Patent Publication Number: US-2022227256-A1

Title: Vehicle traction battery circuit and control system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a circuit for a traction battery, and to a control system. In particular, but not exclusively it relates to a circuit for a traction battery of a vehicle, and to a control system for the vehicle. 
     BACKGROUND 
     Electric vehicles and hybrid electric vehicles comprise traction motors, and traction batteries for supplying electrical energy to the traction motors. Some traction batteries can be recharged with electrical energy from outside the vehicle, such as electrical energy from an electrical grid. 
     A traction battery of known design comprises a plurality of cell packs. Each cell pack comprises a set of one or more battery cells (‘cells’ herein). Each battery cell should provide a nominal (design) voltage when fully charged. However, the actual voltage at full charge may differ from the nominal voltage within manufacturing tolerances, or due to aging, or due to imperfect balancing during recharging. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to address one or more disadvantages associated with the prior art. 
     Aspects and embodiments of the invention provide a circuit, a control system, a system, a vehicle, a method, and computer software, as claimed in the appended claims. 
     According to a further aspect of the invention there is provided a circuit for a traction battery of a vehicle, the circuit comprising: switching means configured to electrically interconnect in series a first cell set comprising one or more battery cells and a second cell set comprising one or more battery cells, and configured to electrically interconnect in parallel the first cell set and the second cell set; and the circuit comprising at least one component configured to control electrical energy transfer between the first cell set and the second cell set associated with the first cell set and the second cell set having unequal voltages. 
     An advantage is improved durability of a traction battery with a fast charging option. This is because the component can reduce voltage imbalance-induced current surge when switching between parallel (normal charge) and series (fast charge) modes. 
     The switching means may be configured to electrically interconnect in parallel the first cell set and the second cell set in a first path that comprises the at least one component, and may be configured to electrically interconnect in parallel the first cell set and the second cell set in a second path that does not comprise the at least one component. 
     An advantage is improved efficiency. This is because the component has a resistance and can be taken out of circuit when it is not required. 
     The at least one component may comprise a resistor. 
     An advantage is reduced cost and complexity than other components such as DC-DC converters or earthed capacitors. 
     A nominal terminal voltage of the traction battery may be a first value greater than 300 volts when the first cell set and the second cell set are electrically interconnected in series. The first value may be greater than 500 volts. The nominal terminal voltage of the traction battery may be a second value less than the first value when the first cell set and the second cell set are electrically interconnected in parallel. 
     An advantage of series interconnection is reduced charging time because higher voltages enable higher charging power. 
     The switching means may be configured to electrically disconnect the first cell set without electrically disconnecting the second cell set and/or the switching means may be configured to electrically disconnect the second cell set without electrically disconnecting the first cell set. The circuit may comprise a first fuse configured to electrically disconnect the first cell set without electrically disconnecting the second cell set and/or the circuit may comprise a second fuse configured to electrically disconnect the second cell set without electrically disconnecting the first cell set. 
     An advantage is improved fault tolerance, because the vehicle can continue to operate on a reduced number of cell sets. 
     The circuit may comprise a first isolator configured to electrically disconnect the first cell set without electrically disconnecting the second cell set and/or the circuit may comprise a second isolator configured to electrically disconnect the second cell set without electrically disconnecting the first cell set. 
     An advantage is improved fault tolerance, because individual cell sets can be isolated manually if automatic means are unavailable. 
     The circuit may comprise a series-breaker fuse configured to break the series electrical interconnection of the first cell set and the second cell set. The series-breaker fuse may comprise a pyrotechnic fuse actuator. 
     An advantage is improved fault tolerance, because the vehicle is less likely to become stuck in a series mode incompatible with driving. 
     The series-breaker fuse may be configured to break the series electrical interconnection of the first cell set and the second cell set without preventing parallel electrical interconnection of the first cell set and the second cell set. 
     An advantage is improved fault tolerance, because the vehicle can continue to operate on a reduced number of cell sets. 
     According to a further aspect of the invention there is provided a traction battery comprising the circuit, and the first cell set and the second cell set. 
     According to a further aspect of the invention there is provided a vehicle comprising the traction battery. 
     According to a further aspect of the invention there is provided a control system for controlling a circuit for a traction battery of a vehicle, the control system comprising one or more controllers, wherein the control system is configured to: control switching means of the circuit to electrically interconnect in series a first cell set comprising one or more battery cells and a second cell set comprising one or more battery cells; and control the switching means of the circuit to electrically interconnect in parallel the first cell set and the second cell set in a first path that comprises at least one component, wherein the at least one component is configured to control electrical energy transfer between the first cell set and the second cell set associated with the first cell set and the second cell set having unequal voltages. 
     The one or more controllers may collectively comprise: at least one electronic processor having an electrical input for receiving information from one or more sensors and/or one or more external controllers; and at least one electronic memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to cause the control system to control the switching means in dependence on the information. 
