Title of the Invention

POWER SYSTEM FOR A VEHICLE Technical Field

The present invention relates to a power system for a vehicle, which includes a first battery, a second battery and a generator that charges the first and second batteries. Background Art

A known power system installed in a vehicle is configured by using two batteries, e.g. a lead battery (first battery) and a lithium ion battery (second battery). Using these batteries properly, electric power is supplied to various electrical loads installed in the vehicle. A patent document JP-A-2012-080706, for example, discloses a configuration of such a power system.

Specifically, a lithium ion battery is electrically connected to a generator and a lead battery via a connection switch configured by a semiconductor switch. In regenerative generation of the generator associated with deceleration of the vehicle, the connection switch is turned on to enable power supply to the lithium ion battery from the generator. Also, in non-regenerative generation, the connection switch is turned off, so that electric power is ensured to be supplied from the lithium ion battery to electrical loads which establish electrical connection on a lithium ion battery side with respect to the connection switch. Controlling the connection switch as mentioned above, the electrical energy generated by regenerative generation can be efficiently used.

In such a configuration, the lead battery and the lithium ion battery may have a different terminal voltage, i.e. the terminal voltage of the lead battery may be made higher than that of the lithium ion battery. With this configuration, the lithium ion battery can be charged in preference to the lead battery. However, in this case, when the connection switch is turned on in regenerative generation to charge both of the lead battery and the lithium ion battery, an unintentional situation would occur. Specifically, in this situation, due to the difference in the terminal voltage of the batteries, the state of the lead battery may transition from a charging state to a discharging state to unintentionally decrease the amount of charge (residual capacity) of the lead battery. In other words, in regenerative generation, electrical charge is applied from the lead battery to the lithium ion battery and thus the amount of charge of the lead battery decreases.

When the amount of charge of the lead battery decreases, it is necessary to charge the lead battery for the compensation of the decrease by allowing the generator to perform power generation in non-regenerative generation.

Summary of Invention

An embodiment provides a power system for a vehicle which includes a first battery, a second battery, and a connection switch that electrically connects and disconnects the first and second batteries to realize efficient application of electrical charge to the first and second batteries.

As an aspect of the embodiment, a power system for a vehicle is provided which includes: a generator; a first battery and a second battery which are connected in parallel with the generator; and a connection switch which is provided on a connection line electrically connecting the first and second batteries, and which electrically connects and electrically disconnects the first battery and the generator to/from the second battery. The power system performs regenerative generation by the generator when the vehicle decelerates. A terminal voltage of the first battery is made larger than a terminal voltage of the second battery. The power system further includes: a first control means which makes the connection switch an electrically connected state to apply electrical charge to both of the first battery and the second battery during regenerative generation by the generator; a discharge monitor means which monitors a discharging state of the first battery during regenerative generation; and a second control means , which disconnects the connection switch based on the discharging state of the first battery monitored by the discharge monitor means during regenerative generation.

Brief Description of Drawings

i In the accompanying drawings:

Fig. 1 is a schematic diagram illustrating an in-vehicle power system according to an embodiment;

Fig. 2 is a flow diagram illustrating a procedure of a charge control process;

Fig. 3 is a diagram illustrating lead battery state of charge PbSOC relative to turn-on vehicle speed and turn-off vehicle speed;

Fig. 4 is a diagram illustrating PbSOC relative to discharge permission value;

Fig. 5 is a time diagram illustrating a state of charge of batteries in regenerative generation; and

Fig. 6 is a diagram illustrating PbSOC and lithium battery state of charge LiSOC relative to turn-off vehicle speed.

Description of Embodiments

With reference to the accompanying drawings, hereinafter is described an embodiment in which a power system of the present invention is implemented. The power system of the present embodiment is applied to a vehicle that has an engine (internal combustion engine). The power system includes two batteries, i.e. a lead battery and a lithium ion battery, and a generator that charges these batteries. First, referring to Fig. 1, an outline of the present system is described.

