a, 2b, 2c, 2d.

[0085] FIG. 3 is a flow-chart illustrating a method according to an embodiment of the invention. In short, the method comprises the steps S1 of obtaining operational data, S2 (optional) of selecting a prioritization strategy, S3 of proposing a connection sequence for connecting the battery packs to the load, S4 of checking feasibility of the proposed connection sequence, and S5 of connecting battery packs to the load in accordance with the proposed connection sequence found to be feasible. The steps S1-S5 are explained in further detail in the following.

[0086] S1: Obtaining operational data relating to present operating conditions of the electric energy storage system 1, wherein the operational data include at least a voltage of each one of the plurality of battery packs 2a, 2b, 2c, 2d in the form of a measured terminal voltage and/or an open circuit voltage, OCV. The operational data may further include measurement data in the form of measured terminal current and temperature of each battery pack 2a, 2b, 2c, 2d, at the present time instant, and/or derived operational data such as state-of-charge (SoC), state-of-capacity (SoQ), internal resistance (state-of-resistance, SoR), state-of-power (SoP), state-of-energy (SoE), etc., of each battery pack 2a, 2b, 2c, 2d, at the present time instant, determined based on measurement data relating to current, voltage and temperature of the battery packs 2a, 2b, 2c, 2d. These derived operational parameters may be derived elsewhere and received by the ESS control unit 3, or they may be calculated/estimated by the ESS control unit 3.

[0087] S2: Setting a prioritization strategy for electrical connection of the battery packs 2a, 2b, 2c, 2d to the load 4. This step may be carried out before or after the first step S1, or it may be omitted so that the same prioritization strategy is always used. The prioritization strategy may for example be set as one of a first prioritization strategy, according to which it is attempted to maximize the state-of-power of the ESS 1, a second prioritization strategy, according to which it is attempted to minimize the total number of connection instants at which at least one battery pack will be connected to the load, and a third prioritization strategy, according to which it is attempted to maximize the state-of-energy of the ESS 1.

[0088] S3: Based on at least an operational mode of the electric energy storage system and on the terminal voltage and/or OCV of each one of the battery packs 2a, 2b, 2c, 2d, and by allowing simultaneous connection of at least two battery packs to the load, proposing a connection sequence for electrically connecting at least a subset of the plurality of battery packs 2a, 2b, 2c, 2d to the load 4. The prioritization strategy is also taken into account when proposing the connection sequence, as will be described by way of example further ahead.

[0089] The step S3 of proposing a connection sequence may be carried out by identifying the subset of battery packs to connect and proposing a point in time at which each battery pack within the subset should be connected, such as simultaneously with and/or subsequently to the other battery packs within the subset. Thus, the proposed connection sequence may include connection of one or more battery packs at one connection instance, i.e., point in time, only, or it may include connection of combinations of battery packs at different connection instances. The battery packs may for example be grouped based on terminal voltage and/or OCV, and/or state-of-power, SoP, in order to identify the subset of battery packs to connect.

[0090] S4: Based on at least the internal resistance of each one of the battery packs 2a, 2b, 2c, 2d within at least the subset of battery packs, determining whether the proposed connection sequence fulfils a predetermined connection condition. The predetermined connection condition may comprise at least a predetermined circulation current connection condition relating to a circulation current expected to flow between the battery packs upon electrical connection of said subset of battery packs, and preferably also a predetermined state-of-power connection condition relating to a total state-of-power of the electric energy storage system after electrical connection of said subset of battery packs in accordance with the proposed connection sequence, and/or a predetermined state-of-energy connection condition relating to a total state-of-energy of the electric energy storage system after electrical connection of said subset of battery packs in accordance with the proposed connection sequence.

[0091] In order to determine if the connection conditions are fulfilled, a magnitude of the circulation current expected to flow between the battery packs 2a, 2b, 2c, 2d upon electrical connection of the subset of battery packs in accordance with the proposed connection sequence may be predicted based on at least the internal resistance and an open circuit voltage of each one of the battery packs 2a, 2b, 2c, 2d. A dynamic multi-battery prediction model may be used to predict the magnitude of the circulation current. It is thereafter determined whether the predicted magnitude of the circulation current is within a predetermined allowable range, in which case the predetermined circulation current connection condition is considered fulfilled, i.e., whether the proposed connection sequence is feasible.