     The control system may be configured to: receive information capable of indicating a detected or expected voltage imbalance between the first cell set and the second cell set; determine whether to electrically interconnect in parallel the first cell set and the second cell set in the first path comprising the at least one component, or in a second path that does not comprise the at least one component, in dependence on the received information; and control the switching means of the circuit in dependence on the determination. 
     The control system may be configured to: receive information indicative of a required vehicle charging voltage; determine whether to electrically interconnect the first cell set and the second cell set in series or in parallel, in dependence on the received information; and control the switching means in dependence on the determination. 
     The control system may be configured to: receive information indicative of a requirement to electrically disconnect one of the first cell set or the second cell set from a terminal while the other of the first cell set or the second cell set remains electrically connected to the terminal; and control the switching means to electrically disconnect the first cell set from the terminal without electrically disconnecting the second cell set from the terminal, in dependence on the requirement being to electrically disconnect the first cell set from the terminal; or control the switching means to electrically disconnect the second cell set from the terminal without electrically disconnecting the first cell set from the terminal, in dependence on the requirement being to electrically disconnect the second cell set from the terminal. 
     According to a further aspect of the invention there is provided a system comprising the control system, the circuit, the first cell set and the second cell set. 
     According to a further aspect of the invention there is provided a vehicle comprising the system. 
     According to a further aspect of the invention there is provided a method of controlling a circuit for a traction battery of a vehicle, the method comprising: controlling switching means of the circuit to electrically interconnect in series a first cell set comprising one or more battery cells and a second cell set comprising one or more battery cells; and controlling the switching means of the circuit to electrically interconnect in parallel the first cell set and the second cell set in a first path that comprises at least one component, wherein the at least one component is configured to control electrical energy transfer between the first cell set and the second cell set associated with the first cell set and the second cell set having unequal voltages. 
     According to a further aspect of the invention there is provided computer software that, when executed, is arranged to perform any one or more of the methods described herein. 
     According to a further aspect of the invention there is provided a non-transitory computer readable medium comprising computer readable instructions that, when executed by a processor, cause performance of any one or more of the methods described herein. 
     According to a further aspect of the invention there is provided a circuit for a traction battery of a vehicle, the circuit comprising: switching means operable to control electrical interconnection of a plurality of cells in a first configuration to provide a first terminal voltage, and operable to control electrical interconnection of the plurality of cells in a second configuration to provide a second terminal voltage different from the first terminal voltage and greater than 300 volts. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in 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/or 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates an example of a vehicle; 
         FIG. 2  illustrates an example of a traction battery, a traction motor and a control system; 
         FIG. 3A  illustrates an example of a control system comprising a controller; 
         FIG. 3B  illustrates an example of a non-transitory computer readable medium storing instructions; 
         FIG. 4  illustrates an example of a circuit; 
         FIG. 5  illustrates an example of modes of the switching means of the circuit of  FIG. 4 ; 
         FIG. 6  illustrates an example of a method; 
         FIG. 7  illustrates an example of another method; and 
         FIG. 8  illustrates an example of a further method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of a vehicle  10  in which embodiments of the invention can be implemented. In some, but not necessarily all examples, the vehicle  10  is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as industrial vehicles. 
     The vehicle  10  may be an electric vehicle (EV) or a hybrid electric vehicle (HEV). If the vehicle  10  is an HEV, the vehicle  10  may be a plug-in HEV or a mild HEV. If the vehicle  10  is a plug-in HEV, the vehicle  10  may be a series HEV or a parallel HEV. 
     In a parallel HEV, a traction motor and an internal combustion engine are operable in parallel to simultaneously provide tractive torque. In a series HEV, the internal combustion engine generates electricity and the traction motor exclusively provides tractive torque. 
       FIG. 2  illustrates a system  200  comprising a traction battery  20  (‘battery’ herein) and a control system  26  for the EV or HEV  10 , which may be supplied together or separately.  FIG. 2  also illustrates a traction motor  28  which could optionally be part of the system  200 . 
     The battery  20  may be a high voltage battery, particularly if the vehicle  10  is an EV or a plug-in HEV. High voltage traction batteries provide nominal voltages in the hundreds of volts, as opposed to traction batteries for mild HEVs which provide nominal voltages in the tens of volts. The battery  20  may have a voltage and capacity to support electric only driving for sustained distances requiring continuous battery power. 
     The battery  20  may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or even over a hundred kilowatt-hours. 
     The battery  20  comprises a positive terminal  22  and a negative terminal  24 . The terminals  22 ,  24  may be configured for connection to a high voltage bus  27 . The high voltage bus  27  may be configured to supply energy from the battery  20  to power electronics, such as an inverter (not shown), and onwards to the traction motor  28 . 
     The high voltage bus  27  may also comprise charging circuitry, for connecting a charging port (not shown) and/or a generator to the battery  20 . 
     Although one battery  20  is shown, the vehicle  10  could comprise additional traction batteries. 
