JP5991016B2 - Vehicle power supply system - Google Patents

Description

  The present invention relates to a vehicle power supply system that starts a motor generator from a power storage device.

  In recent years, there are vehicle power supply systems that regenerate, store, and reuse deceleration energy to improve vehicle fuel efficiency.

  In a conventional vehicle power supply system, a high-voltage power storage device and a DC / DC converter that converts voltage between the high-voltage power storage device and the low-voltage battery are inserted between the generator and the low-voltage battery. It has become. In such a configuration, since the generator can generate power with the voltage of the high-voltage power storage device, it is possible to obtain a larger amount of generated power and further improve fuel efficiency.

  In addition, in the deceleration energy regeneration system, when a motor generator is used as a generator, not only the regeneration operation but also a power running operation such as engine start and engine assist becomes possible. In engine restart from idle stop, when a motor generator is used, it is possible to start silently from a conventional starter.

  Furthermore, in such a conventional vehicle power supply system, there are a high-voltage power storage device and a low-voltage battery as a source of driving power for the motor generator, but a high voltage that does not affect on-vehicle electrical equipment due to voltage fluctuations. The system configuration becomes simpler when the power storage device is used as the supply source (see, for example, Patent Document 1).

JP 2010-104123 A (page 4-7, FIG. 1)

  However, in the conventional vehicle power supply system, when the rotational speed of the motor generator at the time of engine recovery from the idle stop is low, the impedance of the motor generator is low. Therefore, if the voltage applied to the motor generator is high, a large current is generated. Therefore, there is a problem that the allowable current of the motor generator has to be increased. As a result, there is a problem that it is difficult to reduce the size and cost of the electrical generator.

  The present invention has been made to solve the above-described problems, and provides a vehicle power supply system capable of starting a motor generator from a high-voltage power storage device without increasing the allowable current of the motor generator. It is.

In the vehicle power supply system according to the present invention, the motor generator, the first power storage device that supplies power to the vehicle-mounted load, the second power storage device that charges and discharges the power generated by the motor generator, and the motor An inverter connected between the generator and the second power storage device and bi-directionally converting an AC voltage of the motor generator and a DC voltage of the second power storage device; the first power storage device; A DC / DC converter connected between a power storage device and converting a DC voltage between the first power storage device and the second power storage device; and connection control means for controlling the second power storage device. In the vehicle power supply system, the second power storage device includes a plurality of power storage units and a plurality of switches, and the connection control unit detects a charging voltage of the power storage unit when the motor generator is started. The motor generator The allowable current Der Ru startup current upper limit value of the inverter the starting current for driving the motor generator and the electric generator with the minimum current value startup current lower limit Ru der required to start the engine A power supply system for a vehicle, wherein the plurality of switches are turned on and off, and the connection state of the plurality of power storage units is switched and controlled so as to be within a range.

  In the present invention, the second power storage device has a plurality of power storage units and a plurality of switches, and the connection control means controls the plurality of switches on and off during operation of the motor generator so that the connection states of the plurality of power storage units are changed. Since the switching is performed, the motor generator can be started from the high voltage power storage device without increasing the allowable current of the motor generator.

It is a circuit diagram which shows the structure of the vehicle power supply system in Embodiment 1 of this invention. It is explanatory drawing which shows the connection of the electrical storage module which comprises the 2nd electrical storage apparatus of the power supply system for vehicles in Embodiment 1 of this invention. The relationship of the starting current of the motor generator with respect to each voltage of the power storage modules when the power storage modules are connected in series and in parallel in Embodiment 1 of the present invention, the starting current upper limit value, the starting current lower limit value, and the operation line FIG. It is a circuit diagram which shows the structure of the vehicle power supply system in Embodiment 2 of this invention. It is explanatory drawing which shows the connection of the electrical storage module which comprises the 2nd electrical storage apparatus of the power supply system for vehicles in Embodiment 2 of this invention. It is a figure which shows the electrical energy stored in the 2nd electrical storage apparatus of Embodiment 2 of this invention, the voltage of each electrical storage module, and the voltage of a 2nd electrical storage apparatus. The relationship of the starting current of the motor generator with respect to each voltage of the power storage module when the power storage modules in the third embodiment of the present invention are connected in series and in parallel, the starting current upper limit value, the starting current lower limit value, and the operation line FIG.