Fig. 1 is a schematic diagram illustrating the power system according to the present embodiment. In Fig. 1, the power system includes as its main components, an alternator 11 (generator), a lead battery 12, and a battery unit 14 that includes a lithium ion battery 13. The lead battery 12 and the lithium ion battery 13 are connected in parallel with the alternator 11. The lead battery 12 corresponds to a first battery, while the lithium ion battery 13 corresponds to a second battery.

The alternator 11 is connected to the crank shaft (output shaft) of the engine to generate power with the rotational energy of the crank shaft. In other words, when the rotor of the alternator 11 is rotated by the crank shaft, AC current is induced in the stator coil in response to excitation current that passes through the rotor core. The AC current is converted to DC current by a rectifier. The excitation current passing through the rotor coil is regulated by a regulator to thereby adjust the voltage of the generated DC current to a set voltage Vreg. The regulator of the alternator 11 is controlled by an engine controller 30 (first control means, discharge monitor means, second control means, reference value setting means, prohibition vehicle speed setting means, permission vehicle speed setting means).

The lead battery 12 is a well-known general-purpose battery. As an electrical load, a starter 15a is connected to the lead battery 12. The engine is started when the starter 15a is driven by the power supplied from the lead battery 12. Electrical loads 15b, such as headlights and a power window motor, are also connected to the lead battery 12.

In the battery unit 14, the lithium ion battery 13 is a high-density battery that has high power density and energy density compared with the lead battery 12. The lithium ion battery 13 is configured by a battery pack in which a plurality of electric cells are connected in series. It should be appreciated that the charge capacity of the lead battery 12 is ensured to be larger than that of the lithium ion battery 13.

The battery unit 14 is provided with an input terminal 16 and an output terminal 17, ι which are connected to each other via a power feeder 18. The alternator 11 and the lead battery 12 are connected to the input terminal 16. Electrical loads 19, to which power is supplied from the lithium ion battery 13, are connected to the output terminal 17. Specific examples of the electrical loads 19 include constant-current loads, such as a navigation system and an audio system, which are driven with constant current.

Other than the lithium ion battery 13, the battery unit 14 includes a MOS switch 21, an SMR switch 22 and a battery controller 23 that performs control, under which these switches are turned on/off (electrically connected/disconnected). The battery controller 23 is a well-known electronic control unit which is configured by a microcomputer having a CPU and memories.

The MOS switch 21 is a semiconductor switch configured by a MOSFET (metal oxide semiconductor field effects transistor). The MOS switch 21 is provided between the input and output terminals 16 and 17. The MOS switch 21 functions as a switch that electrically connects (turns on) and electrically disconnects (turns off) the lithium ion battery 13 to/from the alternator 11 and the lead battery 12. Similar to the MOS switch 21, the SMR switch 22 is a semiconductor switch configured by a MOSFET. The SMR switch 22 is arranged between a connecting point (indicated by XI in Fig. 1) and the lithium ion battery 13, the connecting point connecting between the MOS switch 21 and the output terminal 17. The SMR switch 22 functions as a switch that electrically connects (turns on) and electrically disconnects (turns off) the lithium ion battery 13 to/from a power feed path that connects between the input and output terminals 16 and 17.

The SMR switch 22 also functions as an opening/closing means in a time of emergency. Normally, or, in periods other than a time of emergency, the SMR switch 22 is retained to be a state of being turned on (on state) by an on signal sent from the battery controller 23. In a time of emergency as exemplified below, the output of the on signal is stopped to bring the SMR switch 22 into a state of being turned off (off state). By bringing the SMR switch 22 into an off state, overcharge or overdischarge of the lithium ion battery 13 is avoided. For example, in the event that the regulator provided at the alternator 11 breaks down to abnormally raise the set voltage Vreg, the lithium ion battery 13 may be overcharged. In such a case, the SMR switch 22 is brought into an off state. Further, in the event that the alternator 11 or the MOS switch 21 breaks down to disable application of electrical charge to the lithium ion battery 13, the lithium ion battery 13 may be overdischarged. In such a case as well, the SMR switch 22 is brought into an off state.

