Patent Publication Number: US-2023163344-A1

Title: Secondary battery pack, charger and discharger

Description:
TECHNICAL FIELD 
     The present invention relates to a secondary battery pack, a charger configured to charge the secondary battery pack, and a discharger configured to discharge the secondary pack. 
     BACKGROUND ART 
     Batteries are a type of device for utilizing, as electric energy, chemical energy which results from a chemical reaction between a positive active material and negative active material through an electrolyte. Batteries are roughly divided into primary and secondary batteries. The primary battery is, so to speak, a single-use battery that cannot be recharged once it has been discharged, whereas the secondary battery is a type of battery that can be repeatedly charged and discharged. Due to such a nature, the secondary battery is also called the storage battery. 
     Secondary batteries are used in various applications. For example, they are used as a power source for small-sized consumer appliances, such as a handheld information terminal, mobile electronics, portable music player or digital camera, or as a battery for an electric bicycle, hybrid electric vehicle, or drone. They are also used as a storage battery device to be embedded in a power generator which uses natural energy (renewable energy), such as solar generation or wind-power generation (Patent Literature 1 or 2). 
     The charging of a secondary battery is performed by setting the secondary battery in a charger which is connected, for example, to a commercial AC power supply. The charger has a power conditioner (converter) configured to convert alternating current into direct current. The device converts the alternating current from the commercial AC power supply into direct current, and passes the resulting current between the positive electrode and negative electrode of the secondary battery. 
     On the other hand, small-sized consumer appliances, electric vehicles and other aforementioned types of products are normally driven by alternating current. Therefore, in order to drive such AC-driven equipment by the direct current released from a secondary battery, it is necessary to convert direct current into alternating current by means of a power conditioner (inverter) or similar device. 
     In summary, a two-stage power conversion including AC-to-DC power conversion and DC-to-AC power conversion is needed in order to supply AC-driven equipment with electric power if a conventional and common type of secondary battery is used. The power conversion efficiency from DC to AC through an inverter as well as the power conversion efficiency from AC to DC through a converter are each within a range from 80% to 90%. It cannot reach 100%. Accordingly, there is the problem that a power loss associated with the power conversion is present, which lowers the power utilization efficiency. 
     With respect to this problem, Patent Literature 3 discloses a new secondary battery capable of the charging and discharging of the alternating current and alternating power. This secondary battery has three electrodes, i.e. the positive electrode and negative electrode as well as a bipolar electrode located between the positive and negative electrodes, having an intermediate electrode potential between the positive and negative electrodes. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2012-221670 A 
     Patent Literature 2: JP 2013-069431 A 
     Patent Literature 3: JP 2016-171075 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     When the secondary battery described in Patent Literature 3 is charged or discharged, the battery is alternately switched between the state in which the positive terminal and bipolar terminal are connected to a pair of output ends of a commercial AC power supply or a pair of input ends of a load, and the state in which the negative terminal and bipolar terminal are connected to the pair of output or input ends. Consequently, cations alternately move between the positive and bipolar electrodes as well as between the bipolar and negative electrodes. During the period in which cations are moving between the positive and bipolar electrodes, electrons flow between the positive and bipolar electrodes through an external circuit. During the period in which cations are moving between the bipolar and negative electrode, electrons flow between the bipolar and negative electrodes through an external circuit. Since the direction of the flow of the electrons is inverted for each period, the charging and discharging of the alternating current is possible. 
     For the previously described charging and discharging operation, it is necessary that the electrons which have come from the positive or negative electrode to the bipolar electrode should subsequently be made to flow toward the negative or positive electrode of the same battery. In other words, the charging and discharging of the alternating current cannot be achieved if the electrons which have come from the positive or negative electrode to the bipolar electrode in one secondary battery are subsequently made to flow to the negative or positive electrode of another secondary battery. This means that two or more secondary batteries having previously described configuration cannot be serially connected for use, which prevents the previously described secondary battery from being used as a high-voltage power source. 
     The problem to be solved by the present invention is to create a secondary battery which allows for the charging and discharging of the alternating current as well as one which produces high voltage. 
     Solution to Problem 
     The first aspect of the present invention developed for solving the previously described problem is a secondary battery pack including: 
     a first battery and a second battery each of which includes a positive terminal and a negative terminal; 
     a third battery including a positive terminal, a negative terminal and a bipolar terminal; 
     a first connector configured to electrically connect the negative terminal of the first battery and the positive terminal of the third battery; and 
     a second connector configured to electrically connect the positive terminal of the second battery and the negative terminal of the third battery, 
     where the third battery includes the following elements, with n being an integer equal to or greater than zero: 
     (n+1) positive electrodes and (n+1) negative electrodes alternately arranged; 
     (2n+1) bipolar electrodes individually located in the spaces between the positive electrodes and the negative electrodes neighboring each other; 
     an electrolyte; 
     a positive-electrode connection member configured to electrically connect the positive terminal of the third battery and the (n+1) positive electrodes; 
     a negative-electrode connection member configured to electrically connect the negative terminal of the third battery and the (n+1) negative electrodes; and 
     a bipolar-electrode connection member configured to electrically connect the bipolar terminal of the third battery and each of the (2n+1) bipolar electrodes. 
