Improvements in or relating to battery packs for electric vehicles

Abstract

A battery pack 10 for an electric vehicle (EV) comprises a plurality of parallel-connected cell strings 121-12n, each cell string including a plurality of series-connected energy-storage cells 141-14m

Description

This invention relates to a battery pack for an electric vehicle, and to an electric vehicle 5 including such a battery pack. Battery packs for electric vehicles typically include a large number of small, individual, energy-storage cells. These individual cells are often connected together in series to define cell strings that are connected in parallel with one another to achieve a desired 10 operating voltage and energy storage capacity. The number of cells in each cell string usually determines the voltage at which the battery is intended to operate, e.g. 100-200V for hybrid / plug-in hybrid electric vehicles or 400-800V and higher for electric-only vehicles, while the number of parallel-connected cell strings usually determines the storage capacity of the battery, e.g. a given number of Amp-hours (Ah). 15 Balancing of the energy stored by each of the series-connected energy-storage cells in a given cell string, i.e. so that each such series-connected energy-storage cell stores essentially the same amount of energy, is highly desirable because it helps to optimise the total energy capacity and service life of a given cell string, and thus optimise the 20 total overall energy capacity and service life of the battery in which the cell strings are located. According to a first aspect of the invention there is provided a battery pack, for an electric vehicle, comprising: 25 a plurality of parallel-connected cell strings, each including a plurality of series- connected energy-storage cells and a balancing energy-storage cell selectively connectable in parallel with each series-connected energy-storage cell thereof; and a battery controller programmed to selectively connect the or each balancing energy-storage cell with a corresponding respective series-connected energy-storage 30 cell to cause the or each said balancing energy-storage cell to exchange energy with the said corresponding respective series-connected energy-storage cell wherein a plurality of the said balancing energy-storage cells are selectively connectable in series with one another, and the battery controller is programmed to selectively switch a plurality of the selectively connectable balancing energy-storage 35 cells into series with one another to define a supplemental cell string. The ability to selectively connect a given balancing energy-storage cell with a corresponding respective series-connected energy-storage cell and thereafter have the given balancing energy-storage cell exchange energy, i.e. charge, with the respective series-connected energy-storage cell is advantageous since it allows the amount of 5 energy stored by the individual series-connected energy-storage cell to be changed. 09 04 25 This, in turn, is beneficial since it provides the ability to modify, selectively, the amount of energy stored by each series-connected energy-storage cell within a cell string having such a balancing energy-storage cell, and thereby arrange for a balancing of the energy stored by each of the series-connected energy-storage cells in the said cell string, e.g. so that each such series-connected energy-storage cell in the said cell string stores essentially the same amount of energy. Such balancing of the energy stored by individual series-connected energy-storage cells can be used to compensate for variations in the energy storage capacity of individual series-connected energy-storage cells, which might arise because of manufacturing variances, cell aging or impurities, or environmental exposure (e.g. additional heating from nearby sources such as motors or electronics), and that over the course of various charge and discharge cycles can lead to individual series-connected energy-storage devices storing different amounts of energy. As a consequence, such energy balancing leads to the desirable optimisation of the total energy capacity and service life of a given cell string, and thus the optimisation of the total overall energy capacity and service life of the associated battery pack within which the cell strings are located. Moreover, utilising a balancing energy-storage cell avoids achieving energy balancing by simply discharging the energy stored by a given series-connected energy-storage cell, e.g. by wasting it as heat, which would otherwise adversely reduce the amount of energy stored and thus the range of an electric vehicle in which the battery pack is incorporated. Preferably at least one balancing energy-storage cell is able, when selectively connected with a corresponding series-connected energy-storage cell, to supply energy to the said corresponding series-connected energy-storage cell or receive energy from the corresponding series-connected energy-storge cell. The ability to both supply energy to and remove energy from each series-connected energy-storage cell in a respective cell string is beneficial because it provides