     The illustrated traction motor  28  may be configured to output tractive torque directly or indirectly to one or more wheels of the vehicle  10 . The traction motor  28  may also be configured to operate as a regenerative brake generator for converting kinetic energy of the vehicle  10  to electrical energy for the battery  20 . 
     Although one traction motor  28  is shown, the vehicle  10  could comprise additional traction motors, for the same or different wheels of the vehicle  10 . 
       FIG. 3A  illustrates how the control system  26  may be implemented. The control system  26  of  FIG. 3A  illustrates a controller  30 . In other examples, the control system  26  may comprise a plurality of controllers on-board and/or off-board the vehicle  10 . 
     The controller  30  of  FIG. 3A  includes at least one electronic processor  32 ; and at least one electronic memory device  34  electrically coupled to the electronic processor and having instructions  36  (e.g. a computer program) stored therein, the at least one electronic memory device  34  and the instructions  36  configured to, with the at least one electronic processor  32 , cause any one or more of the methods described herein to be performed. 
     According to an example implementation, the controller  30  of  FIG. 3A  is a battery management system (BMS). The BMS may be internal to or external from a protective housing of the battery  20 . 
       FIG. 3B  illustrates a non-transitory computer-readable storage medium  38  comprising the instructions  36  (computer software). 
       FIG. 4  illustrates a battery  20  comprising an example of the circuit  400  described herein. In some examples, the battery  20  of  FIG. 4  may be the battery  20  of  FIG. 2 , for a vehicle  10  such as the vehicle  10  of  FIG. 1 . 
     The battery  20  comprises a first cell pack  402  and a second cell pack  404 . 
     The first cell pack  402  corresponds to a first cell set of one or more cells  402   a,    402   b  . . . . The first cell set may comprise a plurality of cells. A series string of two cells  402   a,    402   b  is shown in  FIG. 4 , but more cells could be provided in the string, in other examples. 
     The first cell pack  402  may be a supplied module (‘pack’) housing the first set of cells  402   a,    402   b.  Alternatively, the first set of cells  402   a,    402   b  may not be supplied as a pack, and just as a cell set  402   a,    402   b.  Although the term cell pack is used in the description below, just cell sets could be used instead, depending on implementation. 
     The individual cells of the first cell pack  402  may have equal or unequal nominal voltages. The cells of the first cell pack  402  may be interconnected in a single series string as illustrated, in parallel strings, or in a combination of parallel and series strings. 
     The nominal voltage of the first cell pack  402  may be in the hundreds of volts. In some examples, the nominal voltages of each of the cell packs is from the range 300-600 volts. In a specific example, the nominal voltage may be 400 volts to one significant figure. The nominal voltage is defined as the voltage between a positive terminal of the last cell of the string and a negative terminal of the first cell of the string. 
     Each cell pack may comprise voltage sensors (not shown) for measuring the voltage of each cell, or between the first and last cells of a string. The voltage sensor may be configured to provide voltage indications to the control system  26  or another control system for controlling pack balancing during charging or for other methods. 
     The second cell pack  404  corresponds to a second cell set of one or more cells  404 a,  404 b. 
     By design, the nominal voltage of the second cell pack  404  may be the same as the nominal voltage of the first cell pack  402 . 
     The second cell pack  404  may have the same components as the first cell pack  402 , in the same arrangement. Alternatively, the second cell pack  404  could have a different number of cells and/or the cells could be interconnected differently. 
     The circuit  400  is configured to connect a positive terminal ++ of the first cell pack  402  to the positive terminal  22  of the battery  20 . The circuit  400  is configured to connect a positive terminal ++ of the second cell pack  404  to the positive terminal  22  of the battery  20 . The circuit  400  is configured to connect a negative terminal −− of the first cell pack  402  to the negative terminal of the battery  20 . The circuit  400  is configured to connect a negative terminal −− of the second cell pack  404  to the negative terminal of the battery  20 . 
     The circuit  400  of  FIG. 4  is further configured in a particular way that solves problems associated with charging. Range anxiety and lengthy charge times challenge the adoption of zero-emissions electric vehicles. Without a fast charge option, the cell packs can take a long time to charge when plugged in to a charging station. Therefore, a fast charge option is available wherein if a compatible fast charger is connected, the cell packs can be charged faster than usual. The fast charge option may utilize a higher voltage than usual, since raising the current instead is less efficient and may exceed current limits for charging cables. Backwards compatibility with slower, lower voltage charging options should be maintained, to improve access to charging stations. 
     To enable fast charging and backwards compatibility, the circuit  400  is configured to facilitate charging at different voltages by comprising switching means. 
     The switching means of  FIG. 4  comprises a first switch A, a second switch B, a third switch C and a fourth switch D. The function of switch A will be described later, and until then the circuit  400  will be described as if switch A is open. 
     The switches may be relays, for automatic operation. The switches may be single pole single throw switches or could be merged while providing equivalent functionality. 
     Switch B is located to control interconnection of the positive terminal ++ of one of the cell packs with the positive terminal  22  of the battery  20 . In  FIG. 4 , but not necessarily in all examples, switch B is between the first cell pack  402  and the positive terminal  22  of the battery  20 . 