     The second aspect of the present invention developed for solving the previously described problem is a secondary battery pack including: 
     a first battery and a second battery each of which includes a positive terminal and a negative terminal; 
     a third battery including a positive terminal, a negative terminal and a bipolar terminal; 
     a first connector configured to electrically connect the negative terminal of the first battery and the positive terminal of the third battery; and 
     a second connector configured to electrically connect the positive terminal of the second battery and the negative terminal of the third battery, 
     where the third battery includes the following elements, with n being an integer equal to or greater than zero: 
     either (n+2) positive electrodes and (n+1) negative electrodes alternately arranged, or (n+2) negative electrodes and (n+1) positive electrodes alternately arranged; 
     (2n+2) bipolar electrodes individually located in the spaces between the positive electrodes and the negative electrodes neighboring each other; 
     an electrolyte; 
     a positive-electrode connection member configured to electrically connect the positive terminal of the third battery and the (n+2) or (n+1) positive electrodes; 
     a negative-electrode connection member configured to electrically connect the negative terminal of the third battery and the (n+1) or (n+2) negative electrodes; and 
     a bipolar-electrode connection member configured to electrically connect the bipolar terminal of the third battery and the (2n+2) bipolar electrodes. 
     In the secondary battery pack according to the first or second aspect of the present invention, the third battery having the positive and negative electrodes as well as the bipolar electrodes arranged between the positive and negative electrodes enables the charging and discharging of alternating current. The serial connection of the third electrode to the first battery as well as to the second battery enables the production of high voltage. 
     The third battery has positive, bipolar and negative electrodes (which may hereinafter be simply called the “electrodes” when no discrimination is made between the positive, bipolar and negative electrodes) arranged in a predetermined order in the electrolyte, with a separator sandwiched between the neighboring electrodes. Being “arranged in order” does not necessarily mean that the electrodes are linearly arranged; it includes various arrangements, such as a U-shaped, V-shaped or zigzag form. It is also possible to prepare each of the positive, negative and bipolar electrodes in the form of a sheet electrode, and to join the electrodes together in a laminated form with a separator sandwiched between the neighboring electrodes. 
     In any of the previously described secondary battery packs, commercially available secondary batteries may be used as the first and second batteries. In that case, a plurality of secondary battery cells may be connected in series or in parallel and used as the first or second battery. Connecting a plurality of secondary battery cells in series enables the first and second batteries to produce high voltage. Connecting a plurality of secondary battery cells in parallel provides the first and second batteries with a high amount of capacity. Accordingly, a plurality of secondary battery cells connected in series, or those connected in parallel, can be used as the first battery and/or second battery depending on the purpose of use of the secondary battery pack. 
     Let Ec denote the electrode potential of the positive electrodes of the third battery, Ea denote the electrode potential of the negative electrodes, and Eb denote the electrode potential of the bipolar electrodes (“Biodes”). The three potentials satisfy the relationship expressed as Ec&gt;Eb&gt;Ea. Such a relationship in potential level reflects the capacity of each electrode to give or receive electrons. The active materials for the positive, negative and bipolar electrodes, the material for the current collector, as well as the electrolyte should be selected to satisfy the aforementioned relational expression. As far as the aforementioned relational expression is satisfied, the secondary battery pack according to the first or second aspect of the present invention can be applied in any type of secondary battery, such as a nickel-hydride secondary battery, nickel-cadmium secondary battery, lead-acid secondary battery, lithium ion secondary battery, or sodium ion secondary battery. 
     The electrode potential of the positive electrodes of the first and second batteries only needs to be higher than the electrode potential Ea of the negative electrodes of the third battery, while it is insignificant whether the electrode potential in question is higher or lower than the electrode potential Ec of the positive electrodes of the third battery. Similarly, the electrode potential of the negative electrodes of the first and second batteries only needs to be lower than the electrode potential Ec of the positive electrodes of the third battery, while it is insignificant whether the electrode potential in question is higher or lower than the electrode potential Ea of the negative electrodes of the third battery. 
     Examples of the active material for the positive electrodes of the first, second and third batteries include, but not limited to, the compounds represented by the following chemical formulae: LiCoPO 4 , LiNiPO 4 , LiNiVO 4 , LiMn 3/2 Ni 1/2 O 4 , LiCoPO 4 , LiPtO 3 , LiCrMnO 4 , LiMn 2 O 4 , LiMnPO 4 , LiNi 1/3 Mn 1/3 Co 1/3 O 2 , LiNi 1/2 Mn 1/2 O 2 , LiNi 4/5 Co 1/5 O 2 , LiCoVO 4 , LiCoO 2 , LINiO 2 , LiFe 2 (SO 4 ) 2 , LIFePO 4 , Li 1+x (Fe 2/5 Mn 2/5 Ti 1/5 ) 1−x O 2 , Li 2 FeSiO 4 , and Li 2 MnSiO 4 . 