the opportunity to maximise the total energy capacity of the said cell string, and thus an opportunity to increase the total overall energy capacity of the associated battery pack. Optionally the at least one balancing energy-storage cell is connected to a first side of each respective series-connected energy-storage cell in the corresponding cell string via a respective first switch and is connected to a second side of each respective series-connected energy-storage cell in the corresponding cell string via a respective second switch. Utilising respective first and second switches to connect each series-connected energystorage cell within a given cell string to the associated balancing energy-storage cell desirably provides, in a practical and readily implementable manner, the associated balancing energy-storage cell with the ability to selectively supply energy individually to each said series-connected energy-storage cell or selectively receive, i.e. remove, energy individually from each said series-connected energy-storage cell. Additionally such an arrangement, i.e. having a single balancing energy-storage cell connected to each of multiple series-connected energy-storage cells by respective pairs of first and second switches, advantageously provides the aforementioned ability to selectively supply or remove energy from each individual series-connected energystorage device while keeping the number of connections to a minimum, e.g. compared to providing similar functionality by selectively connecting each series-connected energy-storage cell via first and second switches with each of the other series-connected energy-storage cells in a given cell string, i.e. such that the series-connected energy-storage cells in a given cell string are all interconnected with one another. The battery controller may be arranged in operative communication with each pair of first and second switches. Such a feature readily permits the battery controller to desirably control operation of each said pair of first and second switches to thereby advantageously affect the addition or removal of energy, i.e. charge, from the corresponding series-connected energy-storage cell. The battery controller may also be additionally arranged in operative communication with each series-connected energy-storage cell having a pair of first and second switches connected therewith. The further feature of having a battery controller in operative communication with each series-connected energy-storage cell having a pair of first and second switches connected therewith beneficially allows the battery controller to obtain information about the amount of energy stored by the or each such series-connected energystorage cell with which it is in operative communication, and thereafter coordinate the selective opening and closing of the associated first and second switches to add or remove energy, i.e. charge, from the corresponding series-connected energy-storage cell to modify a reported amount of energy stored and thereby balance the energies stored across the series-connected energy-storage cells in a given cell string. In a preferred embodiment of the invention the battery controller is programmed to connect the or each balancing energy-storage cell to only a single series-connected energy-storage cell at a time. Such programming of the battery controller helps to prevent short-circuits between respective series-connected energy-storage cells within a cell string which might otherwise damage those cells, and / or reduce their performance. Optionally the battery controller is programmed to introduce a delay between disconnecting at least one balancing energy-storage cell from one series-connected energy-storage cell and connecting the said at least one balancing energy-storage cell to another series-connected energy-storage cell. Programming of the battery controller in this manner ensures a given balancing energy-storage cell is connected to only one series-connected energy-storage cell at any given time, and thus is a practical and readily implementable way of avoiding short circuits between respective series-connected energy-storage cells. Preferably the battery controller includes a monitor to monitor the rate of energy exchange of the or each balancing energy storage-cell with a corresponding respective series-connected energy-storage cell. The ability to monitor the rate of energy exchange of the or each balancing energystorage cell with a corresponding respective series-connected energy-storage cell, i.e. with a respective series-connected energy-storage cell which a given balancing energystorage cell is selectively connected, is beneficial because it can be used to initiate remedial action, e.g. if the rate of energy exchange is deemed to high, as might be the case when the amount of energy stored by the corresponding respective series-connected energy-storage cell differs considerably from the amount of energy stored by the balancing energy-storage cell. 09 04 25 In accordance with the invention, a plurality of the said balancing energy-storage cells are selectively connectable in series with one another, and the battery controller is programmed to selectively switch a plurality of the selectively connectable balancing energy-storage cells into series with one another to define a supplemental cell string. 