     Switch D controls interconnection of the negative terminal of other of the cell packs with the negative terminal of the battery  20 . In  FIG. 4 , switch D is between the second cell pack  404  and the negative terminal. The first cell pack  402  may be connected to the negative terminal regardless of the state of switch D. 
     Switch C resides in an additional electrical path that connects the positive terminal ++ of one of the cell packs to the negative terminal −− of the other of the cell packs. When switch C is closed, the positive terminal ++ of one of the cell packs is connected to the negative terminal −− of the other of the cell packs, forming a series string of cell packs. In  FIG. 4 , switch C controls whether the positive terminal ++ of the first cell pack  402  is connected to the negative terminal −− of the second cell pack  404 . 
     In another implementation, switch C may interconnect intermediate cells of the cell packs, rather than connecting the last cell of the first cell pack  402  to the first cell of the second cell pack  404 . This enables greater control of the voltage at the terminals of the battery  20 . 
     In  FIG. 4 , but not necessarily in all examples, the electrical path comprising switch C branches from the electrical path comprising switch B, at node  442 . The node  442  is between the positive terminal ++ of the first cell pack  402  and switch B. At the negative side, the electrical path comprising switch C branches from the electrical path comprising switch D, at node  444 . The node  444  is between switch D and the negative terminal −− of the second cell pack  404 . 
     Therefore, the switches B-D of  FIG. 4  are arranged so that closing switches C and D short-circuits the first cell pack  402 , and closing switches B and C short-circuits the second cell pack  404 . 
       FIG. 5  is a table illustrating operation modes M 1 -M 5  enabled by controlling the switches B-D of  FIG. 4  in different open (0) and closed (1) states. 
     The switching means is configured to enable a higher voltage charging mode M 3  by electrically interconnecting the first cell pack  402  and the second cell pack  404  in series. In  FIG. 4 , this would comprise closing switch C. Switches B and D may be opened to prevent a short circuit. Switches B and D may be opened before switch C is closed, to prevent a short circuit. 
     In this high voltage charging mode M 3 , the nominal voltage at the terminals of the battery  20  becomes greater than the nominal voltages of the cell packs. For example, if the nominal cell pack voltages are 400 volts to one significant figure, the nominal voltage at the terminals may be greater than or equal to 500 volts to one significant figure. 
     In  FIG. 4 , the cell packs are connected end-to-end in the high voltage charging mode M 3 , so the nominal voltage at the terminals of the battery  20  in mode M 3  is the sum of the nominal voltages of the cell packs  402 ,  404 . If the nominal voltages of each of the cell packs is from the range 300-600 volts, then the nominal voltage at the terminals will become from the range 600-1200 volts. The exact voltage depends on implementation. 
     Based on an example mentioned earlier, the nominal cell pack voltages are 400 volts to one significant figure. Further, the nominal voltage at the terminals of the battery  20  may be 800 volts to one significant figure in mode M 3 . The battery  20  may therefore support 800-volt charging. 
     The switching means is also configured to enable one or more lower voltage charging modes M 2 , M 4 , M 5 . In  FIG. 4 , this would involve opening switch C and closing only switch B (mode M 4 ) or closing only switch D (mode M 2 ) or closing both of switches B and D (mode M 5 ). Mode M 5  is a parallel mode in which the first cell pack  402  and the second cell pack  404  are interconnected in parallel. Switch C may be opened first to prevent a short circuit. 
     In this lower voltage charging mode (mode M 2  or M 4  or M 5  in  FIG. 5 ), the nominal voltage at the terminals would be identical to the nominal voltages of the individual cell packs, assuming that the cell packs have identical nominal voltages to each other. 
     Therefore, if the nominal voltages of each of the cell packs is from the range 300-600 volts, then the nominal voltage at the terminals will be from the same range in mode M 2 , M 4  and M 5 . Based on an example mentioned above, the cell pack voltages are 400 volts to one significant figure. The nominal voltage at the terminals of the battery  20  may be 400 volts to one significant figure. The battery  20  may therefore support 400-volt charging. 
     In mode M 5 , the capacity of the battery  20  is double that of mode M 2  or M 4 , because in mode M 5  two cell packs are interconnected in parallel to the same terminals. The greater capacity enables full power operation. 
     Modes M 2  and M 4  are ‘half-pack’ modes in which just one of the two cell packs is operational. In mode M 2 , the switching means electrically disconnects the first cell pack  402  without electrically disconnecting the second cell set. The first cell pack  402  is open-circuit and the second cell pack  404  is closed-circuit. In  FIG. 4 , only the second cell pack  404  is operational in mode M 2 . In mode M 4 , the switching means electrically disconnects the second cell pack  404  without electrically disconnecting the first cell set. The first cell pack  402  is closed-circuit and the second cell pack  404  is open-circuit. In  FIG. 4 , only the first cell pack  402  is operational in mode M 4 . 