[0092] 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. Connection or disconnection of any battery pack within this multi-battery model can be achieved by instantaneously toggling between low and high resistance values. In short, a disconnected battery pack, i.e., open contactors, is emulated by putting its resistance infinitely high, or an order of magnitude higher than other connected battery packs. When it is desired to connect that battery pack, i.e., closed contactors, its resistance is lowered to its actual value in a single step. This will enable using the same state-space model for emulating current transients just after the connection instance as well as power split between battery packs during normal steady-state operation long after connection. The following multi-battery state-space model may be used:

[00001]$\begin{array}{cc}\stackrel{.}{x}\left(t\right)=A_I\left(t\right).Math.x\left(t\right)+B_I\left(t\right).Math.u\left(t\right)y\left(t\right)=C_I\left(t\right).Math.x\left(t\right)+D_I\left(t\right).Math.u\left(t\right)& \mathrm{Eq}.\left(1\right)\end{array}$

[0093] Herein, full state of the complete ESS 1 is represented by x=[x.sub.1 . . . x.sub.n].sup.T, wherein a state of each constituent battery pack i of the ESS is represented by x.sub.i=[V.sub.1i V.sub.2i V.sub.oci SoC.sub.i T.sub.i].sup.T, T herein denoting vector transpose. The output of the system is represented by y=[I.sub.1 . . . I.sub.n].sup.T where I.sub.i is the output current of each battery pack i. The control input of the state-space model is given by u=[I.sub.ESS U.sub.h,1 . . . U.sub.h,1].sup.T wherein I.sub.ESS is the total (e.g., total demanded) input current of ESS and U.sub.h,I is the total heat generated inside a battery pack i.

[0094] The system matrix A.sub.I, the input matrix B.sub.I, the output matrix C.sub.I and the feedthrough matrix D.sub.I are nonlinear functions of system parameters (R.sub.0i,R.sub.1i,R.sub.2,i,C.sub.2i,Q.sub.i,R.sub.ci-1,i,R.sub.ci) and system electro-thermal and ageing states (SoC.sub.i,T.sub.i,SoQ.sub.i,SoR.sub.i). Note that as compared to a single battery model, the open circuit voltage of each battery unit has been included as an additional state in order to derive a complete model for parallel multi-battery systems.

[0095] Note that above the state-space model is a vector-valued linear differential equation in the time domain (by virtue of the time derivative z) and such a differential equation may be solved using a standard method, such as the forward/backward Euler method.

[0096] As may be gleaned from the above, various system parameters, including battery impedances, capacities, cable connection resistances, temperatures etcetera enter into this model through the above matrices. This enables prediction of load, e.g. power or current, split between battery units with an appropriately high accuracy under given operating conditions.

[0097] The full expression for the above state-space multi-battery model is very cumbersome to show here. However, to give some idea about the internal model structure, a simpler version (so-called zero-order multi-battery model) is shown below. It is derived by dropping internal slow polarization states (i.e., V.sub.1i=V.sub.2i=0 or merging them with OCV) and temperature states from the above-mentioned model such that each element in the above-mentioned matrices A.sub.I, B.sub.I, C.sub.I and D.sub.I relating to internal slow polarizations and temperature is set to zero. Using such an approach for the matrices discussed hereinabove for the above example, viz a system with two battery packs such that n=2, the following equations are obtained for the electric currents for each one of the battery packs:

[00002]$\begin{array}{cc}{I}_1=\frac{R_{02}+R_{c12}}{\Omega }.Math.I_{\mathrm{ESS}}+\frac{1}{\Omega }.Math.\left(V_{\mathrm{oc}2}-V_{\mathrm{oc}1}\right)I_2=\frac{R_{01}}{\Omega }.Math.I_{\mathrm{ESS}}+\frac{1}{\Omega }.Math.\left(V_{\mathrm{oc}1}-V_{\mathrm{oc}2}\right)\Omega =R_{01}+R_{02}+R_{c12}& \mathrm{Eq}.\left(2\right)\end{array}$