     Examples of the active material for the negative electrodes of the first, second and third batteries include, but not limited to, the materials represented by the following names: lithium metal, lithium alloy, graphite, graphene (two-dimensional graphite sheet), hard carbon, soft carbon, crystal(line) silicon, amorphous silicon, silica (silicon dioxide), silicene (two-dimensional silicon sheet), tin metal, and tin alloy. 
     For example, when the secondary battery pack according to the first or second aspect of the present invention is applied in a lithium ion secondary battery, LiFePO 4  can be used as the active material of the positive electrodes, a carbon-based material as the active material of the negative electrodes, and Li 4/3 Ti 5/3 O 4  as the active material of the bipolar electrodes. 
     When such a secondary battery pack is charged, one of the two output terminals of an AC power source is selectively connected to either the positive terminal of the first battery or the negative terminal of the second battery, while the other output terminal is connected to the bipolar terminal of the third battery. When the former output terminal of the AC power source is connected to the positive terminal of the first battery, electrons flow from the positive terminal of the first battery toward the bipolar terminal (bipolar electrode) of the third battery through an external circuit, causing the oxidization of the active material of the positive electrode in the first battery. The resulting cations (positive ions, Li + ) move toward the negative electrode, to be eventually reduced to C 6 Li. Meanwhile, the active material of the positive electrode in the third battery undergoes oxidization, and the resulting cations move toward the bipolar electrode. 
     The former output terminal of the AC power source is subsequently connected to the negative terminal of the second battery. Then, electrons flow from the bipolar terminal of the third battery toward the negative terminal of the second battery through the external circuit, and cations move from the bipolar electrode toward the negative electrode in the third battery, to be eventually reduced to C 6 Li. Meanwhile, the active material of the positive electrode in the second battery undergoes oxidization, and the resulting cations (positive ions, Li + ) move toward the negative electrode, to be eventually reduced to C 6 Li. 
     The chemical reactions which respectively occur at the electrodes in the first, second and third batteries in the charging process are expressed by the following formulae (1) and (2): 
       Cathode: 
       LiFe (III) PO 4 →Fe (III) PO 4 +Li + +e −   (1)
 
       Biode: 
       Li[Li 1/3 Ti (IV)   5/3 ]O 4 +Li + +e − →Li 2 [Li 1/3 Ti (III)   3/3 Ti (IV)   2/3 ]O 4 
 
       Biode: 
       Li 2 [Li 1/3 Ti (III)   3/3 Ti (IV)   2/3 ]O 4 →Li[Li 1/3 Ti (IV)   5/3 ]O 4 +Li + +e −   (2)
 
       Anode: 
       6C+Li + +e − →C 6 Li
 
     Thus, the secondary battery pack according to the first or second aspect of the present invention is charged through the two-stage chemical reactions. The flow of the electrons occurs in the opposite direction in each stage. Accordingly, the battery pack can be charged with the alternating current by alternately connecting the bipolar terminal of the third battery to either the positive terminal of the first battery or the negative terminal of the second battery according to the direction of the alternating current taken from the AC power supply. 
     In the discharging process, the chemical reactions at the electrodes in the first, second and third batteries occur in the direction opposite to the charging process. Initially, the active material of the negative electrode in the second battery undergoes oxidization. The resulting cations (positive ions, Li + ) move toward the positive electrode, to be eventually reduced to LiFePO 4 . Meanwhile, the active material of the negative electrode in the third battery undergoes oxidization, and the resulting cations (positive ions, Li + ) move toward the bipolar electrode. During this process, electrons flow from the negative terminal of the second battery toward the bipolar electrode of the third battery through the external circuit. Subsequently, the cations move from the bipolar electrode toward the positive electrode in the positive electrode, to be eventually reduced to LiFePO 4 . Meanwhile, the active material of the negative electrode in the first battery undergoes oxidization, and the resulting cations (positive ions, Li + ) move toward the positive electrode, to be eventually reduced to LiFePO 4 . During this process, electrons flow from the bipolar terminal of the third battery toward the positive terminal of the first battery through the external circuit. Accordingly, alternating current can be released by switching the electrode to be connected to the bipolar electrode between the negative terminal of the second battery and the positive terminal of the first battery at appropriate timings. 
     The secondary battery pack according to the first or second aspect of the present invention does not only allow for the charging and discharging of the alternating current but can also be modified to allow for the charging and discharging of the direct current by providing a switching circuit at each of the positive and negative sides of a direct-current battery and operating the switching circuits in such a manner as to switch the battery pack at appropriate timings between the state in which the positive terminal of the first battery and the bipolar terminal of the third battery are respectively connected to the positive electrode and the negative electrode of the direct-current battery, and the state in which the bipolar terminal of the third battery and the negative terminal of the second battery are respectively connected to the positive electrode and the negative electrode of the direct-current battery. 
     The previously described secondary battery packs can be charged and discharged by the following device. 