The on state and the off state of the MOS switch 21 and the SMR switch 22 are constantly monitored by the battery controller 23. The results of the monitoring are transmitted from the battery controller 23 to other components, such as the engine controller 30, at predetermined intervals. The electric power generated by the alternator 11 is supplied to various in-vehicle electrical loads, while being supplied to the lead battery 12 and the lithium ion battery 13. When the driving of the engine is stopped and no generation is performed by the alternator 11, electric power is supplied from the lead battery 12 and the lithium ion battery 13 to the in-vehicle electrical loads. An amount of discharge from the lead battery 12 and the lithium ion battery 13 to the in-vehicle electrical loads and an amount of charge applied from the alternator 11 to the batteries 12 and 13 are controlled so that SOC (state of charge: percentage (%) of an actual amount of charge with respect to an amount of charge in a fully-charged state) of the batteries 12 and 13 will fall in a rage that would not cause overcharge or overdischarge (proper range). In other words, the engine controller 30 is ensured to control the set voltage Vreg so as not to cause overcharge or overdischarge, while the battery controller 23 is ensured to control the operation of the MOS switch 21.

In regenerative generation of the alternator 11 associated with deceleration of the vehicle, the MOS switch 21 and the SMR switch 22 are both turned on to charge both of the lead battery 12 and the lithium ion battery 13. In the present embodiment, terminal voltages of the lead battery 12 and the lithium ion battery 13 are determined such that the terminal voltage of the battery 12 is higher than that of the battery 13. Accordingly, in a state where the switches 21 and 22 are turned on for mutual electrical connection of the batteries 12 and 13, the lithium ion battery 13 is ensured to be charged by both the alternator 11 and the lead battery 12.

The engine controller 30 has an idle reduction function that automatically stops the engine when predetermined automatic stop conditions are met while the vehicle runs, and automatically restarts the engine when predetermined restart conditions are met while the engine is automatically stopped. The automatic stop conditions include, for example, that: the vehicle speed is not more than a predetermined level; and the manipulated variable of the accelerator is zero (or the brake is applied). The engine restart conditions include, for example, that: the accelerator has been manipulated; and the brake has been released.

When the engine is automatically stopped under the idle reduction control, the battery controller 23 brings both of the MOS switch 21 and the SMR switch 22 into an on state, so that the lithium ion battery 13 is charged (regeneratively charged) while the engine revolution decreases. When the engine is restarted, the battery controller 23 brings the MOS switch 21 into an off state from an on state, so that the starter 15a is driven by the electric power supplied from the lead battery 12 under the condition where the lead battery 12 is electrically disconnected from the lithium ion battery 13.

In regenerative generation associated with deceleration of the vehicle, the MOS switch 21 is turned on to charge both of the lead battery 12 and the lithium ion battery 13. In this case, gradual decrease of the vehicle speed causes decrease in the amount of regenerative generation performed by the alternator 11. When the amount of regenerative generation decreases in this way, the state of the lead battery 12 may transition from a charging state to a discharging state. Accordingly, the amount of charge of the lead battery 12 (PbSOC) unintentionally decreases. This may result in the alternator 11 being forced to charge the battery 12 in a period other than the period of regenerative generation. In addition, this may also result in impairing the fuel efficiency. In this regard, in the present embodiment, the discharging state of the lead battery 12 is ensured to be monitored during regenerative generation and the MOS switch 21 is ensured to be turned off (electrically disconnected) based on the discharging state. Hereinafter is specifically described control of charge/discharge of the batteries during regenerative generation.

Fig. 2 is a flow diagram illustrating a procedure of a charge control process performed by the engine controller 30. In the present process, the procedure is repeatedly performed by the engine controller 30 at a predetermined cycle. It should be appreciated that, as an alternative to the engine controller 30, the battery controller 23 may perform the present process.