[0098] As obvious from the above, the load—here exemplified as the electric current—imparted on each one of the battery units may be determined using total internal ohmic resistance values R.sub.0i and open circuit voltage value V.sub.oci (we may also call this internal slowly varying voltage if we assume that slow polarizations are not zero but instead merged with OCV) for each battery unit as well as the connective resistances relating to the position of each battery in the electric energy storage system 1. In the above equations, the connective resistance R.sub.c12 between the battery units—generally used for a daisy-chain configuration of battery packs—has been employed. However, the above equations could easily be modified so as to also, or instead, include a connective resistance R.sub.c1, R.sub.c2 between each battery unit and a reference/connection point in the electric energy storage system 1—which connective resistance generally is employed for a so-called star configuration of battery packs. For the sake of completeness, it should be noted that there may be a connective resistance R.sub.c01 between the first battery pack and the load but such a connective resistance R.sub.c01 may be omitted in the above equations. This is since the connective resistance R.sub.c01 will form a common resistance in the path of each one of the battery packs and may thus not have any influence of the load distribution among the battery packs.

[0099] Moreover, the equation presented hereinabove using the internal ohmic resistance values R.sub.01, the open circuit voltage (internal slowly varying voltage) value V.sub.oci and zero connective resistances could easily be generalized to the following two equivalent representations of zero-order multi-battery models for any number n of battery packs.

[00003]$\begin{array}{cc}{I}_i=\frac{1}{\theta }.Math.\underset{k\ne i}{\overset{n}{\Pi }}{R}_{0k}.Math.I_{\mathrm{ESS}}+\frac{1}{\theta }\underset{k\ne i}{\overset{n}{.Math.}}\left(\left(V_{\mathrm{ock}}-V_{\mathrm{oci}}\right).Math.\underset{\underset{j\ne i}{j\ne k}}{\overset{n}{\Pi }}{R}_{0j}\right),\forall i=1,\mathrm{.Math.},n\theta =\underset{i=1}{\overset{n}{.Math.}}\left(\underset{k\ne i}{\overset{n}{\Pi }}{R}_{0k}\right)& \mathrm{Eq}.\left(3\right)\\ I_i=\frac{1}{R_{0i}}.Math.\left(\frac{I_{\mathrm{ESS}}+\underset{j=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{ocj}}}{R_{0j}}}{\underset{j=1}{\overset{n}{.Math.}}\frac{1}{R_{0j}}}\right)-\frac{V_{\mathrm{oci}}}{R_{0i}},\forall =1,\mathrm{.Math.},n& \mathrm{Eq}.\left(4\right)\end{array}$

[0100] Using Eq. (4) above, the feasibility based on current circulation criterion can, for example, be checked using the following conditions, which basically say that the magnitude of current split/circulation in each battery pack under external load should be less than a certain percentage of its maximum SoP just after connection time instant.

[00004]$\begin{array}{cc}.Math.\frac{1}{R_{0i}}.Math.\left(\frac{I_{\mathrm{ESS},\mathrm{cha}}^{\mathrm{peak}}+\underset{j=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{ocj}}}{R_{0j}}}{\underset{j=1}{\overset{n}{.Math.}}\frac{1}{R_{0j}}}\right)-\frac{V_{\mathrm{oci}}}{R_{0i}}.Math.\le \left(1-\delta \right).Math..Math.I_{i,\mathrm{cha}}^{\max }.Math.,\forall i=1,\mathrm{.Math.},n& \mathrm{Eq}.\left(5\right)\\ .Math.\frac{1}{R_{0i}}.Math.\left(\frac{I_{\mathrm{ESS},\mathrm{dch}}^{\mathrm{peak}}+\underset{j=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{ocj}}}{R_{0j}}}{\underset{j=1}{\overset{n}{.Math.}}\frac{1}{R_{0j}}}\right)-\frac{V_{\mathrm{oci}}}{R_{0i}}.Math.\le \left(1-\delta \right).Math..Math.I_{i,\mathrm{dch}}^{\max }.Math.,\forall i=1,\mathrm{.Math.},n& \mathrm{Eq}.\left(6\right)\end{array}$