     That is to say, another aspect of the present invention provides a charger configured to charge one of the previously described secondary battery packs, the charger including: 
     first and second input lines to be connected to a pair of output ends of an AC power supply; 
     a positive-electrode terminal and a negative-electrode terminal located on the first input line and configured to be connected to the positive terminal of the first battery and the negative terminal of the second battery of the secondary battery pack, respectively; 
     a bipolar-electrode terminal located on the second input line and configured to be connected to the bipolar terminal of the third battery of the secondary battery pack; 
     a switching means located between the first input line and the positive-electrode terminal as well as between the first input line and the negative-electrode terminal, and configured to switch between a first connection state in which the first input line is connected to the positive-electrode terminal and a second connection state in which the first input line is connected to the negative-electrode terminal; and 
     a controller configured to control the switching means based on the frequency of the AC power supply. 
     Still another aspect of the present invention is a discharger configured to discharge one of the previously described secondary battery packs, the discharger including: 
     first and second output lines to be connected to a pair of input terminals of a load; 
     a positive-electrode terminal and a negative-electrode terminal located on the first output line and configured to be connected to the positive terminal of the first battery and the negative terminal of the second battery of the secondary battery pack, respectively; 
     a bipolar-electrode terminal located on the second output line and configured to be connected to the bipolar terminal of the third battery of the secondary battery pack; 
     a switching means located between the first output line and the positive-electrode terminal as well as between the first output line and the negative-electrode terminal, and configured to switch between a first connection state in which the first output line is connected to the positive-electrode terminal and a second connection state in which the first output line is connected to the negative-electrode terminal; and 
     a controller configured to control the switching means. 
     As for the switching means, for example, a bipolar transistor can be used. 
     Yet another aspect of the present invention is a secondary battery module including: 
     one of the previously described secondary battery packs; 
     first and second input/output lines; 
     a positive-electrode terminal and a negative-electrode terminal located on the first input/output line and configured to be connected to the positive terminal of the first battery and the negative terminal of the secondary battery pack, respectively; 
     a bipolar-electrode terminal located on the second input/output line and configured to be connected to the bipolar terminal of the third battery of the secondary battery pack; 
     a switching means located between the first input/output line and the positive-electrode terminal as well as between the first input/output line and the negative-electrode terminal, and configured to switch between the state in which the first input/output line is connected to the positive-electrode terminal and the state in which the first input/output line is connected to the negative-electrode terminal; and 
     a controller configured to control the switching means. 
     Advantageous Effects of Invention 
     In the secondary battery pack according to the present invention, the third battery having the positive, negative and bipolar electrodes enables the charging of the AC power from an AC power supply without the help of a power conditioner, as well as the discharging of the AC power without the help of a power conditioner. Furthermore, the serial connection of the first battery and the second battery, which are common types of secondary batteries, to the third battery enables the secondary battery pack to produce high voltage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic configuration diagram of the first embodiment of the secondary battery pack according to the present invention. 
         FIG.  2 A  is an illustration of the charging operation by the reaction in the first stage of the secondary battery pack shown in  FIG.  1   . 
         FIG.  2 B  is an illustration of the charging operation by the reaction in the second stage of the secondary battery pack shown in  FIG.  1   . 
         FIG.  3 A  is an illustration of the discharging operation by the reaction in the first stage of the secondary battery pack shown in  FIG.  1   . 
         FIG.  3 B  is an illustration of the discharging operation by the reaction in the second stage of the secondary battery pack shown in  FIG.  1   . 
         FIG.  4 A  is a diagram showing one example of the voltage value at each section of the secondary battery pack in the first embodiment. 
         FIG.  4 B  is a diagram showing another example of the voltage value at each section of the secondary battery pack in the first embodiment. 
         FIG.  5    is a schematic configuration diagram of the second embodiment of the secondary battery pack according to the present invention. 
         FIG.  6    is a schematic configuration diagram of an embodiment of the secondary battery module according to the present invention. 
         FIG.  7    is a schematic configuration diagram of a high-voltage generator using a secondary battery pack according to the present invention. 
         FIG.  8    is a diagram showing an example of the arrangement of the positive electrodes, bipolar electrodes and negative electrodes of the third battery. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the secondary battery according to the present invention are hereinafter described. 
     First Embodiment 
       FIG.  1    is a schematic configuration diagram of a secondary battery pack according to the first embodiment of the present invention. The secondary battery pack  1  includes a case  11  as well as a first battery  2 , second battery  3  and third battery  4  contained in the case  11 . The first battery  2  includes two secondary battery cells  12  and  13 . The second battery  3  also includes two secondary battery cells  14  and  15 , while the third battery  4  includes one secondary battery cell  16 . The five secondary battery cells  12 - 16  each include a closed container  20  as well as a plurality of electrodes  21 , separators  22  and an electrolyte  23  which are contained in the closed container  20 . The closed container  20  is provided with external terminals  24  electrically connected through connection members  25  to the electrodes  21  contained in the closed container  20 . 
     The case  11  includes, for example, a metallic case body  111  having an upper opening sealed with a metallic cover  112 . The cover  112  has a positive terminal  113  and a negative terminal  114 , while the case body  111  has a bipolar terminal  115  on its bottom side. 