As shown in Fig. 2, it is determined, in step Sll, whether or not regenerative generation associated with deceleration of the vehicle is currently performed by the alternator 11. In this case, the determination is made according the condition of the driver's accelerator manipulation, the vehicle speed, and the like. If regenerative generation is currently performed, control proceeds to step S12. In step S12, it is determined whether or not the process performed at the present cycle is the one immediately after the start of regenerative generation.

If the determination is YES in step S12, control proceeds to step S13 where the residual capacity PbSOC of the lead battery 12 is calculated. The method of calculating PbSOC is well known. If briefly explained, PbSOC is calculated from an open-circuit voltage of the lead battery 12 in an open state and an integrated value of current (hereinafter also referred to as current integrated value) of the lead battery 12 in a charging/discharging state.

Then, in step S14, the engine controller 30 sets a turn-on vehicle speed and a turn-off vehicle speed. The turn-on vehicle speed is used as a criterion for determining whether to turn on the MOS switch 21 in regenerative generation. The turn-off vehicle speed is used as a criterion for determining whether to turn off the MOS switch 21 if it has been turned on in regenerative generation. In this case, the turn-on vehicle speed corresponds to a vehicle speed threshold (determination value) (connection permission vehicle speed) for determining whether to charge both of the lead battery 12 and the lithium ion battery 13 or to charge only the lead battery 12 during regenerative generation. Thus, on condition that the vehicle speed at the start of regenerative generation is equal to or more than the turn-on vehicle speed, the MOS switch 21 is ensured to be turned on in regenerative generation.

The turn-off vehicle speed corresponds to a vehicle speed threshold (determination value) (connection prohibition vehicle speed) for making a transition from a situation where both of the lead battery 12 and the lithium ion battery 13 are charged during regenerative generation to a situation where only the lead battery 12 is charged. Thus, on condition that the vehicle speed decreases below the turn-off vehicle speed, the state of the MOS switch 21 is ensured to transition from an on state to an off state.

In the present embodiment, the turn-on vehicle speed and the turn-off vehicle speed are set based on PbSOC (PbSOC at the start of regenerative generation) calculated in step S13. For example, the turn-on vehicle speed and the turn-off vehicle speed are set based on a relationship shown in Fig. 3. Fig. 3 is a diagram illustrating PbSOC relative to turn-on vehicle speed and turn-off vehicle speed. In Fig. 3, the relationship is established such that, the smaller the PbSOC is, the higher the turn-on vehicle speed and turn-off vehicle speed become. In other words, when PbSOC is small, the lead battery 12 is desired to be charged in preference to the lithium ion battery 13. Accordingly, the turn-on vehicle speed is set to a higher level so that the MOS switch 21 is hardly turned on. Further, when PbSOC is small, it is desirable that the charge of the lithium ion battery 13 is finished early compared with the case where PbSOC is large. Accordingly, the turn-off vehicle speed is set to a higher level so that the MOS switch 21 is turned off comparatively early.

After that, in step S15, the engine controller 30 sets a discharge permission value that is a criterion for determining whether to permit discharge from the lead battery 12 in regenerative generation. The discharge permission value corresponds to a threshold (determination value) for determining whether to permit or to immediately stop continuation of discharge of the lead battery 12 in regenerative generation. For example, the discharge permission value is set based on the relationship shown in Fig. 4. Fig. 4 is a diagram illustrating PbSOC relative to discharge permission value. A large discharge permission value here means that a comparatively large amount of discharge is permitted to the lead battery 12 in regenerative generation.

The relationship shown in Fig. 4 is established such that, when

"PbSOC Al" is satisfied, "Discharge permission value=0" is established, and that when "PbSOC≥A2" is satisfied, "Discharge permission value=B" is established. Further, the relationship is determined such that, when "PbSOC=Al to A2" (i.e. when PbSOC ranges from Al to A2), a larger PbSOC establishes a larger discharge permission value. In other words, when PbSOC at the start of regenerative generation is small, the discharge permission value is set to a small value accordingly (0 in the present embodiment) so that the lead battery 12 is preferentially charged. Further, when PbSOC at the start of regenerative generation is large, the discharge permission value is set to a comparatively large value so that the lithium ion battery 13 is preferentially charged.

