JP2011103746A - Charging method of battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrict peak current caused at timing when a battery is connected to a power supply to stably charge the battery while a plurality of batteries are charged with the power supply of small output by sequentially switching the plurality of batteries. <P>SOLUTION: In a charging method of the battery, a plurality of battery units 2 are time-divisionally and sequentially switched to the output side of a DC/DC converter 3 having a constant current characteristic in a supply state of power to perform charging. In the charging method of the battery, the battery unit 2 stopping charging is detached from the output side of the DC/DC converter 3 while a state where at least one battery unit 2 is connected to the output side of the DC/DC converter 3 in an output supply state is kept, and the plurality of battery units 2 are sequentially switched and charged. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a charging method for charging a plurality of battery units by sequentially switching to the output side of a DC / DC converter having constant current characteristics.

A charging method for switching a plurality of battery units in order and charging them has been developed. (See Patent Document 1)
In the charging method described in Patent Document 1, a plurality of batteries are connected to a power source via a switching element. The switching element is controlled by a control circuit. The control circuit turns on one switching element in turn to charge the battery. The battery is charged by being connected to a power source through an on-state switching element. In this charging method, as shown in FIG. 1, the batteries are charged in order at a predetermined timing.

JP 2001-2111558 A

  The charging method shown in FIG. 1 has a problem that the load of the power source becomes temporarily unstable and the peak current flows through the battery at the timing of switching the switching element on and off. This problem cannot be solved even if a power supply having a constant current characteristic for controlling the output current to be constant is used. This is because the load of the power supply changes abruptly at the timing of switching the switching element on and off, and the constant current characteristic cannot respond to this change. In particular, a battery connected to a power source at the moment when the switching element is turned on has a drawback that a peak current temporarily flows from the power source because the voltage is low and the impedance is also low. For example, this peak current is as extremely large as 3 A in a power source that charges a battery with a constant current of 1 A or less. The peak current not only harms the battery, but the power supply is also temporarily overloaded, causing various harmful effects.

  The present invention has been developed for the purpose of solving the above drawbacks. An important object of the present invention is that a plurality of batteries can be charged by switching a plurality of batteries in order, and in addition to charging a large number of batteries with a small output power supply, the peak current generated at the timing of connecting the batteries to the power supply is limited An object of the present invention is to provide a charging method that can be stably charged.

Means for Solving the Problems and Effects of the Invention

  According to the battery charging method of the present invention, a plurality of battery units 2 are switched in order in a time-sharing manner to the output side of the DC / DC converter 3 having a constant current characteristic in a power supply state. In the battery charging method, at least one battery unit 2 is connected to the output side of the DC / DC converter 3 in the output supply state, and the battery unit 2 that stops charging is output from the DC / DC converter 3. The battery units 2 are switched in order and charged.

  With the above charging method, multiple batteries can be switched in order and charged, and a large number of batteries can be charged with a small output power source, while limiting the peak current generated at the timing of connecting each battery to the power source. There is a feature that can be charged stably. This is because at the timing of switching the battery to be connected to the power supply, the battery is switched while being held in a state in which any battery is connected, so that the battery can be switched without always making the power supply unloaded.

  The method for charging a battery according to the present invention stops the charging of the battery unit 2 that has started charging before the specific battery unit 2 while holding the specific battery unit 2 in a charged state. Next, a plurality of battery units 2 can be charged in order by switching the battery unit 2 to be charged so as to start charging the battery unit 2 to be charged next.

  In the battery charging method of the present invention, one of the battery units 2 is connected to the output side of the DC / DC converter 3 to start charging, and then connected to the output side of the DC / DC converter 3 for charging. The battery unit 2 can be charged in order by repeating the operation of disconnecting the battery unit 2 from the output side of the DC / DC converter 3 and stopping the charging.

  In the battery charging method of the present invention, the time for charging one battery unit 2 can be set to 1 sec to 5 sec.

  In the battery charging method of the present invention, a plurality of battery units 2 charged by the DC / DC converter 3 can be connected in parallel with each other in a state where they are disconnected from the DC / DC converter 3 to equalize the remaining capacity. .

  In the battery charging method of the present invention, the DC / DC converter 3 having constant current characteristics can be changed to the DC / DC converter 3 having constant voltage / constant current characteristics.

  In the battery charging method of the present invention, the battery unit 2 can be a battery formed by connecting a plurality of secondary batteries 20 in series.