5 The ability to selectively connect a plurality of balancing energy-storage cells in series with one another and thereby define a supplemental cell string is advantageous because such a supplemental cell string provides the battery pack with a degree of redundancy that could be used, e.g. to replace or augment a damaged or poorly 10 performing other, normal cell string. Preferably the battery controller is programmed to selectively switch the supplemental cell string into parallel with the other cell strings. 15 Having a battery controller so programmed provides the option of increasing the total amount of energy that can be provided by the battery pack, i.e. in addition to that available ordinarily from the other cell strings. Such an increase in the total amount of energy that the battery pack can provide can 20 be used, as desired, to, e.g. increase the range of an associated electric vehicle in which the battery pack is configured to operate, or to provide such an electric vehicle with a "power boost" functionality, e.g. to temporarily increase the rate at which the vehicle is able to accelerate or the maximum velocity at which it can travel. 25 Also, utilising the extra balancing energy-storage cells needed to provide the aforementioned energy balancing functionality within each cell string to additionally, optionally, provide an increase in the total amount of energy that the battery pack can supply helps to justify the additional cost of the extra balancing energy-storage cells, and thereby goes towards keeping the cost per unit of energy storage, e.g. per kWh, 30 of the battery pack of the invention comparable to conventional battery packs without such extra balancing energy-storage cells and associated energy balancing functionality. Additionally, utilising the extra balancing energy-storage cells to provide an increase 35 in the total amount of energy similarly helps to justify the added weight of the extra balancing energy-storage cells, which is particularly important in a relatively small and compact electric vehicle such as an electric motorcycle. In another preferred embodiment of the invention the battery controller is programmed to selectively connect the or each balancing energy-storage cell with a corresponding respective series-connected energy-storage cell while there is no energy exchange with the cell string in which the said corresponding respective series-connected energystorage cell is included. Such programming of the battery controller helps to ensure that selective connection of the or each balancing energy-storage cell and subsequent balancing of the energies stored within a given cell string desirably takes place during a rest time of the battery pack, i.e. when the battery pack is not providing energy to the electric vehicle, e.g. not when the vehicle is being driven or ridden, and when the battery pack is not being charged. Avoiding energy balancing when the battery pack is being used, i.e. when the battery pack is supplying energy, is advantageous because it ensures that all of the extra balancing energy-storage cells are available within the supplemental cell string when it is switched into parallel with the other cell strings to increase the total amount of energy that can be provided by the battery pack, and thus such a total energy increase can be maximised. Meanwhile, avoiding energy balancing when the battery pack is being charged, i.e. when the battery pack is receiving energy, is advantageous because it allows the balancing energy-storage cells in the supplemental cell string to be charged too. Optionally each balancing energy-storage cell and each of the plurality of series-connected energy-storage cells in each cell string has the same nominal energy storage capacity. Such an arrangement allows each of the balancing and series-connected energystorage cells to be manufactured from the same, essentially identical, basic cell which provides economies of scale and associated cost savings and manufacturing simplification. The number of balancing energy-storage cells in the supplemental cell string may be the same as the number of series-connected energy-storage cells in each other cell string. Having the same number of balancing energy-storage cells in the supplemental cell string as the number of series-connected energy-storage cells in each other cell string advantageously ensures that when the supplemental cell string is selectively switched into parallel with the other cell strings there is no transfer of energy between the supplemental cell string and any of the other cell strings, and thus the full amount of additional energy stored in the supplemental cell string remains available to increase the total amount of energy that can be supplied by the battery pack. According to a second aspect of the invention there is provided an electric vehicle including a battery pack as described hereinabove. It will be appreciated that the use of the terms "first" and "second", and the like, in this patent specification is merely intended to help distinguish between similar features, and is not intended to indicate the relative importance of one feature over another feature, unless otherwise specified. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, and the claims and / or the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and all features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. There now follows a brief description of preferred embodiments of the invention, by way of non-limiting examples, with reference being made to the following figures in which: Figure 1 shows a schematic view of energy-storage cell interconnections within a battery pack according to a first embodiment of the invention; Figures 2(a) and 2(b) show different examples of energy balancing within a string of series-connected energy-storage cells in the battery pack shown in Figure 1; and Figure 3 shows a schematic view of further interconnections between the energy-storage cells of the battery pack shown in Figure 1. A battery pack according to a first embodiment of the invention is designated generally by reference numeral 10, as shown schematically in Figure 1. The battery pack 10 is for an electric vehicle (not shown), and in particular for an electric motorcycle, although this need not necessarily be the case. The battery pack 10 includes a plurality of parallel-connected cell strings 12i, 12?, 12n, with respective first, second and nth cell strings 12i, 12?, 12n, being shown by way of example. The total number of cell strings 12i, 12?, 12n in the battery 10 defines the nominal energy storage capacity, e.g. in kWh, of the battery. Each cell string 12i, 12?, 12n includes a plurality of series-connected energy-storage cells with, by way of example, respective first, second, third, fourth, fifth, sixth and mth first energy-storage cells 14i, 14?, 14s, 144, 14s, 14e, 14m being shown in the first cell string 12i, respective first, second, third, fourth, fifth, sixth and mth second energy-storage cells 16i, I62, I63, I64, I65, 16e, 16m being shown in the second cell string 12?, and respective first, second, third, fourth, fifth, sixth and mth nth energystorage cells I81, I82, I83, I84, I85, 18e, 18m being shown in the nth cell string 12n. The number of energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m , I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in each cell string 12i, 122, 12n, along with each of their individual energy storage capacities, defines the nominal operating voltage of the battery pack 10. Each energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m preferably is or includes a lithium ion cell or a lithium polymer cell. Other types of rechargeable cell may also be used, however, such as lead-acid, nickel-cadmium, nickel-metal hydride, or less commonly zinc-air and sodium nickel chloride cells. In any event, each energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m has the same nominal, individual, energy storage capacity, although again that need not necessarily be the case. In this context the various energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m, 16i, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m having the same nominal energy storage capacity is intended to mean that they have the same specified or "rated" energy storage capacity, not that their actual individual energy storage capacities are exactly equal. In that regard it is known that the actual energy storage capacity of individual energy-storage cells can vary because of, e.g. manufacturing variances, cell aging or impurities, or environmental exposure (e.g. additional heating from nearby sources such as motors or electronics). In addition to the foregoing, each cell string 12i, 122, 12n includes a balancing energystorage cell 20i, 2O2, 20n. More particularly, a first balancing energy-storage cell 20i is selectively connectable in parallel with each series-connected first energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m in the first cell string 12i, a second balancing energy-storage cell 2O2 is selectively connectable in parallel with each series-connected second energy-storage cell I61, I62, I63, I64, I65, 16e, 16m in the second cell string 122, and a nth balancing energy-storage cell 20n is selectively connectable in parallel with each series-connected nth energy-storage cell I81, I82, I83, I84, I85, 18e, 18m in the nth cell string 12n. More particularly still: the first balancing energy-storage cell 20i is connected to a first side of each respective series-connected first energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m in the first cell string 12i by a corresponding first switch 22i, 222, 22s, 224, 22s, 22e, 22m and is connected to a second side of each respective series-connected first energystorage cell 14i, 142, 143, 144, 14s, 14e, 14m in the first cell string 12i, by a corresponding second switch 24i, 242, 24s, 244, 24s, 24e, 24m; the second balancing energy-storage cell 2O2 is connected to a first side of each respective series-connected second energy-storage cell I61, I62, I63, I64, I65, 16e, 16m in the second cell string 