Embodiment 1 FIG.
1 is a circuit diagram showing a configuration of a vehicle power supply system according to Embodiment 1 of the present invention. First, the overall configuration of the vehicle power supply system according to the first embodiment will be described. As shown in FIG. 1, the vehicle power supply system 100 includes a motor generator 1, an inverter 2, a second power storage device 3, a DC / DC converter 4, a first power storage device 5, an in-vehicle load 6, a field control device 14, and an inverter control. It comprises a device 28, a changeover switch control device 36 which is a connection control means, a converter control device 46, and a host control device 7.

  The motor generator 1 is driven by an engine (not shown) that is an internal combustion engine to generate an alternating voltage. The inverter 2 rectifies the AC voltage generated by the motor generator 1 into a DC voltage and outputs it. Second power storage device 3 is connected between inverter 2 and DC / DC converter 4, and charges and discharges DC power of inverter 2. The DC / DC converter 4 is connected between the second power storage device 3 and the first power storage device 5, and converts a DC voltage between the first power storage device 5 and the second power storage device 3. The first power storage device 5 is connected between the DC / DC converter 4 and the vehicle load 6 and supplies power to the vehicle load 6. The motor generator 1 is controlled by the field control device 14, the inverter 2 is controlled by the inverter control device 28, the second power storage device 3 is controlled by the changeover switch control device 36, and the DC / DC converter 4 is controlled by the converter control device 46. Further, each of these control devices is controlled by the host control device 7.

  Next, details of each component device will be described. The motor generator 1 includes a stator 11 having a three-phase AC winding, a rotor 12 having a field winding, and a field power source 13. Inverter 2 includes a smoothing capacitor 27, a U-phase arm, a V-phase arm, and a W-phase arm. The motor generator 1 is controlled by the field controller 14, and the inverter 2 is controlled by the inverter controller 28. Each phase arm is arranged in parallel between the power supply line 9 and the earth line 10 of the vehicle power supply system. The U-phase arm has MOSFETs 21 and 22 connected in series, the V-phase arm has MOSFETs 23 and 24 connected in series, and the W-phase arm has MOSFETs 25 and 26 connected in series. An intermediate point of each phase arm is connected to each phase end of each phase coil of stator 11 having a three-phase AC winding. During the regenerative operation of the motor generator 1, the AC voltage induced in the three-phase AC winding is rectified to a DC voltage by the inverter 2. During the power running operation of the motor generator 1, the energy stored in the second power storage device 3 is converted into an AC voltage by the inverter 2 to drive the motor generator 1 to generate a predetermined torque.

  The second power storage device 3 absorbs fluctuations in the generated power from the motor generator 1 and accumulates the power supplied to the first power storage device 5, and supplements the power shortage of the first power storage device 5 with the accumulated power. For example, the power supplied to the first power storage device 5 is leveled. In addition, it becomes an energy supply source at the time of powering operation such as engine restart from idling stop or engine assist that gives torque at low engine speed. As the second power storage device 3, a large-capacity capacitor such as an electric double layer capacitor capable of charging / discharging large power or a secondary battery such as a lithium ion battery is applied.

  The second power storage device 3 includes power storage modules 31 and 32 that are power storage units, and MOSFETs 33, 34, and 35 that are switches for switching the power storage modules 31 and 32 between series connection and parallel connection. The second power storage device 3 is controlled by the changeover switch control device 36. The power storage modules 31 and 32 are each configured by connecting a plurality of power storage cells. The power storage module 31 has a negative electrode connected to the earth line 10 of the vehicle power supply system, and a positive electrode connected to the source of the MOSFET 33 and the drain of the MOSFET 34. The power storage module 32 has a positive electrode connected to the power supply line 9 of the vehicle power supply system and a negative electrode connected to the source of the MOSFET 34 and the drain of the MOSFET 35. The drain of the MOSFET 33 is connected to the power supply line 9 of the vehicle power supply system, and the source of the MOSFET 35 is connected to the earth line 10 of the vehicle power supply system.

  For example, the rated voltage of the power storage module is 14 V when a configuration in which five electric double layer capacitors with a rated voltage of 2.8 V are connected in series.