[0101] where I.sub.ESS,cha.sup.peak is a maximum peak (i.e., maximum possible) charging load on ESS, I.sub.ESS,dch.sup.peak is a peak discharging load on ESS (not necessarily same as ESS SoPs I.sub.ESS,cha.sup.max and I.sub.ESS,dch.sup.max), δ∈[0,1] is a tuning parameter determining the margin until a SoP limit is violated, and i represents an individual battery pack. The sums,

[00005]$\underset{j=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{ocj}}}{R_{0j}}\mathrm{and}\underset{j=1}{\overset{n}{.Math.}}\frac{1}{R_{0j}},$

will De same Tor all battery packs and it is therefore enough to calculate them once for each combination. It is also possible to save computations by utilizing previous calculations of the sums.

[0102] Furthermore, a total expected state-of-power, SoP, of the ESS 1 upon electrical connection of said subset of battery packs 2a, 2b, 2c, 2d in accordance with the proposed connection sequence may be predicted according to known methods, e.g., based on the SoP of individual battery packs or using an ESS-level multi-battery model, taking also interactions and current distribution between individual battery packs into account. For example, the above zero-order multi-battery model [see Eq. (3) and Eq. (4)] can also be used to compute ESS SoP. In short, using given chargeable SoPs, dischargeable SoPs, ohmic resistances, and OCVs of each individual battery pack i, we solve the model for maximum value of chargeable current ability I.sub.ESS,cha.sup.max and dischargeable current ability I.sub.ESS,dch.sup.max (i.e., ESS SoP Charge or Discharge magnitude) respectively as shown below.

[00006]$\begin{array}{cc}{I}_{\mathrm{ESS},\mathrm{cha}}^{\max }=\left(\underset{i=1}{\overset{n}{.Math.}}\frac{1}{R_{0i}}\right).Math.\underset{i\in 1,\mathrm{.Math.},n}{\min }\left\{\left(R_{0i}.Math.I_{i,\mathrm{cha}}^{\max }+V_{\mathrm{oci}}\right)\right\}-\underset{i=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{oci}}}{R_{0i}}& \mathrm{Eq}.\left(7\right)\\ I_{\mathrm{ESS},\mathrm{dch}}^{\max }=\left(\underset{i=1}{\overset{n}{.Math.}}\frac{1}{R_{0i}}\right).Math.\underset{i\in 1,\mathrm{.Math.},n}{\min }\left\{\left(R_{0i}.Math.I_{i,\mathrm{dch}}^{\max }+V_{\mathrm{oci}}\right)\right\}-\underset{i=1}{\overset{n}{.Math.}}\frac{V_{\mathrm{oci}}}{R_{0i}}& \mathrm{Eq}.\left(8\right)\end{array}$

[0103] In Eq. (8), a minus sign is used with R.sub.oi.Math.I.sub.i,dch.sup.max to address the fact that I.sub.i,dch.sup.max is a magnitude of SoP and is thus not a signed quantity as typically used for discharge currents. Note that the above equations for I.sub.ESS,cha.sup.max and I.sub.ESS,dch.sup.max can also be derived as a solution to a constrained Linear Programming problem using zero-order multi-battery model. The full-order multi-battery state-space model [see Eq. (1)] can also be used, but then it is necessary to solve a constrained Nonlinear Programming problem that is relatively hard. Regardless of the methods used, once the predicted SoP values for a selected candidate of future ESS configuration (i.e., a proposed connection sequence) are known, the predetermined state-of-power connection condition may be considered to be fulfilled if the predicted total expected SoP is above a predetermined total SoP threshold.