     The two secondary battery cells  12  and  13  forming the first battery  2 , as well as the two secondary battery cells  14  and  15  forming the second battery  3 , each have two electrodes  21 , i.e. a positive electrode (cathode) “C” and a negative electrode (anode) “A” arranged within the closed container  20 . The secondary battery cell  16  forming the third battery  4  has three electrodes  21  arranged within the closed container  20 . The three electrodes  21  include a positive electrode “C” a negative electrode “A” as well as a bipolar electrode (Biode) “B” located between the positive electrode C and the negative electrode A. 
     The first battery  2  is formed by the secondary battery cells  12  and  13  connected in series. The external terminal  24  of the negative electrode A of the secondary battery cell  12  is electrically connected to that of the positive electrode C of the secondary battery cell  13  by a connection line  241 . The external terminal  24  of the positive electrode C of the secondary battery cell  12  is electrically connected to the positive terminal  113  by a connection line  242 . In the present embodiment, the external terminal  24  of the positive electrode C of the secondary battery cell  12  and the external terminal  24  of the negative electrode A of the secondary battery cell  13  correspond to the positive terminal and the negative terminal of the first battery  2 , respectively. 
     The second battery  3  is formed by the secondary battery cells  14  and  15  connected in series. The external terminal  24  of the negative electrode A of the secondary battery cell  14  is electrically connected to that of the positive electrode C of the secondary battery cell  15  by a connection line  243 . The external terminal  24  of the negative electrode A of the secondary battery cell  15  is electrically connected to the negative terminal  114  by a connection line  244 . In the present embodiment, the external terminal  24  of the positive electrode C of the secondary battery cell  14  and the external terminal  24  of the negative electrode A of the secondary battery cell  15  correspond to the positive terminal and the negative terminal of the second battery  3 , respectively. 
     The third battery  4  is serially connected to each of the first and second batteries  2  and  3 . Specifically, the external terminal  24  of the negative electrode A of the secondary battery cell  13  is electrically connected to that of the positive electrode C of the secondary battery cell  16  by a connection line  245 , while the external terminal  24  of the positive electrode C of the secondary battery cell  14  is electrically connected to that of the negative electrode A of the secondary battery cell  16  by a connection line  246 . The external terminal of the bipolar electrode B of the secondary battery cell  16  is electrically connected to the bipolar terminal  115  by a connection line  247 . In the present embodiment, the external terminals  24  connected to the positive electrode C, negative electrode A and bipolar electrode B correspond to the positive terminal, negative terminal and bipolar terminal of the third battery  4 , respectively. The connection members  25  which connect the positive electrode C, negative electrode A and bipolar electrode B to the corresponding external terminals  24  respectively correspond to the positive-electrode connection member, negative-electrode connection member and bipolar-electrode connection member. The connection lines  245  and  246  respectively correspond to the first connector and the second connector. 
     The electrodes  21  (positive electrodes C, bipolar electrode B and negative electrodes A) each include a current collector and an active-material layer formed on the surface of the current collector. The active-material layer is made of an active material and an adhesive for adhering the active material to the current collector. As for the active materials contained in the positive electrode C and the negative electrode A, active materials commonly used for the positive and negative electrodes in secondary batteries can be used. The active material contained in the active-material layer of the bipolar electrode B is not limited to any specific material and may be any material which gives the bipolar electrode B an electrode potential between the electrode potential of the positive electrode C and that of the negative electrode A. 
     For example, the active-material layer of the positive electrode C should preferably contain an active material whose electrode potential changes within a range from 2 V (vs. Li/Li + ) to 5 V (vs. Li/Li + ), while the active-material layer of the negative electrode A should preferably contain an active material whose electrode potential changes within a range from 0 V (vs. Li/Li + ) to 2 V (vs. Li/Li + ). The active-material layer of the bipolar electrode B should preferably contain an active material whose electrode potential changes within a range from 1 V (vs. Li/Li + ) to 4 V (vs. Li/Li + ). 
     As for the electrolyte  23 , an appropriate kind of electrolyte compatible with the active materials of the electrodes may be used. It may be an aqueous or non-aqueous electrolyte. It may be an electrolytic solution, polyelectrolyte gel or solid polyelectrolyte. 
     An operation principle of the secondary battery pack  1  is hereinafter described with reference to  FIGS.  2 A and  2 B .  FIGS.  2 A and  2 B  show the movement of cations in the charging process under the condition that the negative electrodes A in the secondary battery cells  12 - 16  are made of a carbon-based material, the positive electrodes C are made of LiFePO 4 , and the bipolar electrode B is made of Li 4/3 Ti 5/3 O 4 . 
     The description is initially concerned with the charging operation for the secondary battery pack  1 . A charger  100  for the secondary battery pack  100  includes a pair of input lines  101  and  102 , a single-pole double-throw switching circuit  103  connected to an end of the input line  101 , two branch lines  104  and  105  connected to the switching circuit  103 , as well as a controller  106  configured to control the operation of the switching circuit  103 . The switching circuit  103  switches between the state in which the input line  101  is connected to one branch line  104  (first state) and the state in which the input line  101  is connected to the other branch line  105  (second state). The switching circuit  103  corresponds to the switching means in the present invention. Various kinds of elements are available for this circuit, such as a bipolar transistor or relay. 