122 by a corresponding further first switch 26i, 262, 263, 264, 265, 26e, 26m and is connected to a second side of each respective series-connected second energy-storage cell I61, I62, I63, I64, I65, 16e, 16m in the second cell string 122 by a corresponding further second switch 28i, 282, 283, 284, 28s, 28e, 28m; and the nth balancing energy-storage cell 20n is connected to a first side of each respective series-connected nth energy-storage cell I81, I82, I83, I84, I85, 18e, 18m in the nth cell string 12n by a corresponding still further first switch 30i, 302, 30s, 304, 305, 30e, 30m and is connected to a second side of each respective series-connected nth energy-storage cell 18i, I82, I83, I84, I85, 18e, 18min the nth cell string 12n by a corresponding still further second switch 32i, 32?, 32s, 324, 32s, 32e, 32m. The battery pack 10 of the invention also includes a battery controller 34 that preferably takes the form of a complete Battery Management System embedded on a microcontroller or other programmable integrated circuit. In any event, the battery controller 34 is programmed to selectively connect each balancing energy-storage cell 20i, 2O2, 20nwith a corresponding respective series-connected energy-storage cell, i.e. selectively connect the first balancing energy-storage cell 20i with a respective first energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m in the first cell string 12i, selectively connect the second balancing energy-storage cell 2O2 with a respective second energy-storage cell I61, I62, I63, I64, I65, 16e, 16m in the second cell string 122, and selectively connect the nth balancing energy-storage cell 20n with a respective nth energy-storage cell I81, I82, I83, I84, I85, 18e, 18m in the nth cell string 12n. Such selective connection, i.e. connection as and when the battery controller 34 determines such a connection should be made, of each balancing energy-storage cell 20i, 2O2, 20n with a corresponding respective series-connected energy-storage cell 14i, 142, 143, 144, 145, 146, 14m, 161, 162, 163, I64, 165, 166, 16m, 181, 182, 183, I84, 185, 18e, 18m causes the said balancing energy-storage cell 20i, 2O2, 20n to exchange energy, i.e. charge, with the said corresponding respective first, second or nth energystorage cell 14i, 142, 143, 144, 14s, 146, 14m, I61, I62, I63, 164, I65, 166, 16m, I81, I82, I83, 184, I85, 186, 18m. More particularly, the ability selectively to connect to the first and second sides of each series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m, means that each balancing energystorage cell 20i, 2O2, 20n is able, when selectively connected with a corresponding series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m, to supply energy, i.e. charge, to the said corresponding series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, 181, I82, I83, I84, I85, 18e, 18m or receive energy, i.e. charge, from the corresponding series-connected energy-storge cell 14i, 142, 143, 144, 145, 146, 14m, 161, 162, 163, I64, 165, 166, 16m, 181, 182, 183, I84, 185, 186, 18m. This permits the battery controller 34 to control charging or discharging of each energystorage cell 14i, 142, 143, 144, 14s, 146, 14m, 16i, I62, I63, 164, I65, 166, 16m, I81, 182, 18s, 184, 18s, 18e, 18m in a given cell string 12i, 12?, 12n according to the state of charge (SoC), i.e. energy storage level, of each such cell 14i, 142, 14s, 144, 14s, 146, 14m, 161, 162, 163, 164, 165, 166, 16m, 181, 182, 183, 184, 185, 186, 18m. Such control of the charging and discharging of each energy-storage cell 14i, 142, 14s, 144, 145, 146, 14m, 161, 162, 163, 164, 165, 166, 16m, 181, 182, 183, 184, 185, 186, 18m in a given cell string 12i, 122, 12n allows the state of charge, i.e. the amount of stored energy, of each energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, 16i, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in a given cell string 12i, 122, 12n to be modified. This, in turn, provides for a balancing of the energy stored by each of the series-connected energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in the said cell string 12i, 122, 12n, e.g. so that each such series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in the said cell string 12i, 122, 12n stores essentially the same amount of energy. This same amount of energy may be based on the maximum state of charge within a given cell string 12i, 122, 12n, i.e. on the level of energy stored by the energy-storage cell 14i, 142, 143, 144, 14s, 146, 14m, I61, I62, I63, 164, I65, 166, 16m, I81, I82, I83, I84, I85, 18e, 18m in a given cell string 12i, 122, 12n which is the largest, e.g. as shown schematically in Figure 2(a) in relation to the varying energy storage levels of the first energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m in the first cell string 12i that, over time, all move towards an energy level, e.g. voltage level, of about 0.49V which is the highest individually-stored energy amount, i.e. highest voltage, of each of the first energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m. The same amount of energy may instead be based on the mean state of charge within a given