  The DC / DC converter 4 includes a smoothing capacitor 41, MOSFETs 42 and 43, a reactor 44, and a smoothing capacitor 45. The DC / DC converter 4 is controlled by a converter control device 46. Smoothing capacitors 41 and 45 are connected in series between power supply line 9 and ground line 10 of the vehicle power supply system, respectively. One end of the reactor 44 is connected to the smoothing capacitor 45, and the other end is an intermediate point between the MOSFET 42 and the MOSFET 43, and is connected to the source of the MOSFET 42 and the drain of the MOSFET 43. MOSFETs 42 and 43 are connected in series between power line 9 and ground line 10 on the second power storage device 3 side. The drain of the MOSFET 42 is connected to the power supply line 9 on the second power storage device 3 side, and the source of the MOSFET 43 is connected to the ground line 10 on the second power storage device 3 side.

  When supplying the electric power generated by the motor generator 1 or the energy stored in the second power storage device to the in-vehicle load 6, the DC / DC converter 4 steps down the voltage of the second power storage device 3 to the voltage of the first power storage device 5. . The step-down operation of the DC / DC converter 4 can be realized by repeatedly turning on and off the MOSFET 42 at a predetermined frequency and on-duty, and repeatedly storing and releasing energy in the reactor 44.

  As the first power storage device 5, a secondary battery having a large energy that can be charged per volume, such as a lead storage battery, a nickel / cadmium storage battery, or a lithium ion battery, is applied, and its rated voltage is, for example, 14V.

  Next, the operation in each operation mode will be described.

1. Engine restart from idle stop (during power running)
FIG. 2 is an explanatory diagram showing connection of power storage modules constituting the second power storage device of the vehicle power supply system according to Embodiment 1 of the present invention. 2A shows a case where the power storage modules 31, 32 of the second power storage device 3 are connected in series, and FIG. 2B shows a case where the power storage modules 31, 32 of the second power storage device 3 are connected in parallel. The on and off states of the MOSFET are shown. In the case of the series connection shown in FIG. 2A, the MOSFETs 33 and 35 are off and the MOSFET 34 is on. In the case of the parallel connection shown in FIG. 2B, the MOSFETs 33 and 35 are on and the MOSFET 34 is off.

  When the rotational speed of the motor generator 1 is zero, no induced voltage is generated in the three-phase AC winding, so the impedance is very small. The currents flowing at this time are the internal resistance and inductance of the power storage modules 31 and 32, the resistance when the MOSFETs constituting the inverter 2 and the second power storage device 3 are turned on, the inductance, the winding resistance of the motor generator 1, the inductance, these It is obtained from the total impedance of the resistance, inductance, and contact resistance of the wiring connecting the components and the voltage of the second power storage device 3.

  FIG. 3 shows the relationship of the starting current of the motor generator with respect to each voltage of the power storage modules when the power storage modules are connected in series and in parallel according to Embodiment 1 of the present invention, and the starting current upper limit value and the starting current lower limit value. And an operation line. Here, the allowable range of the starting current of the motor generator defined by the starting current upper limit value and the starting current lower limit value necessary for starting the motor generator is defined as within a predetermined current range.

  The starting current upper limit value is determined by the allowable currents of the MOSFETs 21, 22, 23, 24, 25, and 26 used in the inverter 2 that drives the motor generator 1, and when more current flows, the MOSFETs 21, 22 , 23, 24, 25, 26 may be destroyed. The starting current lower limit value is a minimum current value necessary for starting the engine by the motor generator 1.

  The working voltage range of the power storage module in which five electric double layer capacitors having a rated voltage of 2.8 V of one power storage cell are connected in series is set to 7 to 14 V in accordance with the output voltage of the second power storage device 3. The charging voltage of the power storage module at the start of the start is 7 to 14 V depending on the state of the stored energy. From this voltage value, the starting current of the motor generator 1 is between the starting current lower limit value and the starting current upper limit value. Thus, it is determined whether the connection state of the power storage modules 31 and 32 is a serial connection or a parallel connection.

  For example, as shown by the thick lines in FIG. 3, when the charging voltage of the power storage modules 31 and 32 is 7 to 10.5 V, the connection state of the power storage modules 31 and 32 is selected as the series connection and the motor generator 1 Start. When the charging voltage of the power storage modules 31 and 32 is 10.5 to 14V, the connection state of the power storage modules 31 and 32 selects parallel connection and starts the motor generator 1. In this way, by appropriately switching the series-parallel connection state of the power storage modules 31 and 32, the motor generator 1 is reliably started, and the MOSFETs 21, 22, 23, 24, 25, and 26 of the inverter 2 are allowed. It is possible to start without exceeding the current.