It is a timing chart figure which shows the charging method of the conventional battery. It is a circuit diagram of the power supply circuit used for the charging method of the battery concerning one Example of this invention. It is a timing chart figure which shows the charging method of the battery concerning one Example of this invention. It is a timing chart figure which shows the charging method of the battery concerning the other Example of this invention. It is a circuit diagram of the power supply circuit used for the charging method of the battery concerning the other Example of this invention.

  Embodiments of the present invention will be described below with reference to the drawings. However, the examples described below exemplify battery charging methods for embodying the technical idea of the present invention, and the present invention does not specify the following charging methods.

  Further, in this specification, in order to facilitate understanding of the scope of claims, numbers corresponding to the members shown in the examples are indicated in the “claims” and “means for solving problems” sections. It is added to the members. However, the members shown in the claims are not limited to the members in the embodiments.

  FIG. 2 shows a power supply circuit used in the charging method of the present invention. This power supply circuit is used as a power supply for the uninterruptible power supply of the server 8. The power supply circuit includes a power control circuit 1 that converts an input commercial power supply 9 into a voltage to be supplied to the server 8, and a charging DC / DC that converts an output voltage of the power control circuit 1 into a charging voltage of the battery unit 2. A DC converter 3, a plurality of battery units 2 charged by the DC / DC converter 3, and each battery unit 2 are connected between the DC / DC converter 3 and each battery unit 2 is connected to the DC / DC. The switching element 4 connected to the output side of the converter 3, the control circuit 5 for controlling the switching element 4 to be turned on and off, and the battery unit 2 are connected in parallel when the commercial power supply 9 is interrupted. And a DC / DC converter 6 for output to be supplied.

  The power control circuit 1 includes a rectifier circuit 10 that rectifies alternating current, which is a commercial power supply 9 of 200 volts, and a booster circuit 13 that is connected to the output side of the rectifier circuit 10.

  The rectifier circuit 10 rectifies the alternating current of the commercial power supply 9 with a bridge diode 11 to generate a pulsating flow, and the pulsating flow is smoothed with an electrolytic capacitor 12. The booster circuit 13 smoothes the pulsating current rectified by the diode 17 that rectifies the alternating current boosted by the switching circuit 14 that is a series circuit of the coil 15 and the switching element 16, and the switching circuit 14. And an electrolytic capacitor 18 of a smoothing circuit.

  The switching circuit 14 switches the switching element 16 on and off at a predetermined cycle, and boosts the direct current input with the energy stored in the coil 15. That is, when the switching element 16 is switched on, the coil 15 is energized, and with the switching element 16 switched off, a voltage is induced across the coil 15 by the energy of the current stored in the coil 15. The energy of the current stored in the coil 15 increases in proportion to the product of the inductance of the coil 15 and the square of the current. The energy stored in the coil 15 with the switching element 16 switched off induces a high voltage across the coil 15.

  The voltage induced in the coil 15 is rectified by the diode 17 and output as a pulsating current. The output pulsating flow is smoothed by the electrolytic capacitor 18 and supplied to the server 8. The switching circuit 14 incorporates a stabilization circuit 19 that stabilizes the output voltage to a constant voltage. The stabilization circuit 19 controls the duty for switching on the switching element 16 by feeding back the output voltage. As described above, since the energy stored in the coil 15 increases in proportion to the square of the current, the output of the switching element 16 is controlled by controlling the energy of the coil 15 with the ratio of the on time to the off time, that is, the duty. it can. That is, if the ON time of the switching element 16 is lengthened, the current of the coil 15 is increased, the energy is increased, and the output voltage can be increased. Therefore, the output voltage can be stabilized by controlling the duty of the switching element 16 by the output voltage. The power control circuit 1 shown in the figure controls the output voltage to 360V to 380V. However, the output voltage of the power control circuit 1 is set to an optimum voltage supplied to the server 8. In a state where the commercial power supply 9 does not fail, the power control circuit 1 supplies operating power to the power supply of the server 8.

  The charging DC / DC converter 3 converts the output voltage of the power control circuit 1 into a voltage for charging the battery unit 2. The DC / DC converter 3 charges, for example, a battery unit 2 having a rated voltage of 48V by connecting 36 nickel hydride batteries in series. The DC / DC converter 3 has an output voltage of 56V. Since the DC / DC converter 3 charges the battery unit 2, it outputs a voltage higher than the rated voltage of the battery unit 2.