     When the secondary battery pack  1  is charged, the secondary battery pack  1  is set in the charger  100 . The positive and negative terminals  113  and  114  of the secondary battery pack  1  are thereby connected to the branch lines  104  and  105  of the charger  100 , respectively, while the bipolar terminal  115  is connected to the input line  102 . Meanwhile, the input lines  101  and  102  of the charger  100  are connected to a pair of output ends of an AC power supply  150 . In such a charging circuit, when the electrons flow through the input lines  101  and  102  in the direction as indicated by the arrows in  FIG.  2 A , the controller  106  switches the switching circuit  103  to the first state in which the input line  101  is connected to the branch line  104 . In this state, cations (positive ions, Li + ) move from the positive electrode C toward the negative electrode A across the electrolyte  23  in each of the secondary battery cells  12  and  13 , while cations (positive ions, Li + ) move from the positive electrode C toward the bipolar electrode B across the electrolyte  23  in the secondary battery cell  16 . 
     When the electrons flow through the input lines  101  and  102  in the direction as indicated by the arrows in  FIG.  2 B , the controller  106  switches the switching circuit  103  to the second state in which the input line  101  is connected to the branch line  105 . In this state, cations move from the bipolar electrode B toward the negative electrode A across the electrolyte  23  in the secondary battery cell  16 , while cations move from the positive electrode C toward the negative electrode A across the electrolyte  23  in each of the secondary battery cells  14  and  15 . As a result, C 6 Li deposits on the negative electrode A. 
     Thus, the secondary battery pack  1  is charged by the two-stage chemical reactions. Accordingly, the charging can be continued by switching the switching circuit  103  to the first connection state or the second connection state every time the direction of the alternating current from the AC power supply  150  changes its direction. 
     A discharging operation for the secondary battery pack  1  is hereinafter described with reference to  FIGS.  3 A and  3 B . A discharger  200  for the secondary battery pack  1  includes a pair of output lines  201  and  202 , a single-pole double-throw switching circuit  203  connected to an end of the output line  201 , two branch lines  204  and  205  connected to the switching circuit  203 , as well as a controller  206  configured to control the operation of the switching circuit  203 . The switching circuit  203  switches between the state in which the output line  201  is connected to one branch line  204  (third state) and the state in which the output line  201  is connected to the other branch line  205  (fourth state). 
     When the secondary battery pack  1  is discharged, the secondary battery pack  1  is set in the discharger  200 . The positive and negative terminals  113  and  114  of the secondary battery pack  1  are thereby connected to the branch lines  204  and  205  of the discharger  200 , respectively, while the bipolar terminal  115  is connected to the output line  202 . Meanwhile, the output lines  201  and  202  of the discharger  200  are connected to a pair of input ends of a load  250 . In such a discharging circuit, the controller  206  switches the switching circuit  203  to the fourth state in which the output line  201  is connected to the branch  205  (see  FIG.  3 A ). Then, the active material on the negative electrode A is dissolved in each of the secondary battery cells  14  and  15 . The resulting cations (positive ions, Li + ) move toward the positive electrode C across the electrolyte  23 . Meanwhile, the active material on the negative electrode A in the secondary battery cell  16  is dissolved, and the resulting cations (positive ions, Li + ) move toward the bipolar electrode B across the electrolyte  23 . During this process, electrons flow from the negative electrode A of the secondary battery cell  15  toward the bipolar electrode B of the secondary battery cell  16  through the external circuit (output lines  201  and  202 ). 
     The controller  206  subsequently switches the switching circuit  203  to the third state in which the output line  201  is connected to the branch line  204  (see  FIG.  3 B ). Then, cations move from the bipolar electrode B toward the positive electrode C across the electrode  23  in the secondary battery cell  16 , and eventually deposit in the form of LiFePO 4 . Meanwhile, the active material on the negative electrode A is dissolved in each of the secondary battery cells  13  and  12 . The resulting cations (positive ions, Li + ) move toward the positive electrode C across the electrolyte  23 . During this process, electrons flow from the bipolar electrode B of the secondary battery cell  16  toward the positive electrode C of the secondary battery cell  12  through the external circuit (output lines  201  and  202 ). Accordingly, alternating current can be released by operating the switching circuit  203  so that the electrode to be connected to the bipolar electrode B of the secondary battery cell  16  is alternately changed between the negative electrode A of the secondary battery cell  15  and the positive electrode C of the secondary battery cell  12  at an appropriate timing (e.g. according to the frequency of the load  250 ). 
     As can be understood from the comparison of  FIGS.  2 A and  2 B  with  FIGS.  3 A and  3 B , the relationship of the input lines  101 ,  102 , switching circuit  103  and secondary battery pack  1  in the charging process is basically the same as that of the output lines  201 ,  202 , switching circuit  203  and secondary battery pack  1  in the discharging process. Accordingly, it is possible to make the charger  100  or discharger  200  be a charging-and-discharger having both the charging and discharging functions by appropriately controlling the timing to switch the switching circuit. 