[0104] In addition, the best battery combination for connection that maximizes the ESS SoP can also be found. This can be achieved by devising a simple algorithm where ESS SoP for each possible battery combination, i.e., ESS configuration, is first found, whereafter the best one that gives the maximum ESS SoP value among all combinations is selected. For example, the algorithm may comprise the following steps: [0105] 1) enumerating each possible ESS configuration and store them in a table; [0106] 2) using Eq. (7) and Eq. (8) to find ESS Sops (i.e., I.sub.ESS,cha.sup.max and I.sub.ESS,dch.sup.max) for each possible ESS configuration; [0107] 3) assigning each ESS SoP value to its corresponding ESS configuration in a table and sort them in a descending order; and [0108] 4) finally choosing the ESS configuration corresponding to the top ESS SoP value.

[0109] Note that Eq. (7) and Eq. (8) pose very low computational burden, so the proposed optimization method based on exhaustive search is practically feasible in real-time for a large of number of battery packs.

[0110] The predetermined state-of-power connection condition may additionally be related to an SoP difference between the battery packs 2a, 2b, 2c, 2d. In this case, an SoP of each one of the battery packs 2a, 2b, 2c, 2d within the subset of battery packs is determined, and it is checked whether a difference in state-of-power between the battery packs 2a, 2b, 2c, 2d within the subset is below a predetermined state-of-power difference threshold, in which case the predetermined state-of-power connection condition may be considered fulfilled.

[0111] S5: Electrically connecting at least the subset of battery packs 2a, 2b, 2c, 2d to the load 4 in accordance with the proposed connection sequence. This step is only performed if the connection condition is considered to be fulfilled in step S4.

[0112] The steps S1-S5 may be repeated until all battery packs 2a, 2b, 2c, 2d are connected to the load 4. Herein, it is differentiated between so-called internal iterations (i.e., iterations based on internal virtual feedback) and so-called external iterations (i.e., iterations based on external real feedback). In short, for each given external feedback data regarding present operating conditions of the battery packs, multiple internal iterations are carried out until a feasible connection sequence is found under those given operating conditions. In the next external iteration, new operating conditions are provided in the form of new measurement data, which may allow a different feasible connection sequence, identified again through multiple internal iterations.

[0113] The method may also comprise disconnecting one or more battery packs 2a, 2b, 2c, 2d from the load 4. In this case, after connection of at least the subset of battery to the load 4, it is determined whether a predetermined disconnection condition is fulfilled for disconnection of at least one battery pack within the subset of battery packs 2a, 2b, 2c, 2d. Only if the predetermined disconnection condition is fulfilled, the at least one battery pack 2a, 2b, 2c, 2d is disconnected from the load 4.

[0114] FIG. 4 is a block diagram illustrating how the method may be carried out in a vehicle powertrain comprising an ESS 1 according to an exemplary embodiment of the method according to the invention, such as the ESS 1 shown in FIG. 2 comprising four battery packs 2a, 2b, 2c, 2d. In this embodiment, the vehicle powertrain comprises a load 4 and a power train controller 6 controlling the load 4 based on input from an ESS control unit 3 and on feedback from the load 4. The ESS control unit 3 is configured to determine a connection sequence CS for connecting battery packs 2a, 2b, 2c, 2d to the load 4 in accordance with the proposed method, to output the determined connection sequence CS to the powertrain controller, and to control the ESS 1 to connect the battery packs in accordance with the connection sequence.

[0115] A first block B1 is provided for performing battery state estimations and predictions, e.g., estimating and/or predicting OCV, SoH, SoR, SoC, SoE, SoQ, SoP, etc., of the individual battery packs 2a, 2b, 2c, 2d of the ESS. This block may also be provided elsewhere, e.g., in battery control units of the individual battery packs, i.e., BMUs.

[0116] A second block B2 is provided for sequencing, i.e., propose a connection sequence for electrically connecting at least a subset of the plurality of battery packs 2a, 2b, 2c, 2d to the load 4. This block virtually tests different connection sequences based on a selected prioritization strategy, and on input received from the first block.