cell string 12i, 122, 12n, i.e. on the average level of energy stored by all of the energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in a given cell string 12i, 122, 12n, e.g. as shown schematically in Figure 2(b) in relation to the varying energy storage levels of the first energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m in the first cell string 12i that, over time, all move towards an energy level, e.g. voltage level, of about 0.51V which is the average of the individually-stored energy amounts, i.e. average voltage, of each of the first energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m. In still further embodiments of the invention the same amount of energy to be stored by each series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, 16i, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in a respective cell string 12i, 122, 12n may be determined differently. In any event, the battery controller 34 is arranged in operative communication, e.g. by a communication bus (not shown), with each pair of first and second switches, i.e. with each of the first and second switches 22i, 222, 22s, 224, 22s, 22e, 22m, 24i, 242, 243, 244, 24s, 24e, 24m, with each of the further first and second switches 26i, 262, 263, 264, 265, 26e, 26m, 28i, 282, 283, 284, 285, 28e, 28m , and each of the still further first and second switches 30i, 302, 30s, 304, 30s, 30e, 30m, 32i, 322, 32s, 324, 32s, 32e, 32m. Additionally, the battery controller 34 is arranged in operative communication, for example by the same communication bus, with each series-connected energy-storage cell having a pair of first and second switches connected therewith, i.e. in the embodiment shown with each series-connected energy-storage cell 14i, 142, 14s, 144, 145, 146, 14m, 161, 162, 163, 164, 165, 166, 16m, 181, 182, 183, 184, 185, 186, 18m. Arranging the battery controller 34 in such operative communication allows the battery controller 34 to obtain information about the amount of energy stored by each series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, 16i, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m, and thereafter to coordinate the selective opening and closing of the associated first and second switches to add or remove energy, i.e. charge, from the corresponding series-connected energy-storage cell 14i, 142, 143, 144, 145, 146, 14m, 161, 162, 163, I64, 165, 166, 16m, 181, 182, 183, I84, 185, 18e, 18m., i.e. selectively charge or discharge the corresponding energy-storage cell 141, 142, 143, 144, 145, 146, 14m, 161, I62, I63, 164, I65, 166, 16m, 181, I82, I83, 184, I85, 18e, 18m, to modify a reported amount of energy stored and thereby balance the energies stored, i.e. voltage, across the series-connected energy-storage cells 14i, 142, 143, 144, 145, 146, 14m, 161, 162, 163, 164, 165, 166, 16m, 181, 182, 183, 184, 185, 186, 18m. in a given cell string 12i, 122, 12n. The battery controller 34 is also programmed to connect each balancing energystorage cell 20i, 202, 20n to only a single, corresponding series-connected energystorage cell 14i, 142, 143, 144, 14s, 146, 14m, I61, I62, I63, 164, I65, 166, 16m, I81, I82, I83, I84, I85, 18e, 18m at a time. In the embodiment shown this is achieved by programming the battery controller 34 to introduce a delay between disconnecting each balancing energy-storage cell 20i, 202, 20n from a given series-connected energy-storage cell 14i, 14?, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, 181, I82, I83, I84, I85, 18e, 18m and then connecting the respective balancing energy-storage cell 20i, 2O2, 20n to another series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m. Other ways of achieving the same result are possible, however. In addition to the foregoing, the battery controller 34 includes a monitor (not shown), such as one or more voltage sensors, the or each of which monitors the rate of energy exchange of a given balancing energy-storage cell 20i, 2O2, 20n with a corresponding respective series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m which the given balancing energy-storage cell 20i, 2O2, 20n may from time to time be connected with. Such monitoring can be used to initiate remedial action, e.g. if the rate of energy exchange is deemed too high, as might be the case when the amount of energy stored by the corresponding respective series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 146, 14m, 161, 162, 163, I64, 165, 166, 16m, 181, 182, 183, I84, 185, 186, 18m differs considerably from the amount of energy stored by the balancing energy-storage cell 20i, 2O2, 20nwith which it is selectively connected. In addition to the foregoing, all but one of the balancing energy-storage cells are configured to be selectively connectable in series with one another, e.g. as shown schematically in Figure 3 which additionally shows third, fourth, fifth, sixth and mth 2O3, 204, 2O5, 20e, 20m balancing energy-storage cells by way of illustration. Furthermore, the battery controller 34 is programmed to selectively switch the all but one selectively connectable balancing energy-storage cells 20i, 2O2, 20s, 204, 20s, 20e, 20m into