2. Deceleration energy regeneration (during regenerative operation)
When regenerating the energy when the vehicle body decelerates, the power storage module 31 and the power storage module 32 of the second power storage device 3 are connected in series, and the power storage modules 31 and 32 are charged using the regenerative energy. During this time, the voltage of the second power storage device 3 is in the range of 14 to 28V, and by generating power at a voltage higher than the voltage 14V of the first power storage device 5, larger generated power can be obtained. Therefore, more regenerative energy can be collected and fuel consumption can be improved.

  In the vehicle power supply system configured as described above, the second power storage device is configured with a plurality of power storage modules and a plurality of switches, and the connection states of the plurality of power storage modules are set in accordance with the charging voltage of the power storage modules. By switching with a switch, the starting current of the motor generator can be controlled. For this reason, the motor generator can be started by the charging voltage from the second power storage device without increasing the allowable current of the motor generator. In addition, the connection state of the power storage module may be switched before or after the AC voltage is supplied to the motor generator. In any case, the same effect can be obtained. is there.

Embodiment 2. FIG.
FIG. 4 is a circuit diagram showing a configuration of a vehicle power supply system according to Embodiment 2 of the present invention. First, the overall configuration of the vehicle power supply system according to the first embodiment will be described. As shown in FIG. 4, the vehicle power supply system 200 includes a motor generator 1, an inverter 2, a second power storage device 8, a DC / DC converter 4, a first power storage device 5, an in-vehicle load 6, a field control device 14, and an inverter control. It comprises a device 28, a changeover switch control device 87 as a connection control means, a converter control device 46, and a host control device 7.

  The second embodiment is different from the first embodiment in that the second power storage device 3 used in the first embodiment is replaced with a second power storage device 8. The rest of the configuration is the same as that of the first embodiment, and the same operation is performed. By using the configuration replaced with the second power storage device 8 in this way, energy use efficiency is improved.

  The second power storage device 8 absorbs fluctuations in the generated power from the motor generator 1 and accumulates power supplied to the first power storage device 5, and supplements the power shortage of the first power storage device 5 with the accumulated power. For example, the power supplied to the first power storage device 5 is leveled. In addition, it becomes an energy supply source at the time of powering operation such as engine restart from idling stop or engine assist that gives torque at low engine speed. As the second power storage device 8, a large-capacity capacitor such as an electric double layer capacitor capable of charging and discharging a large amount of power or a secondary battery such as a lithium ion battery is applied.

  In FIG. 4, the second power storage device 8 switches the power storage modules 81 and 82, which are power storage units, to the power storage modules 81 and 82 in series connection, the single use connection of the power storage module 81, and the single use connection of the power storage module 82 MOSFETs 83, 84, 85, and 86, which are switches of the above. The second power storage device 8 is controlled by the changeover switch control device 87.

  The power storage modules 81 and 82 are each configured by connecting a plurality of power storage cells. The power storage module 81 has a negative electrode connected to the ground line 10 of the vehicle power supply system and the source of the MOSFET 84, and a positive electrode connected to the drain of the MOSFET 83. The power storage module 82 has a positive electrode connected to the drain of the MOSFET 85 and a negative electrode connected to the source of the MOSFET 83, the drain of the MOSFET 84, and the source of the MOSFET 86. The source of the MOSFET 85 and the drain of the MOSFET 86 are connected to the power supply line 9 of the vehicle power supply system. The configuration of the power storage module is, for example, that the power storage module 81 has five electric double layer capacitors with a rated voltage of 2.8V in one power storage cell in series, the rated voltage of the power storage module is 14V, and the power storage module 82 has one power storage cell. 10 electric double layer capacitors having a rated voltage of 2.8V are connected in series, and the rated voltage of the power storage module is set to 28V.

Next, the operation in each operation mode will be described.
1. Deceleration energy regeneration (during regenerative operation)
FIG. 5 is an explanatory diagram showing connection of power storage modules constituting the second power storage device of the vehicle power supply system according to Embodiment 2 of the present invention. FIG. 5A shows the case where the power storage modules 81 and 82 of the second power storage device 8 are connected in series, FIG. 5B shows the case where the power storage module 81 is connected for single use, and FIG. ) Shows a case where the power storage module 82 is connected for single use. In the case of the series connection shown in FIG. 5A, the MOSFETs 83 and 85 are on and the MOSFETs 84 and 86 are off. In the case of the single use connection of the power storage module 81 shown in FIG. 5B, the MOSFETs 83 and 86 are turned on and the MOSFETs 84 and 85 are turned off. In the case of the single use connection of the power storage module 82 shown in FIG. 5C, the MOSFETs 83 and 86 are off and the MOSFETs 84 and 85 are on.