     A final mode M 1  shown in  FIG. 5  corresponds to switches B-D being open, so the voltage at the terminals of the battery  20  is OV. This may be useful for vehicle maintenance or deep sleep, or may be unused. 
     Although  FIG. 4  shows two cell packs, the battery  20  may comprise more cell packs in other implementations, and the circuit  400  may be extended to accommodate the additional cell packs in a straightforward manner in light of this description. 
     A difficulty can arise when the first cell pack  402  and the second cell pack  404  do not have identical voltages, even if they are rated to the same nominal voltage. The actual voltages may differ within manufacturing tolerances, or due to aging, or due to imperfect balancing during charging. 
     If the actual voltages of the cell packs differ when entering mode M 5  or when starting to draw a load while in mode M 5 , then the higher voltage cell pack will drive a short circuit current through the lower voltage cell pack until the voltages are self-equalised. As cell packs have a very low internal resistance, the short circuit current could be thousands of amps, which could cause undesirable resistive heating. 
     To mitigate this problem, the circuit  400  comprises at least one component  407  configured to control electrical energy transfer between the first cell pack  402  and the second cell pack  404 , associated with the first cell set and the second cell set having unequal voltages. In  FIG. 4 , the component  407  reduces the current magnitude of the voltage imbalance-induced equalizing short circuit current. 
     In  FIG. 4 , the component  407  comprises a resistor  408 . The resistor  408  of  FIG. 4  is an ohmic resistor. The resistance of the resistor  408  may be significantly higher than the internal resistance of the cell packs. In an example, the average or nominal resistance of the resistor  408  may be from the range approximately  1  ohm to approximately  100  ohms. The resistor  408  may be a fixed resistor or a variable resistor. 
     If the resistor  408  is a variable resistor, the resistor  408  may be configured for its resistance to increase as current rises. For example, the resistor  408  could be a positive temperature coefficient (PTC) thermistor resistor, wherein its resistance increases in proportion to ohmic heating. A variable resistor is useful for regulating the rate of voltage equalization. 
     Although one resistor  408  is shown, multiple resistors could be provided in series, and/or multiple resistors could be provided in selectable parallel branches for different voltage imbalances. The component  407  of  FIG. 4  consists of resistor(s) and no more. 
     In other examples, the component  407  could be different from a resistor. The component  407  could comprise an earthed capacitor, e.g. earthed to the vehicle chassis. If the battery is electrically floating, earthing can be inconvenient. In a further example, the component  407  may comprise a DC-DC converter. DC-DC converters are more efficient than resistors, but are complex, bulky and expensive compared to resistors, for high power applications. 
     In  FIG. 4 , the resistor  408  and switch A is on the positive side of the circuit  400 . Alternatively, the resistor  408  and switch A could be on the negative side of the circuit  400 , parallel to switch D, to achieve the same result. The resistor  408  is not in the path from node  442  to node  444 , because said path is only active in mode M 3  in which equalizing short circuit current is not a problem. 
     Since the resistor  408  is a parasitic, it would be efficient to only use the resistor  408  when it is needed, and to bypass the resistor  408  via switch B when it is not needed. 
     Therefore, in  FIG. 4 , the circuit  400  comprises a first path  409  that comprises the resistor  408 , and a second path  410  that does not comprise the resistor  408 . The first path  409  and the second path  410  are parallel to each other. In  FIG. 4 , the paths  409 ,  410  split and rejoin each other between the node  442  and the positive terminal  22  of the battery  20 . 
     The second path  410  comprises switch B. The first path  409  comprises switch A. 
     If the resistor  408  is instead on the negative side of the circuit  400 , the paths would split and rejoin between node  444  and the negative terminal  24  of the battery  20 . The second path  410  would comprise switch D instead of switch B. 
     When switch A is closed, current flows through the first path  409 . When switch B is closed, current flows through the second path  410 . The switches A and B may optionally be controlled so that only one of them is closed at any given time. Although switches A and B are illustrated as two single pole single throw switches, they could be implemented as a single pole dual throw switch or as another known type of switch for selecting which of the first path  409  and the second path  410  is active. 
     When entering mode M 5 , switch A could be closed and switch B could be opened or kept open. Switch D is closed and switch C is open. Switch C could be opened first, then switch D could be closed, then switch A could be closed. 
     When the resistor  408  is no longer required, e.g. the cell pack voltages are equalized, switch A can be opened and switch B can be closed, to avoid parasitic ohmic losses. Switch B may be closed before switch A is opened, for example if the circuit  400  is under load and a brief open circuit is to be avoided. 
     Although switch A is illustrated as a separate hardware component from the resistor  408 , the switch A and resistor  408  could be implemented using a single solid-state device. 
     Based on the foregoing, the above-described circuit  400  is able to solve the above-described problems using a very small number of generic components. 
     The circuit  400  shown in  FIG. 4  comprises some additional optional components that are advantageous for improving fault tolerance. These additional components will now be described. 