     In the secondary battery pack  1  having the previously described configuration, the voltage value of the entire secondary battery pack  1  is determined by the voltage difference between the positive electrode C and the negative electrode A in the secondary battery cells  12 - 15 , voltage difference between the positive electrode C and the bipolar electrode B in the secondary battery cell  16 , as well as voltage difference between the bipolar electrode B and the negative electrode A in the secondary battery cell  16 .  FIG.  4 A  shows an example of the voltage values in the case where the positive electrodes C made of the same electrode material and the negative electrodes A made of the same electrode material are used in all secondary battery cells  12 - 16 . On the other hand,  FIG.  4 B  show an example of the voltage values in the case where the negative electrode of the secondary battery cell  12  and that of the secondary battery cell  14  are each made of the same electrode material as the bipolar electrode B of the secondary battery cell  16 . Those examples demonstrate that secondary battery packs  1  with various voltage values can be obtained by appropriately selecting the electrode materials. 
     Second Embodiment 
       FIG.  5    is a schematic configuration diagram of a secondary battery pack  1 A according to the second embodiment of the present invention. In  FIG.  5   , the outer shape of the secondary battery pack  1 A is shown by the long dashed short dashed line. The portions which are identical or correspond to those of the secondary battery pack  1  according to the first embodiment are denoted by the same reference signs. A difference of this secondary battery pack  1 A from the secondary battery pack  1  exists in the configuration of the third battery  4 A. Specifically, the third battery  4 A is formed by a secondary battery cell  16 A having five electrodes  21  arranged within the closed container  20 . The five electrodes include two negative electrodes A at both extremities, one positive electrode C located between the two negative electrodes A, and two bipolar electrodes B respectively located in the two spaces formed between the two negative electrodes A and the positive electrode C. 
     The two negative electrodes A are electrically connected to each other by a connection line  301 . This connection line  301  is electrically connected to the positive electrode C of the secondary battery cell  14  by a connection line  246 . The two bipolar electrodes B are electrically connected to each other by a connection line  302 . This connection line  302  is electrically connected to the bipolar terminal  115  by a connection line  247 . 
     The secondary battery pack  1 A having the previously described configuration also allows for the charge and discharge of alternating current, as with the secondary battery pack  1 . 
     Third Embodiment 
       FIG.  6    shows one embodiment of the secondary battery module according to the present invention. This secondary battery module  400  includes a case  410  having a pair of terminals  411  and  412 , a secondary battery pack  420  contained in the case  410 , a switching circuit  430  functioning as the switching means, and a controller  440  configured to control the switching circuit  430 . The secondary battery pack  420  has almost the same configuration as the previously described secondary battery pack  1  according to the first embodiment. Therefore, the portions which are identical or correspond to those of the secondary battery pack  1  are denoted by the same reference signs, and detailed descriptions of the secondary battery pack  420  will be omitted. 
     The terminal  411  of the secondary battery module  400  is connected to the switching circuit  430  by a first input/output line  401 . The terminal  412  of the secondary battery module  400  is connected to the bipolar terminal  115  of the secondary battery pack  420  by a second input/output line  402 . One of the two contacts of the switching circuit  430  is connected to the positive terminal  113  of the secondary battery pack  420  by a first line  404 , while the other contact is connected to the negative terminal  114  by a second line  405 . 
     In the secondary battery module  400 , when the pair of terminals  411  and  412  are connected to a pair of output ends of an AC power supply, a charging circuit for the secondary battery pack  420  is formed, and the secondary battery pack  420  is thereby charged. The direction of the flow of the electrons and the timing to switch the switching circuit  430  in this charging process are the same as in the charging operation performed by the charger  100  when the secondary battery pack  1  according to the first embodiment is set in the charger  100 . 
     When the pair of terminals  411  and  412  of the secondary battery module  400  are connected to a pair of input ends of a load, a discharging circuit for the secondary battery pack  420  is formed, and alternating current is supplied from the secondary battery pack  420  to the load. The direction of the flow of the electrons and the timing to switch the switching circuit  430  in this discharging process are the same as in the charging operation performed by the discharger  200  in which when the secondary battery pack  1  according to the first embodiment is set in the discharger  200 . 
     Fourth Embodiment 
       FIG.  7    is an embodiment of a high-voltage generator using a secondary battery pack according to the present invention. This high-voltage generator  500  includes a secondary battery pack  501 , a switching unit  510 , and a high-voltage generation unit  520  including a multistage rectification capacitor circuit. The secondary battery pack  501  has the same configuration as the secondary battery pack  1  according to the first embodiment. Therefore, the portions which are identical or correspond to those of the secondary battery pack  1  are denoted by the same reference signs, and detailed descriptions of the secondary battery pack  501  will be omitted. 