series with one another to define a supplemental cell string 36. In that regard, the exemplary battery pack 10 of the invention shown, having n cell strings, has n-1, i.e. m, balancing energy-storage cells 20i, 2O2, 20s, 204, 20s, 20e, 20m in the supplemental cell string 36. Having one less balancing energy-storage cell 20i, 2O2, 2O3, 204, 2O5, 20e, 20m in the supplemental cell string 36 than there are cell strings 12i, 122, 12n overall is desirable since it ensures the maximum utilisation in the supplemental cell string 36 of as many of the extra, added, balancing energy-storage cells 20i, 2O2, 2O3, 204, 2O5, 20e, 20m as possible, thus helping to justify the additional cost and weight of the added balancing energy-storage cells 20i, 20?, 20s, 204, 20s, 206, 20m. Moreover, the number of balancing energy-storage cells 20i, 20?, 20s, 204, 20s, 20e, 20m in the supplemental cell string 36 is the same, e.g. m, as the number of series-connected energy-storage cells 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m in each of the other, main cell strings 12i, 122, 12n noting, as set out above, that the balancing energy-storage cells 20i, 2O2, 2O3, 204, 2O5, 20e, 20m and the series-connected energy-storage cells 14i, 142, 14s, 144, 145, 146, 14m, 161, 162, 163, 164, 165, 166, 16m, 181, 182, 183, 184, 185, 186, 18m all have the same nominal energy storage capacity. Having the aforementioned same number, e.g. m, of balancing energy-storage cells 20i, 202,203, 204, 2O5, 20e, 20m in the supplemental cell string 36 is similarly beneficial because it ensures the maximum utilisation of both the series-connected energystorage cells 14i, 142, 143, 144, 14s, 146, 14m, 16i, I62, I63, 164, I65, 166, 16m, I81, I82, I83, I84, I85, 18e, 18m and the balancing energy-storage cells 20i, 202,20s, 204, 2O5, 206, 20m. Meanwhile, the balancing energy-storage cells 20i, 2O2, 20s, 204, 20s, 20e, 20m are selectively connectable in series with one another by respective first, second, third, fourth, fifth, sixth and mth supplemental switches 38i, 382, 38s, 384, 38s, 38e that are interposed between respective balancing energy-storage cells 20i, 2O2, 20s, 204, 20s, 20e, 20m. The supplemental switches 38i, 382,38s, 384, 38s, 38eare, in turn, arranged in operative communication with the battery controller 34, e.g. via the aforementioned communication bus (not shown), such that they can be switched on and off, as needed, by the battery controller 34. Additionally, the battery controller 34 is programmed to selectively switch the supplemental cell string 36 into parallel with the other, main cell strings 12i, 122, 12n. Such selective switching of the supplemental cell string 36 into parallel is achieved, as shown in Figure 3, via the existing first switch 22i of the first energy-storage cell 14i in the first cell string 12i and the existing second switch 40m of the mth energy-storage cell 42m in the mth cell string 44m, thus further helping with optimum utilisation of existing switch hardware by avoiding the need for one or more additional switches. Preferably the battery controller 34 is programmed to selectively connect the or each balancing energy-storage cell 20i, 20?, 20s, 204, 20s, 20e, 20m with a corresponding respective series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m only while there is no energy exchange with the cell string 12i, 122, 12n in which the said corresponding respective series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m is included. In other words, the battery controller 34 preferably only has a given balancing energystorage cell 20i, 2O2, 2O3, 204, 2O5, 20e, 20m exchange energy with a respective series-connected energy-storage cell 14i, 142, 14s, 144, 14s, 14e, 14m, I61, I62, I63, I64, I65, 16e, 16m, I81, I82, I83, I84, I85, 18e, 18m, and thereby preferably only carries out energy balancing within a given cell string 12i, 122, 12n when that particular cell string 12i, 122, 12n is at "rest", i.e. when the battery pack 10 is not being used, which typically occurs when the electric vehicle in which the battery pack 10 is located is similarly not being used, e.g. is not being ridden in the case of an electric motorcycle, or when the battery pack 10 is not being charged. This need not necessarily be the case, however, and energy balancing within a given cell string 12i, 122, 12n can take place when that particular cell string 12i, 122, 12n is discharging energy, e.g. when an electric motorcycle in which the battery pack 10 is located is being ridden, although this has the drawback of reducing the energy increase that is available from the supplemental cell string 36. Similarly, energy balancing within a given cell string 12i, 122, 12n can take place when that particular cell string 12i, 122, 12n is receiving energy, e.g. when the battery pack 10 is being charged, although this has the downside of making it difficult to also charge the balancing energy-storage cells 20i, 202,20s, 204, 20s, 20e, 20m in the supplemental cell string 36. In use, e.g. as a power source for an electric motorcycle, the ability of the battery 10 of the invention to have the supplemental cell string 36 selectively