[0117] A third block B3 is provided for predicting power sharing dynamics by simulating the ESS using a multi-battery model. This block predicts e.g., an expected total SoP of the ESS 1 and expected circulation currents as a result of connecting the proposed connection sequence received from the second block. The third block uses input data from the first block as well as from the second block.

[0118] A fourth block B4 is provided for performing a feasibility check, i.e., checking whether the proposed connection sequence fulfils predetermined connection conditions. In this block, an analysis of the predicted power sharing dynamics is performed. An internal feedback loop IFL is for this purpose provided, feeding results from the simulations performed in the third block to the fourth block. If the proposed connection sequence is not found to be feasible, information is provided to the second block, which proposes another connection sequence in an iterative manner

[0119] In addition to the internal feedback loop IFL, an external feedback loop EFL is also provided for feeding real-time measurement data and/or derived operational data from the ESS 1 as input to at least the fourth block B4 and the first block B1. This enables continuous updating and adaptation so that the algorithm used to determine the connection sequence may be improved over time.

EXAMPLE

[0120] According to an example, a method for connecting four battery packs 2a, 2b, 2c, 2d to a load 4 according to two different prioritization strategies will be described in the following. The operational mode of the ESS 1 is a discharging mode, so the battery packs 2a, 2b, 2c, 2d are sorted in ascending order according to open circuit voltage V.sub.oci of each battery pack i∈{2a, 2b, 2c, 2d}. Note that open circuit voltage can be assumed almost equal to terminal voltage when battery packs are disconnected and fairly relaxed. For simplicity, it is assumed that the battery pack 2a has the highest terminal voltage and/or open circuit voltage V.sub.oc,2a and the battery pack 2d has the lowest terminal voltage and/or open circuit voltage V.sub.oc,2d, i.e., V.sub.oc,2a>V.sub.oc,2b>V.sub.oc,2c>V.sub.oc,2d. Since the operational mode is a discharging mode, the battery pack 2a having the highest terminal voltage should according to both prioritization strategies be included in the subset of battery packs to be connected at a first connection instant.

[0121] According to a first prioritization strategy illustrated in FIG. 5, the aim is firstly to maximize the power ability, SoP, of the ESS, and secondly to minimize the number of connection instances. The overall aim is thus to maximize the power ability of the ESS. Thus, it is preferable to connect as many battery packs as possible at once to the load. In step S3, a connection sequence involving simultaneous connection of all four battery packs 2a, 2b, 2c, 2d is therefore proposed, as illustrated in the lower left corner of FIG. 6. Step S4 is thereafter carried out to check if the proposed connection sequence fulfils the predetermined connection condition. If yes, all four battery packs are connected at once. If not, step S3 is repeated to propose a next best connection sequence and its feasibility is checked in step S4. In this example, the next best connection sequence involves simultaneous connection of the three battery packs 2a, 2b, 2c having the highest terminal voltage, and/or open circuit voltage. If this connection sequence does not fulfil the predetermined connection sequence, steps S3-S4 are repeated until a feasible connection sequence is proposed. The proposed connection sequences are illustrated by following the arrows in FIG. 5. Thus, the least preferable connection sequence involves connection of the single battery pack 2d having the lowest terminal voltage, and/or open circuit voltage V.sub.oc,2d.

[0122] According to a second prioritization strategy illustrated in FIG. 6, the aim is firstly to minimize the number of connection instances so as to improve driveability of the vehicle, and secondly to maximize the power ability, SoP, of the ESS. Thus, also in this case, it is preferable to connect as many battery packs as possible at once to the load, i.e., all battery packs 2a, 2b, 2c, 2d. If that connection sequence is not feasible, other sequences are checked in order of preference. As can be seen by following the arrows in FIG. 6, the order of preference differs slightly from the one of the first prioritization strategy.

[0123] The same connection sequences can be used for the charging mode. The only difference is that the sorting should be in ascending order, i.e., V.sub.oc,2a<V.sub.oc,2b<V.sub.oc,2c<V.sub.oc,2d.

[0124] 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, the method may be adapted for controlling also disconnection of battery packs from a load, based on the same strategies as used for controlling connection.