  The DC / DC converter 3 of FIG. 2 is output to the transformer 31 that steps down the voltage of the power control circuit 1, the switching element 32 that controls the supply current on the primary side of the transformer 31, and the secondary side of the transformer 31. A diode 33 that rectifies the alternating current, and an electrolytic capacitor 34 that is a smoothing circuit that smoothes the pulsating current output from the diode 33. The switching element 32 is switched on and off at a predetermined period, for example, a frequency of 30 kHz to 500 kHz, and inputs alternating current to the primary side of the transformer 31.

  The switching element 32 controls the duty that is switched on and off by the feedback circuit 35 to stabilize the output voltage and the output current. The feedback circuit 35 detects the output voltage and the output current, and controls the duty for switching the switching element 32 on and off so that the output voltage and the output current are constant. Therefore, the DC / DC converter 3 has a constant voltage / constant current characteristic. The voltage of the constant voltage characteristic is a voltage for charging the battery unit 2, and the current of the constant current characteristic is a current for charging the battery unit 2. Controlled.

  The DC / DC converter 3 for charging is kept in a state in which the output is always supplied by switching the switching element 32 on and off at a predetermined cycle in a time zone in which the battery units 2 are sequentially charged. When charging of all the battery units 2 is completed, the switching element 32 is held in an off state, and the DC / DC converter 3 is held in a state in which no output is supplied.

  The battery unit 2 has a plurality of secondary batteries 20 connected in series to increase the output voltage. For example, the battery unit 2 used in the uninterruptible power supply device has a rated voltage set to 48V. However, the battery unit does not necessarily need to set the rated voltage to this voltage, and can be, for example, 12V to 60V. By making the voltage of the battery unit 2 lower than 60V, it is not necessary to use a high breakdown voltage semiconductor element for the DC / DC converter 3 for charging and the DC / DC converter 6 for output. The power efficiency can be increased by reducing the voltage drop of the semiconductor element while reducing the cost. However, if the voltage of the battery unit is too low, the current with respect to the output increases. Therefore, the voltage of the battery unit is preferably set higher than 12V to reduce the output current. However, since the voltage of the battery unit is set from the output, the battery unit can be made lower than 12 V in an application where the output power is small.

  The battery unit 2 adjusts the output voltage by the number of secondary batteries 20 set in series. The 48V battery unit 2 has 36 nickel hydrogen batteries connected in series. In applications where the required output is small, it is not always necessary to connect a plurality of secondary batteries in series. For example, a battery unit can be constituted by a single secondary battery.

  Further, the power supply circuit of FIG. 2 is provided with five sets of battery units 2 including first to fifth. The number of battery units 2 is also specified from the required output. Therefore, six or more battery units are used for applications that require higher output, and two or four battery units may be used for applications that require less output. it can.

  As the secondary battery 20 constituting the battery unit 2, all rechargeable batteries such as nickel metal hydride batteries and lithium ion batteries can be used. Nickel metal hydride batteries are low in cost and have a long life, and are lighter than lead batteries and can have a large charge capacity. Further, the lithium ion battery is lighter and can increase the charging capacity.

  Each battery unit 2 is connected to the output side of the charging DC / DC converter 3 in the output supply state via the switching element 4. The switching element 4 is a semiconductor switching element such as a transistor or FET. The power supply circuit of FIG. 2 connects each battery unit 2 to a charging DC / DC converter 3 via a charging diode 41 and a switching element 4, and further outputs via an output diode 42. The DC / DC converter 6 is connected. The charging diode 41 and the output diode 42 are connected in series, and are connected in a direction in which a current flows from the charging DC / DC converter 3 to the output DC / DC converter 6. The battery unit 2 is connected to the connection point between the charging diode 41 and the output diode 42 via the switching element 4. According to this circuit configuration, each battery unit 2 can stably output current to the DC / DC converter 6 for output. Further, backflow from the battery unit 2 to the charging DC / DC converter 3 can be prevented.

  The switching element 4 is controlled to be turned on / off by the control circuit 5. The control circuit 5 charges the battery units 2 in order, that is, connects the first to fifth battery units 2 to the output side of the charging DC / DC converter 3 in order. When the battery unit 2 is connected to the DC / DC converter 3 for charging in order, the DC / DC converter 3 switches the switching element 4 on and off at a predetermined cycle and can supply an output, that is, an output supply state. Retained.