  FIG. 6 is a diagram showing the amount of power stored in the second power storage device, the voltage of each power storage module, and the voltage of the second power storage device in the vehicle power supply system according to Embodiment 2 of the present invention.

  FIG. 6 shows the relationship between the amount of power stored in the second power storage device 8, the voltage of the power storage module 82, the voltage of the power storage module 81, and the voltage of the second power storage device when regenerating the energy when the vehicle body decelerates. Show. In the region A in FIG. 6, the power storage modules 81 and 82 are connected in series, and both power storage modules are charged using regenerative energy. A storage module in which 5 electric double layer capacitors having a rated voltage of 2.8V of one storage cell are used in a series of 7 to 14V, and 10 electric double layer capacitors having a rated voltage of 2.8V in one storage cell are connected in series. The working voltage range is set to 7 to 28V. During this time, the voltage of the second power storage device 8 is in the range of 14 to 28V, and by generating power at a voltage higher than the voltage 14V of the first power storage device 5, larger generated power can be obtained. In the region B in FIG. 6, only the power storage module 82 is connected, and the power storage module 82 is charged using regenerative energy. During this time, the voltage of the second power storage device 8 is in the range of 14 to 28V, and by generating power at a voltage higher than the voltage 14V of the first power storage device 5, larger generated power can be obtained.

  Here, in order to examine the effect of the second power storage device in the second embodiment, the case where only one electric double layer capacitor module is used and the second power storage device 8 in the second embodiment are used. Compare the energy utilization rate. In order to make the comparison conditions uniform, the energy from 0 V to the rated voltage is made the same. When only one electric double layer capacitor module is used, a module having a capacity of 100 F and a rating of 28 V is used. In this case, the energy from 0 V to the rated voltage is 0.5 × 100 F × 28 V × 28 V = 39.2 kJ. The power storage module 82 constituting the second power storage device 8 of the second embodiment is assumed to have a capacity of 80F and a rating of 28V, and the power storage module 81 has a capacity of 80F and a rating of 14V. In this case, the energy from 0 V to the rated voltage is 0.5 × 80 F × 28 V × 28 V + 0.5 × 80 F × 14 V × 14 V = 39.2 kJ. When the voltage of the second power storage device 8 is used from 14 to 28V, the energy utilization rate when only one electric double layer capacitor module is used is (28V × 28V-14V × 14V) / (28V × 28V) = 75%. Similarly, when the voltage of the second power storage device 8 is used from 14 to 28 V, the energy utilization rate in the second embodiment is {(28 V × 28 V−7 V × 7 V) + (14 V × 14 V−7 V × 7 V) } / {(28V × 28V) + (14V × 14V)} = 90%. Therefore, the energy utilization rate is higher when the latter second power storage device 8 of the second embodiment is used.

2. When the engine is restarted from idle stop (powering operation)
The basic idea is that the voltage of the second power storage device 8 is set so that the starting current of the motor generator is between the starting current upper limit value and the starting current lower limit value shown in FIG. Series connection or single use connection is selected depending on the charging voltage of the power storage modules 81 and 82. When the charging voltage of the power storage modules 81 and 82 is in the region A in FIG. 6, for example, when the charging voltage of the power storage module is 7 to 10.5 V, the power storage module 81 and the power storage module 82 are connected in series. Starting is started, and when the charging voltage of the power storage module is between 10.5 and 14V, the power storage module 81 or the power storage module 82 is used as a single-use connection and starts.

  When the charging voltage of the power storage modules 81 and 82 is in the region B in FIG. In this way, by appropriately switching the connection state of the power storage modules, the motor generator 1 can be reliably started and the start-up that does not exceed the allowable current of the MOSFETs 21, 22, 23, 24, 25, and 26 of the inverter 2 can be performed. It becomes possible.

  In the vehicle power supply system configured as described above, the second power storage device is configured with a plurality of power storage modules and a plurality of switches, and the connection states of the plurality of power storage modules are set in accordance with the charging voltage of the power storage modules. By switching with a switch, the starting current of the motor generator can be controlled. For this reason, the motor generator can be started by the charging voltage from the second power storage device without increasing the allowable current of the motor generator. In addition, energy use efficiency can be improved.

Embodiment 3 FIG.
FIG. 7 shows the relationship of the starting current of the motor generator with respect to each voltage of the power storage modules when the power storage modules are connected in series and in parallel according to Embodiment 3 of the present invention, and the starting current upper limit value and the starting current lower limit value. FIG.