     The circuit  400  comprises one or more fuses configured to electrically disconnect one or more cell packs from a terminal of the battery  20 . A difference between fuses and switches is that the switches can be reversibly automatically actuated, whereas fuses need to be manually reset or replaced after they actuate. 
     The fuses comprise a first fuse  412  configured to electrically disconnect the first cell pack  402  without electrically disconnecting the second cell pack  404 . The first cell pack  402  becomes open-circuit. The fuses comprise a second fuse  414  configured to electrically disconnect the second cell pack  404  without electrically disconnecting the first cell set. The second cell pack  404  becomes open-circuit. The fuses additionally comprise a third, series-breaker fuse  406  configured to break the series electrical interconnection of the first cell pack  402  and the second cell pack  404 , e.g. mode M 5 . 
     The first fuse  412  and the second fuse  414  may be rated to open the circuit  400  if a cell pack is short circuited. A cell pack may be short circuited when one of the ‘not used’ states of the table of  FIG. 5  occur, e.g. if switches [B C D] are [0 1 1], [1 1 0] or [1 1 1]. The first fuse  412  and the second fuse  414  therefore protect the cell packs from short circuits. Short circuits may occur if a switch becomes stuck, for example. 
     The fuses may be rated for sufficiently high currents that they will not open the circuit  400  for normal in-use loads, and will not open the circuit  400  when the resistor  408  is in-circuit for controlling cell pack voltage equalization. 
     In  FIG. 4 , the first fuse  412  is between the negative terminal  24  of the battery  20  and the negative terminal −− of the first cell pack  402 . Alternatively, the first fuse  412  may be between the positive terminal ++ of the first cell pack  402  and the positive terminal  22  of the battery  20 , or more specifically between the positive terminal ++ of the first cell pack  402  and the node  442 . 
     In  FIG. 4 , the second fuse  414  is between the negative terminal  24  of the battery  20  and the negative terminal −− of the second cell pack  404 . More specifically, the second fuse  414  may be between the node  444  and the negative terminal −− of the second cell pack  404 , as shown in  FIG. 4 . Alternatively, the second fuse  414  may be between the positive terminal ++ of the second cell pack  404  and the positive terminal  22  of the battery  20 . 
     The series-breaker fuse  406  is between the nodes  442  and  444 , to one side of switch C. This enables the series electrical interconnection of mode M 3  to be broken without preventing parallel electrical interconnection of mode M 3 . This is useful if switch C is stuck closed and a mode change to mode M 2 , M 4  or M 5  is required. 
     In an alternative implementation, just one fuse may be provided if a location exists where all cell packs can be disconnected by one fuse. However, an advantage of having multiple fuses such as in  FIG. 4  is that the battery  20  can continue to operate with a reduced number of cell packs. For example, the battery  20  could operate in modes M 2  or M 4  if the first or second fuse  412 ,  414  blows. In another example, the battery  20  could operate in a mode other than M 3  if the series-breaker fuse  406  blows. Therefore, fault tolerance is improved. 
     One or more of the fuses may be non-resettable. One or more of the fuses may be passive and self-actuating. The first fuse  412  and the second fuse  414  may be passive and self-actuating. A passive self-actuating fuse may be actuated by melting of a load-bearing internal conductor due to excessive current flow. 
     The series-breaker fuse  406  may have a different construction from the first fuse  412  and the second fuse  414 . The series-breaker fuse  406  may be actuated by an external controller. The series-breaker fuse  406  may be actuated by a pyrotechnic fuse actuator or other externally controllable actuator. External control is useful for faults in which excess currents do not occur. The fuse can be designed with lower internal resistance than the first fuse  412  and the second fuse  414 , to reduce parasitic losses. An externally controlled fuse is well-suited to the higher voltage associated with the series mode M 3 . 
     The series-breaker fuse  406  can be used in a fault situation when switch C is stuck closed. If the traction motor  28  of the vehicle  10  is only rated for lower voltage (e.g.  400 V) operation, it may be necessary to open switch C and transition out of high voltage mode M 3  when charging is complete and the vehicle  10  is to be driven. Otherwise, the battery  20  would be unable to supply the required lower voltage for the traction motor  28 , and the vehicle  10  would be undriveable. 
     The circuit  400  also comprises one or more isolators configured to electrically disconnect one or more cell packs from a terminal of the battery  20   
     A difference between an isolator and a fuse is that fuse actuation may be non-reversible whereas isolator actuation is reversible. Further, a fuse may actuate automatically, whereas an isolator is manually actuated. The isolators may be for manual service disconnection of cell packs. The isolators may comprise an externally accessible button, lever, staple, rotary switch, or other interface for manual actuation of the isolator. The interface may be on a housing of the battery  20 , as opposed to elsewhere on the vehicle  10 . A fire service isolator could be provided elsewhere on the vehicle. 