switched into parallel with the other, main cell strings 12i, 122, 12n, i.e. switched into parallel when deemed appropriate by the battery controller 34 and / or as desired by a user, e.g. rider of the electric motorcycle, is particularly advantageous because it provides the option of selectively increasing the total amount of energy that can be provided by the battery 10, i.e. in addition to that available ordinarily from the other, main cell strings 12i, 122, 12n. Such an increase in the total amount of energy that the battery 10 is able to provide can be used, as desired, to: 5 - increase the range of an associated electric vehicle, e.g. an electric motorcycle, in which the battery 10 is configured to operate; and / or - provide such an electric vehicle with a "power boost" functionality, e.g. to temporarily increase the rate at which the vehicle is able to accelerate or the 10 maximum velocity at which it can travel. These resulting increases in range and power boost thus help to offset the cost and weight penalty that adding the extra balancing energy-storage cells 20i, 20?, 20s, 204, 15 2O5, 20e, 20m, 20n would otherwise impose. 09 04 25

Claims

1. A battery pack, for an electric vehicle, comprising:a plurality of parallel-connected cell strings, each including a plurality of series-5 connected energy-storage cells and a balancing energy-storage cell selectively connectable in parallel with each series-connected energy-storage cell thereof; anda battery controller programmed to selectively connect the or each balancing energy-storage cell with a corresponding respective series-connected energy-storage cell to cause the or each said balancing energy-storage cell to exchange energy with 10 the said corresponding respective series-connected energy-storage cell,wherein a plurality of the said balancing energy-storage cells are selectively connectable in series with one another, and the battery controller is programmed to selectively switch a plurality of the selectively connectable balancing energy-storage cells into series with one another to define a supplemental cell string.

152. A battery pack according to Claim 1 wherein at least one balancing energystorage cell is able, when selectively connected with a corresponding series-connected energy-storage cell, to supply energy to the said corresponding series-connected energy-storage cell or receive energy from the corresponding series-connected energy-20 storge cell.

3. A battery pack according to Claim 2 wherein the at least one balancing energystorage cell is connected to a first side of each respective series-connected energystorage cell in the corresponding cell string via a respective first switch and is25 connected to a second side of each respective series-connected energy-storage cell in the corresponding cell string via a respective second switch.

4. A battery pack according to Claim 3 wherein the battery controller is arranged in operative communication with each pair of first and second switches.

305. A battery pack according to Claim 4 wherein the battery controller is additionally arranged in operative communication with each series-connected energystorage cell having a pair of first and second switches connected therewith.35 6. A battery pack according to any preceding claim wherein the battery controlleris programmed to connect each balancing energy-storage cell to only a single series-connected energy-storage cell at a time.09 04 257. A battery pack according to Claim 6 wherein the battery controller is programmed to introduce a delay between disconnecting at least one balancing energy-storage cell from one series-connected energy-storage cell and connecting the said at least one balancing energy-storage cell to another series-connected energy-5 storage cell.

8. A battery pack according to any preceding claim wherein the battery controller includes a monitor to monitor the rate of energy exchange of each balancing energy storage cell with a corresponding respective series-connected energy-storage cell.

109. A battery pack according to any preceding claim wherein the battery controller is programmed to selectively switch the supplemental cell string into parallel with the other cell strings.15 10. A battery pack according to Claim 9 wherein the battery controller isprogrammed to selectively connect each balancing energy-storage cell with a corresponding respective series-connected energy-storage cell while there is no energy exchange with the cell string in which the said corresponding respective series-connected energy-storage cell is included.2011. A battery pack according to any proceeding claim wherein each balancing energy-storage cell and each of the plurality of series-connected energy-storage cells in each cell string has the same nominal energy storage capacity.25 12. A battery according to Claim 11 wherein the number of balancing energystorage cells in the supplemental cell string is the same as the number of series-connected energy-storage cells in each other cell string.

13. An electric vehicle including a battery pack according to any preceding claim.

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