Following step S15, it is determined, in step S16, whether or not the vehicle speed of the moment is equal to or higher than the turn-on vehicle speed. If a relation "(Vehicle speed) > (Turn-on vehicle speed)" is satisfied, or if the determination is YES in step S16, control proceeds to step S17 where the MOS switch 21 is turned on. Thus, electrical charge is started to be applied to both of the lead battery 12 and the lithium ion battery 13.

In the subsequent step S18, the engine controller 30 calculates an integrated value of current that flows through the lead battery 12. In this case, if the calculation immediately follows the start of regenerative generation, the engine controller 30 calculates a current integrated value starting from an initial value (=0). If the calculation is performed at some point after the start of regenerative generation, a current detection value! of the present cycle is added to the current integrated value of the previous cycle. For example, a current integrated value is calculated by temporally integrating a current detection value (Pb current) at every cycle, with a Pb current in charging being a positive current and a Pb current in discharging being a negative current. After finishing step S18, the present process is halted until the next iteration.

The current integrated value corresponds to a difference, i.e. a charge-discharge balance, between an amount of charge of the lead battery 12 in a charging state after the start of regenerative generation, and an amount of discharge of the battery 12 in a discharging state after transition thereto from the charging state. In other words, in regenerative generation, the amount of charge is calculated by integrating a Pb charge current in a charging state after the start of regenerative generation, while the amount of discharge is calculated by integrating a Pb discharge current in the following discharging state. In this case, in the present embodiment, one current integrated value is calculated through a period of regenerative generation, resultantly calculating a charge-discharge balance. It should be appreciated that the discharge permission value calculated in step S15 corresponds to the "predetermined discharge reference value".

Alternatively to the above calculation, an amount of charge and an amount of discharge of the lead battery 12 may be separately calculated after the start of regenerative generation and then the difference therebetween may be used as a current integrated value (charge-discharge balance).

In step S16, if a relation "(Vehicle speed) < (Turn-on vehicle speed)" is satisfied, or if the determination is NO, the present process is, halted until the next iteration, without turning on the MOS switch 21. Alternatively, if the relation "(Vehicle speed) < (Turn-on vehicle speed)" is satisfied and thus the MOS switch 21 is not turned on, a current i integrated value may be calculated.

If the determination is NO in step S12, i.e. if regenerative generation is being performed and some time has passed after the start • of the regenerative generation, control proceeds to step S19 where the engine controller 30 determines whether or not the MOS switch 21 is turned on. If a relation "MOS switch 21=On state" is satisfied, or if the determination is YES in step S19, control proceeds to the subsequent step S20. If a relation "MOS switch 21=Off state" is satisfied, or if the determination is NO in step S19, the present process is immediately halted until the next iteration.

In step S20, it is determined whether or not the current integrated value (absolute value) of the moment has become equal to or more than the discharge permission value. This processing is performed for the purpose of determining whether or not an amount of discharge of the lead battery 12 during regenerative generation has exceeded a predetermined amount. Considering that a negative current is integrated in a discharging state of the lead battery 12, this processing is performed for the purpose of determining whether or not the absolute value of the negative current integrated value has become equal to or larger than the discharge permission value. In step S21, it is determined whether or not the vehicle speed of the moment has become less than the turn-off vehicle speed.

If a relation "(Current integrated value) < (Discharge permission value)" is satisfied and a relation "(Vehicle speed) > (Turn-off vehicle speed)" is also satisfied (if the determination is NO in steps S20 and S21), control proceeds to step S18 where the engine controller 30 calculates _: a current integrated value. Then, the present process is halted until the next iteration.

If a relation "(Current integrated value) > (Discharge permission value)" or a relation "(Vehicle speed) < (Turn-off vehicle speed)" is satisfied (if the determination is YES in either step S20 or S21), control proceeds to step S22 where the MOS switch 21 is turned off. Then, the present process is halted until the next iteration.