     The isolators comprise a first isolator  432  configured to electrically disconnect the first cell set  402  without electrically disconnecting the second cell set. The first cell set  402  becomes open-circuit. The isolators comprise a second isolator  434  configured to electrically disconnect the second cell set  404  without electrically disconnecting the first cell set. The second cell set  404  becomes open-circuit. The isolators may be interconnected to ensure that a single service action will disconnect both the first and second cell set. For example the isolators may be included in a single physical unit where both cell sets are simultaneously disconnected by the operation of the single physical unit. 
     The isolators  432 ,  434  are shown on the negative side of the battery  20 , but they could be on the positive side instead or even within the cell packs  402 ,  404 . 
     In an alternative implementation, just one isolator may be provided if a location exists where all cell packs can be disconnected by one isolator. 
     The circuit  400  may further comprise one or more current sensors. The circuit  400  of  FIG. 4  comprises a first current sensor  422  and a second current sensor  424 . The current sensors may be configured to monitor equality of current flow in the parallel mode M 5 . 
     The first current sensor  422  is between the first cell pack  402  and the positive or negative terminal  24  of the battery  20 . The second current sensor  424  is between the second cell pack  404  and the positive or negative terminal  24  of the battery  20 . 
     The current sensors may be configured to provide current indications to the control system  26  for controlling the switching means and/or the series-breaker fuse  406 . 
       FIGS. 6 to 8  describe various control methods that may be performed by the control system  26  and applied to the switching means of the circuit  400  as described above. 
       FIG. 6  is a method  60  for obviating or reducing detected or expected voltage imbalance-induced equalizing short circuit current. 
     In some examples, the method  60  is not performed while the vehicle is charging. The method  60  is not a cell pack balancing method. 
     In a specific example, the method  60  may be performed when the cell packs  402  and  404  are interconnected to the battery terminals  22 ,  24  in parallel, such as in mode M 5 . 
     The method  60  may be performed when switching to mode M 5 , for example on completion of plug-in charging or when the vehicle enters a travelable state. 
     The method  60  may be performed before a load is to be drawn and/or before charging is to commence while in mode M 5 , or may be performed when entering mode M 5  from a different mode. For example, a load is to be drawn by the traction motor when the vehicle  10  enters a travelable state and is to be driven. 
     At block  62  of the method  60 , the method comprises receiving information capable of indicating a detected or expected voltage imbalance between the first cell pack  402  and the second cell pack  404 . 
     Information indicating an expected voltage imbalance may simply be a command to switch to parallel mode M 5  and/or that a load is to be drawn while in/entering mode M 5 . This is because some imbalance of an unknown value is inherently expected. The command may be issued on completion of charging and/or when an event associated with entering a travelable state occurs, such as entering an ignition power mode associated with entering a travelable state. 
     Information indicating an expected voltage imbalance may be from voltage sensors of the cell packs. 
     Information indicating a detected voltage imbalance may be from the current sensors. This is less desirable because current is already flowing. 
     At decision block  64  of the method  60 , the method comprises determining whether to electrically interconnect in parallel the first cell pack  402  and the second cell pack  404  in the first path  409  comprising the resistor  408 , or in the second path  410  that does not comprise the resistor  408 , in dependence on the received information. 
     The decision may be always yes (first path  409 ), in the case of receiving a command. If the decision is based on voltage or current, the decision may be yes if the voltage difference is above a threshold, or if the current is above a threshold. If the threshold is not exceeded, the decision may be no (second path  410 ). 
     At blocks  66  and  68  of the method  60 , the method comprises controlling the switching means of the circuit  400  in dependence on the determination. At block  66 , the first path  409  is used. In  FIG. 4 , using the first path  409  would involve closing switch A (if open), opening switch B (if closed) and closing switch D (if open). At block  68 , the second path  410  is used. In  FIG. 4 , using the second path  410  would involve opening switch A (if closed), closing switch B (if open) and closing switch D (if open). 
     Although not shown, the first path  409  may be used for a limited time, before switching to the second path  410 . This is because the short circuit equalizing current is only a brief inrush, and then the cell packs are equalized. This reduces parasitic ohmic losses through the resistor  408 . A timer may be implemented, with a duration calibrated based on the rate of equalization controlled by the resistor  408 . Alternatively, the voltage and/or current may be monitored in a closed loop, and the second path  410  may be switched to once the voltage difference or current falls below a threshold, which may be the same or different as the threshold of block  64 . 
     The control system  26  may be configured to use the first path  409  only while the battery  20  is not being charged or discharged, so that the resistor  408  does not cause voltage imbalance between the first and second cell packs  402 ,  404 . 