  3 and 4 show a method in which the control circuit 5 controls the switching element 4 to be turned on and off to charge the battery units 2 in order. As shown in these drawings, the control circuit 5 always outputs one switching element 4 in an on state and at least one battery unit 2 connected to the output side of the charging DC / DC converter 3. The battery units 2 to be charged are sequentially switched without setting the output of the charging DC / DC converter 3 in the supply state to a no-load state.

  The charging method of FIG. 3 stops the charging of the battery unit 2 that started charging before the specific battery unit 2 while holding the specific battery unit 2 in the charged state, and next to the specific battery unit 2. The battery unit 2 to be charged is switched so as to start charging the battery unit 2 to be charged, and the plurality of battery units 2 are charged in order. That is, in the figure, at the timing of (1), the second battery unit 2B is held in the charged state, and the charging of the first battery unit 2A that has started charging before the second battery unit 2B is performed. Is stopped, the charging of the third battery unit 2C, which starts charging after the second battery unit 2B, is started, and further, at the timing of (2), the third battery unit 2C is charged. In this state, charging of the second battery unit 2B is stopped, and charging of the fourth battery unit 2D is started. Further, at the timing of (3), the fourth battery unit 2D is held in the charged state, the charging of the third battery unit 2C is stopped, and the charging of the fifth battery unit 2E is started. . Furthermore, at the timing of (4), the fifth battery unit 2E is held in the charged state, the charging of the fourth battery unit 2D is stopped, and the charging of the first battery unit 2A is started. To do. The battery unit 2 is charged by switching the connected switching element 4 on and is not charged by switching the switching element 4 off.

  In the charging method of FIG. 3, the time for charging each battery unit 2, that is, the time interval between (1) and (3), (2) and (4), (3) and (5) is set to 2 sec. The time intervals of (1) and (2), (2) and (3), (3) and (4), and (4) and (5), which are timings for switching the switching element 4, are set to 1 / of the charging time. 2 is set to 1 sec. However, the time interval for charging each battery unit 2 is not specified, and may be, for example, 0.1 sec to 1 minute, preferably 0.5 sec to 30 sec, and more preferably 1 sec to 10 sec.

  In the above charging method, two sets of battery units 2 are connected to the output side of the DC / DC converter 3 for charging, and the two sets of battery units 2 can be connected in parallel to be charged together. Therefore, the battery unit 2 can be fully charged in a shorter time.

  In the charging method of FIG. 4, after a specific battery unit 2 is connected to the output side of the DC / DC converter 3 for charging and charging is started, another battery that is charged by connecting to the DC / DC converter 3 is charged. The operation of separating the unit 2 from the output side of the DC / DC converter 3 and stopping the charging is repeated to charge the plurality of battery units 2 in order. In this figure, at the timing having a predetermined time width of (1), after the charging of the second battery unit 2B is started, the charging of the first battery unit 2A is stopped, and at the timing of (2). Starts charging the third battery unit 2C, stops charging the second battery unit 2B, and further starts charging the fourth battery unit 2D at the timing (3). Then, charging of the third battery unit 2C is stopped. Furthermore, at the timing (4), the charging of the fifth battery unit 2E is started and the charging of the fourth battery unit 2D is stopped. Further, at the timing (5), the first battery unit 2E is charged. Charging of the battery unit 2A is started, and charging of the fifth battery unit 2E is stopped.

  In the charging method of FIG. 4, the time for charging each battery unit 2 is set to 2 seconds, and the switching element 4 of the battery unit 2 that starts charging is switched on at the timings (1) to (5). The time interval (t) until the switching element 4 of the battery unit 2 that stops charging is switched off is set to 1 msec to 100 msec. This time interval (t) peaks at the battery unit 2 that starts charging by switching one switching element 4 on and switching the other switching element 4 off, and the load of the DC / DC converter 3 changes suddenly. The interval is set so that no current flows.

  In the charging method of FIG. 4, the charging DC / DC converter 3 charges a set of battery units 2 in most time zones. Therefore, the charging DC / DC converter 3 can set the output current to a current for charging one set of battery units 2, reduce the output current, and sequentially charge the battery units 2. However, although there is a short time, there is a timing at which the two switching elements 4 are simultaneously turned on. This state occurs only when the battery unit 2 to be charged is switched. At this timing, the charging DC / DC converter 3 limits the output current with a constant current characteristic, so that the charging current of the battery unit 2 becomes small when charging the two battery units 2. However, this time is short, and the time for charging the battery unit 2 is hardly delayed.