     The switching unit  510  includes a single-pole double-throw switching circuit  511  and a controller  512  configured to control the operation of the switching circuit  511 . The two contacts of the switching circuit  511  are respectively connected to the positive terminal  113  and the negative terminal  114  of the secondary battery pack  501  in a removable manner. 
     The high-voltage generator  520  includes a Cockcroft-Walton (CW) circuit having a plurality of serially connected diodes D 1  to D n  and capacitors C 1  to C n  forming a multistage circuit. The CW circuit has a pair of input terminals  521  and  522 , to which the switching circuit  511  of the switching unit  510  and the bipolar terminal  515  of the secondary battery pack  501  are respectively connected. The connection between the input terminal  522  of the CW circuit and the bipolar terminal  115  of the secondary battery pack  501  is removable. The pair of output terminals  523  and  524  of the CW circuit serve as the output terminals of the high-voltage generator  500 . 
     When a pair of input terminals of a load (not shown) is connected to the output terminals  523  and  524  of the high-voltage generator  500 , the controller  512  operates the switching circuit  511  so that the circuit is alternately switched between the first state indicated by the solid line in  FIG.  7    (i.e. the state in which the CW circuit is connected between the positive terminal  113  and the bipolar terminal  115 ) and the second state indicated by the dashed line (i.e. the state in which the CW circuit is connected between the negative terminal  114  and the bipolar terminal  115 ). As a result, AC power is supplied from the secondary battery pack  501  to the high-voltage generation unit  520 , which produces a high-voltage DC-power output to the load connected to the output terminals  523  and  524  of the CW circuit. It is possible to configure the controller  512  so that it disconnects the two contacts from both the positive terminal  113  and the negative terminal  114  upon detecting that all capacitors of the CW circuit have been fully charged. 
     When the amount of charges stored in the secondary battery pack  501  has been decreased to a certain level, the secondary battery pack  501  can be removed from the high-voltage generator  500  and set in a charger. For example, the previously described charger  100  (see  FIG.  2 A ) can be used for this purpose. The operation for charging the secondary battery pack  501  is the same as the charging operation for the secondary battery pack  1  using the charger  100 , and therefore, will not be described in this embodiment. 
     MODIFIED EXAMPLES 
     The previously described embodiments are mere examples and can be appropriately changed or modified along with the gist of the present invention. 
     In the first to fourth embodiments, the plurality of electrodes included in the third battery in the secondary battery pack are horizontally arranged in a row (see  FIGS.  1  and  5   ). If there are a considerable number of electrodes, those electrodes may be arranged in a 
     V-shaped, U-shaped or zigzag form.  FIG.  8    shows an example in which  13  electrodes are arranged in a zigzag form. Such an arrangement makes the positive electrodes C be closer to each other, the bipolar electrodes B be closer to each other, and the negative electrodes C be closer to each other in the third battery, thereby reducing the connection distance between the positive electrodes C, between the bipolar electrodes B as well as between the negative electrodes A. 
     In the first to fourth embodiments, the five secondary battery cells  12 - 16  forming the secondary battery pack are horizontally arranged in a row. Those secondary battery cells  12 - 16  may be arranged in any way as long as their electrical connection is the same. 
     In the first to fourth embodiments, a plurality of secondary battery cells is connected in series to construct the first and second batteries. It is also possible that one or both first and second batteries include a plurality of secondary battery cells connected in parallel. 
     In the first embodiment, the secondary battery pack  1  is separated from the charger  100  or discharger  200 . The secondary battery pack  1  and a charging-and-discharging circuit may be contained in one case to form a secondary battery module. 
     The secondary battery pack included in the secondary battery module according to the third embodiment, as well as the secondary battery pack included in the high-voltage generator according to the fourth embodiment, may be configured in the same manner as the secondary battery pack shown in  FIG.  5   . 
     REFERENCE SIGNS LIST 
       1 ,  1 A,  420 ,  501  . . . Secondary Battery Pack 
       2  . . . First Battery 
       3  . . . Second Battery 
       4 ,  4 A . . . Third Battery 
       100  . . . Charger 
       101 ,  102  . . . Input Line 
       103  . . . Switching Circuit 
       104 ,  105  . . . Branch Line 
       106  . . . Controller 
       11  . . . Case 
       111  . . . Case Body 
       112  . . . Cover 
       113  . . . Positive Terminal 
       114  . . . Negative Terminal 
       115  . . . Bipolar Terminal 
       12 - 16 ,  16 A . . . Secondary Battery Cell 
       20  . . . Closed Container 
       200  . . . Discharger 
       201  . . . Output Line 
       202  . . . Output Line 
       203  . . . Switching Circuit 
       204 ,  205  . . . Branch Line 
       206  . . . Controller 
       21  . . . Electrode 
       22  . . . Separator 
       23  . . . Electrolyte 
       24  . . . External Terminal 
       25  . . . Connection Member 
       241 - 247 ,  301 ,  302  . . . Connection Line 
       150  . . . AC Power Supply 
       250  . . . Load 
       400  . . . Secondary Battery Module 
       500  . . . High-Voltage Generator