       FIG. 7  is a method  70  for controlling a charging mode of the battery  20 . At optional block  72  of the method  70 , the method comprises enabling a charging voltage selection. The selection may be by a user. If a charging station supports low voltage and high voltage mode operation (e.g. 400V and 800V), the control system  26  may cause a user interface of the vehicle  10  or of the charging station to provide the user with a user interface element to specify which voltage is desired. Alternatively, the required voltage may be automatically determined based on signals automatically transmitted between the vehicle  10  and the charging station upon connecting the vehicle  10  to the charging station. At block  74  of the method  70 , the method comprises receiving information indicative of a required vehicle charging voltage. The information may be from the user selection or from the automatically transmitted signal. At block  76  of the method  70 , the method comprises determining whether to electrically interconnect the first cell pack  402  and the second cell pack  404  in series or in parallel, in dependence on the received information. At blocks  78  and  79  of the method  70 , the method comprises controlling the switching means in dependence on the determination. If a higher voltage charging mode is required, the cell packs are connected in series mode M 3  as per block  78 . In  FIG. 4 , this comprises closing switch C and opening switches B and D. If a lower voltage charging mode is required, the cell packs are connected in parallel mode M 5  as per block  79 . Mode M 5  may be the standard for lower voltage charging, to enable all cell packs to be charged. 
     The methods  60  and  70  of  FIGS. 6 and 7  together enable the functionality of a method of robustly switching between at least modes  3  and  5 , the method comprising: controlling switching means of the circuit  400  to electrically interconnect in series a first cell pack  402  comprising one or more battery cells and a second cell pack  404  comprising one or more battery cells (block  78 ); and controlling the switching means of the circuit  400  to electrically interconnect in parallel the first cell pack  402  and the second cell pack  404  (block  79 ) in a first path  409  that comprises at least one component  407  (block  66 ), wherein the at least one component  407  is configured to control electrical energy transfer between the first cell pack  402  and the second cell pack  404  associated with the first cell pack  402  and the second cell pack  404  having unequal voltages. When the timer expires or the voltages are equalized, the electrical interconnection of the first cell pack  402  and the second cell pack may be switched from the first path  409  to the second path  410 , to reduce parasitic losses. 
       FIG. 8  is a method  80  for controlling operation of the battery  20  in a reduced-pack mode. This enables the vehicle  10  to be charged and/or driven with reduced power. At block  82  of the method  80 , the method comprises receiving information indicative of a requirement to electrically disconnect one of the first cell pack  402  or the second cell pack  404  from a terminal of the battery  20  while the other of the first cell pack  402  or the second cell pack  404  remains electrically connected to the terminal. The information may be based on readings by the voltage sensors and/or by the current sensors, or from any other sensor that monitors whether a cell pack should be used. The information may comprise the sensor readings, or a flag if another control system has analysed the readings. The information may be indicative of a requirement to electrically disconnect a cell pack when a fault condition associated with the monitored readings is satisfied. An example of a fault condition being satisfied is when a switch is stuck while trying to change modes. At block  84  of the method  80 , the method comprises controlling the switching means to electrically disconnect the required cell pack from the terminal without disconnecting at least one other cell pack from the terminal. For example, if the first cell pack  402  should be removed, mode  2  can be entered by closing switch D and opening switches B and C. If the second cell pack  404  should be removed, mode  4  can be entered by closing switch B and opening switches C and D. The method  80  may correspond to a limp home mode. A warning may be output to a human-machine interface, prompting a vehicle service. 
     In an alternative implementation of the method  80 , the battery  20  is operated in a reduced-pack mode for reasons other than faults. For example, the reduced-pack mode may act as a power restrictor or range restrictor function. For example, a vehicle administrator user could use this function to restrict an inexperienced driver or borrower of the vehicle  10 . 
     Although examples in the preceding description refer to series/parallel charging, the series/parallel concepts could be applied in a similar way for a discharging use case. For example, if one or more traction motors are capable of operation at different voltages, the circuit  400  could be controlled as described above to provide the different voltages when needed. Examples in the preceding description refer to changing modes. A mode change may be a direct change, or an indirect change via mode M 1 . 
     For purposes of this disclosure, it is to be understood that the controller(s)  30  described herein can each comprise a control unit or computational device having one or more electronic processors. A vehicle  10  and/or a system thereof may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the described method(s)). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on one or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present disclosure is not intended to be limited to any particular arrangement. In any event, the set of instructions described above may be embedded in a computer-readable storage medium (e.g., a non-transitory computer-readable storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions. 
     It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 
     As used here ‘module’ refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user. As used here, ‘connected’ means ‘electrically interconnected’ either directly or indirectly. Electrical interconnection does not have to be galvanic. Where the control system is concerned, connected means operably coupled to the extent that messages are transmitted and received via the appropriate communication means. 
     The term ‘current’ means electrical current. The term ‘Voltage’ means potential difference. The term ‘series’ means electrical series. The term ‘parallel’ means electrical parallel. ‘Active’ and ‘operational’ generally mean closed circuit. A ‘resistor’ is an electrical resistor. The term ‘power’ means electrical power. The term ‘charging’ means electrical recharging of the battery. 
     The blocks illustrated in the  FIGS. 6-8  may represent steps in a method and/or sections of code in the computer program  36 . The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, the nominal voltage of the first cell pack  402  may differ from the nominal voltage of the second cell pack  404 . The component  408  may therefore have a higher resistance and may be always in-circuit. 
     Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.