  The control circuit 5 sequentially turns on the switching element 4 to charge the first to fifth battery units 2. When any one of the battery units 2 is fully charged, the fully charged battery unit 2 is charged. The connected switching element 4 is held in the off state to stop charging. When any one of the battery units 2 is fully charged, the switching elements 4 connected to the other battery units 2 other than the battery unit 2 are turned on in order as shown in FIG. 3 or FIG. All battery units 2 are fully charged.

  3 and FIG. 4, each battery unit 2 is charged for a certain time (2 sec in the figure), but the charging time of the battery unit 2 can be changed by the remaining capacity of the battery. For example, the charging time of a battery unit with a small remaining capacity can be made longer than the charging time of a battery unit with a large remaining capacity, and charging can be performed while equalizing the remaining capacity of the battery unit. The remaining capacity of each battery unit 2 is detected by an open voltage that is the voltage of the battery unit 2 when charging is stopped.

  The control circuit 5 detects the full charge of the battery unit 2 being charged. When a battery unit made of a nickel metal hydride battery is fully charged, the voltage of the battery rises to the peak voltage, and when further charged, ΔV is reduced from the peak voltage or the battery temperature rises. Therefore, the control circuit that controls the charging of the battery unit made of a nickel metal hydride battery detects the peak voltage of the battery unit, or detects a decrease in ΔV from the peak voltage, or also detects an increase in the battery temperature, Fully charged battery unit can be detected. In addition, when a battery unit made of a lithium ion battery is fully charged, the battery unit is charged at a constant voltage and the charging current is reduced. Therefore, the control circuit that controls the charging of the battery unit made of the lithium ion battery can detect full charge from the voltage and current of the battery unit.

  By the way, the power supply circuit that sequentially charges the plurality of battery units 2 may not always fully charge all the battery units 2 under uniform conditions. For example, if the electrical characteristics become unbalanced due to a temperature difference in the battery unit 2 or a change in the degree of deterioration, the remaining capacity of all the battery units 2 cannot be controlled uniformly. In particular, since the battery unit 2 used in the uninterruptible power supply device has a very low discharge timing and is held for a long time in a state close to full charge, the remaining capacity is unbalanced even by self-discharge.

  In addition, a solar cell can be used as a power source of the power circuit instead of the commercial power source. Although not shown, this power supply circuit connects the output of the solar cell to the primary side of the DC / DC converter, and sequentially charges a plurality of battery units with electric power supplied from the solar cell. The output of the solar cell is affected by natural conditions such as the weather, and the output becomes unstable. Since the charging and discharging are repeated for a short time, the remaining capacity of the charged battery unit tends to be unbalanced.

  The power supply circuit of FIG. 5 controls the switching element 4 by the control circuit 5 to eliminate the unbalance of the battery unit 2. When the imbalance is eliminated, the switching element 32 of the charging DC / DC converter 3 is held in an off state, and the DC / DC converter 3 is in a state of not supplying an output. This power supply circuit has a circuit configuration in which a plurality of battery units 2 can be connected in parallel via a switching element 4 connected in series with the battery unit 2. When the control circuit 5 switches on the plurality of switching elements 4 simultaneously, a plurality of sets of battery units 2 are connected in parallel via the switching elements 4. In the battery units 2 connected in parallel, the charging current flows from the high voltage battery unit 2 to the low voltage battery unit 2 so that the voltage becomes uniform and the remaining capacity is made uniform. This power supply circuit can turn on all the switching elements 4 to make the remaining capacity of the battery unit 2 uniform. The control circuit 5 can also make the voltage and the remaining capacity of the battery unit 2 uniform while switching the switching element 4 on and off in a pulsed manner and controlling the current between the battery units 2.

  In the power supply circuit of FIG. 5, a charging diode 41 is connected to the output side of the charging DC / DC converter 3, and the output of the charging DC / DC converter 3 is connected via the charging diode 41. A plurality of battery units 2 are supplied. Further, in the power supply circuit shown in the figure, an output diode 42 is connected to the input side of the output DC / DC converter 6, and outputs from a plurality of battery units 2 are output via the output diode 42. Is supplied to the DC / DC converter 6 for use. According to this circuit configuration, the backflow from the battery unit 2 to the DC / DC converter 3 for charging can be prevented while connecting a plurality of battery units 2 in parallel with all the switching elements 4 switched on. .

  Furthermore, when the commercial power supply 9 that supplies power to the power control circuit 1 fails, the control circuit 5 switches on the switching elements 4 connected in series with all the battery units 2 to turn on each battery unit 2. To supply power to the DC / DC converter 6 for output. Therefore, the control circuit 5 includes a circuit (not shown) that detects a power failure of the commercial power supply 9.