  The third embodiment relates to application to the vehicle power supply system of the first embodiment shown in FIG. 1 when the difference between the starting current upper limit value and the starting current lower limit value as shown in FIG. 7 is small. is there. In the third embodiment, it is possible to keep the starting current of the motor generator within an allowable range by alternately switching the connection state of the power storage modules between series connection and parallel connection at a predetermined timing. It is a different point.

  As shown in FIG. 7, the difference between the starting current upper limit value and the starting current lower limit value becomes smaller than in the case shown in FIG. 3. However, the starting current of the motor generator may not be within the allowable range. In this case, by repeating ON / OFF of the series connection and the parallel connection, the average voltage of the second power storage device becomes between the voltage at the time of series connection and the voltage at the time of parallel connection, and the starting current is within an allowable range. Can fit inside. If the switching time of the series-parallel connection is exactly halved, the voltage becomes an intermediate voltage between the voltage at the serial connection and the voltage at the parallel connection, and the average voltage can be freely changed by changing the ratio of the switching time.

  In the vehicle power supply system configured as described above, the second power storage device is configured with a plurality of power storage modules and a plurality of switches, and the connection states of the plurality of power storage modules are set in accordance with the charging voltage of the power storage modules. The starting current of the motor generator can be controlled by alternately switching between series connection and parallel connection by the switch. For this reason, the motor generator can be started by the charging voltage from the second power storage device without increasing the allowable current of the motor generator. In addition, even when the allowable range of the starting current of the motor generator is narrow and the available charging voltage becomes discontinuous with respect to the charging voltage of the storage module, by switching the combination of the plurality of storage modules, The motor generator can be continuously started with respect to the charging voltage. Furthermore, if such a method for controlling the starting current of the motor generator is used, it is possible to start within the allowable range of the starting current of the electric motor even before or after supplying the AC voltage of the motor generator. Become.

  DESCRIPTION OF SYMBOLS 1 Motor generator, 2 Inverter, 3, 8 2nd electrical storage apparatus, 4 DC / DC converter, 5 1st electrical storage apparatus, 6 Car load, 7 High-order control apparatus, 9 Power supply line, 10 Earth line, 11 3 phase alternating current winding Stator with wire, rotor with 12 field windings, 13 field power supply, 14 field controller, 21, 22, 23, 24, 25, 26, 33, 34, 35, 42, 43, 83 , 84, 85, 86 MOSFET, 27, 41, 45 Smoothing capacitor, 28 Inverter control device, 31, 32, 81, 82 Power storage module, 36, 87 Changeover switch control device, 44 Reactor, 46 Converter control device, 100, 200 Power supply system for vehicles.

Claims (5)

A motor generator,
A first power storage device for supplying electric power to a vehicle-mounted load;
A second power storage device that charges and discharges the electric power generated by the motor generator;
An inverter connected between the motor generator and the second power storage device and bi-directionally converting an AC voltage of the motor generator and a DC voltage of the second power storage device;
A DC / DC converter connected between the first power storage device and the second power storage device and converting a DC voltage between the first power storage device and the second power storage device;
A vehicle power supply system comprising connection control means for controlling the second power storage device,
The second power storage device includes a plurality of power storage units and a plurality of switches,
Said connection control means, upon starting of the motor generator, is determined by detecting the charging voltage of the power storage unit, Ru permissible current der of the inverter the starting current of the motor generator for driving the motor generator to fall within the range between the minimum current value startup current lower limit Ru der required to start the engine by startup current upper limit value and the motor generator, and off the plurality of switches, said plurality of A power supply system for a vehicle, wherein the connection state of the power storage unit is switched and controlled. 2. The vehicle power supply according to claim 1, wherein the connection control unit turns on and off the plurality of switches, and controls the connection state of the plurality of power storage units by switching to at least one of serial connection and parallel connection. system. The connection control means is configured to control by turning on and off the plurality of switches and switching a connection state of the plurality of power storage units to a serial connection or one single connection among the plurality of power storage units. The vehicle power supply system according to claim 1. 4. The vehicle power supply system according to claim 2, wherein the connection control unit switches the plurality of switches before supplying an AC voltage to the motor generator. 5. The vehicle power supply system according to claim 2, wherein the connection control unit performs switching of the plurality of switches after supplying an AC voltage to the motor generator.