  The output DC / DC converter 6 also includes a circuit (not shown) for detecting a power failure of the commercial power source 9. When the power failure detection circuit detects a power failure, the output DC / DC converter 6 is switched to an operating state. The output DC / DC converter 6 that is in an operating state at the time of a power failure boosts the voltage of the battery unit 2 to the output voltage of the power control circuit 1 and supplies it to the server 8.

  The output DC / DC converter 6 includes a transformer 61 that boosts the voltage of the battery unit 2, a switching element 62 that controls a current on the primary side of the transformer 61, and an alternating current that is output to the secondary side of the transformer 61. And an electrolytic capacitor 64 that is a smoothing circuit that smoothes the pulsating current output from the diode 63. The switching element 62 is switched on and off at a predetermined cycle, for example, a frequency of 30 KHz to 500 KHz, and inputs alternating current to the primary side of the transformer 61.

  The switching element 62 controls the duty that is switched on and off by the feedback circuit 65 to stabilize the output voltage and the output current. The feedback circuit 65 detects the output voltage and the output current, and controls the duty for switching the switching element 62 on and off so that the output voltage and the output current are constant. Therefore, the DC / DC converter 3 has a constant voltage / constant current characteristic, the voltage of the constant voltage characteristic is 360 V to 380 V which is a voltage for supplying power to the server 8, and the constant current characteristic operates the server 8. The current is controlled to a current that allows the battery unit 2 to be discharged.

DESCRIPTION OF SYMBOLS 1 ... Power control circuit 2 ... Battery unit 2A ... 1st battery unit
2B ... Second battery unit
2C ... Third battery unit
2D ... Fourth battery unit
2E ... 5th battery unit 3 ... DC / DC converter 4 ... Switching element 5 ... Control circuit 6 ... DC / DC converter for output 8 ... Server 9 ... Commercial power supply 10 ... Rectifier circuit 11 ... Bridge diode 12 ... Electrolytic capacitor 13 ... Boosting circuit 14 ... Switching circuit 15 ... Coil 16 ... Switching element 17 ... Diode 18 ... Electrolytic capacitor 19 ... Stabilizing circuit 20 ... Secondary battery 31 ... Transformer 32 ... Switching element 33 ... Diode 34 ... Electrolytic capacitor 35 ... Feedback circuit 41 ... Charging diode 42 ... Output diode 61 ... Transformer 62 ... Switching element 63 ... Diode 64 ... Electrolytic capacitor 65 ... Feedback circuit

Claims (7)

A battery charging method for charging by switching a plurality of battery units (2) in order of time division on the output side of a DC / DC converter (3) having a constant current characteristic in a power supply state,
While maintaining the state where at least one battery unit (2) is connected to the output side of the DC / DC converter (3) in the output supply state, the battery unit (2) for stopping charging is connected to the DC / DC converter ( A method for charging a battery, characterized in that the battery unit (2) is switched in order and charged by being disconnected from the output side of 3).   While maintaining the specific battery unit (2) in the charged state, the charging of the battery unit (2) that started charging before the specific battery unit (2) is stopped and the next of the specific battery unit (2) is stopped. The battery charging method according to claim 1, wherein the battery unit (2) to be charged is switched so that the battery unit (2) to be charged is started and the plurality of battery units (2) are sequentially charged.   Any battery unit (2) connected to the output side of the DC / DC converter (3) to start charging, and then connected to the output side of the DC / DC converter (3) to charge the battery unit The battery charging method according to claim 1, wherein (2) is disconnected from the output side of the DC / DC converter (3) and the operation of stopping charging is repeated to charge the plurality of battery units (2) in order.   The battery charging method according to any one of claims 1 to 3, wherein the time for charging one battery unit (2) is 1 sec to 5 sec.   The plurality of battery units (2) charged by the DC / DC converter (3) are connected in parallel with each other in a state of being disconnected from the DC / DC converter (3) to equalize the remaining capacity. The battery charging method described in any of the above.   6. The battery charging method according to claim 1, wherein the DC / DC converter (3) having a constant current characteristic is a DC / DC converter (3) having a constant voltage / constant current characteristic.   The battery charging method according to any one of claims 1 to 6, wherein the battery unit (2) is a battery formed by connecting a plurality of secondary batteries (20) in series.

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