Fig. 5 is a time diagram illustrating a state of charge of the batteries 12 and 13 in regenerative generation. In Fig. 5, for the sake of convenience, PbSOC at the start of regenerative generation is regarded to be the same, and the turn-on vehicle speed and the turn-off vehicle speed are regarded to be constant. Also, the SMR switch 22 is regarded to remain in an on state. In Fig. 5, the period between time tl and time t2 and the period between time t3 and time t6 are the periods of regenerative generation associated with deceleration of the vehicle. In the diagram showing current, Pb current is indicated by the solid line, Li current is indicated by the dash-dot line and generated current is indicated by the dash-dot-dot line.

At time tl, regenerative generation is started, triggered by deceleration of the vehicle. However, in this case, the vehicle speed is less than the turn-on vehicle speed (e.g., less than 30 km/h). Accordingly, the MOS switch 21 is not turned on and thus only the lead battery 12 is charged with regenerative generation. In other words, the lithium ion battery 13 is prohibited from being charged. In Fig. 5, on or after time tl, generated current of the alternator 11 increases. With this increase, electrical charge is started to be applied to the lead battery 12. Accordingly, as shown in Fig. 5, Pb current of the lead battery 12 on a discharge side switches to a charge side.

After that, as the vehicle speed decreases, the generated current decreases. With this decrease, Pb current switches to discharge current. At time t2, regenerative generation is finished.

On the other hand, at time t3, regenerative generation is started again, triggered by deceleration of the vehicle. In this case, the vehicle speed is equal to or more than the turn-on vehicle speed (e.g., 30 km/h or more). Accordingly, the MOS switch 21 is turned on. In other words, both of the lead battery 12 and the lithium ion battery 13 are permitted to be charged. On or after time t3, the current generated by the alternator 11 increases. With this increase, electrical charge is started to be applied to the lead battery 12 and the lithium ion battery 13. Thus, charging currents of the batteries 12 and 13, in the form of Pb current and Li current, respectively, flow through the power feed path. Compared with the start of regenerative generation at time tl, the vehicle speed at time t3 is higher and therefore the generated current is larger accordingly.

On or after time t3, for a while, the generated current, Pb current and Li current slightly increase or are substantially retained to be constant, but gradually decrease thereafter with the decrease of the vehicle speed. At time t4, Pb current turns from charging current to discharging current. In essence, in a state where the MOS switch 21 is turned on, the batteries 12 and 13 are mutually electrically connected. Under the condition, a relation "(Terminal voltage of the lead battery 12) > (Terminal voltage of the lithium ion battery 13)" is satisfied. Accordingly, the lithium ion battery 13 is preferentially charged. In this case, the electric power of the lead battery 12 is used for charging the lithium ion battery 13 or for driving electrical loads. Thus, with the decrease of the generated current, the lead battery 12 transitions from a charging state to a discharging state.

On or after time t3, a current integrated value is calculated by integrating Pb currents. In this case, in an interval between time t3 and time t4, a current integrated value is calculated with the integration of charging currents to gradually increase the current integrated value. In an interval between time t4 and time t5, a current integrated value is calculated with the integration of discharging currents to gradually decrease the current integrated value. Then, at time t5, the current integrated value reaches a predetermined value (discharge permission value) on a negative side, or satisfies a relation "(Integrated value of charging currents) < (Integrated value of discharging currents + a)" . At this point, the MOS switch 21 is turned off to stop application of charge to the lithium ion battery 13.

The period from time t3 to time t5 may be explained in other words as follows. Specifically, in the interval between time t3 and time t4, the engine controller 30 calculates an amount of charge of the lead battery 12 during regenerative generation. Then, in the interval between time t4 and time t5, the engine controller 30 calculates an amount of discharge of the lead battery 12 during regenerative generation. Then, at time t5, the engine controller 30 turns off the MOS switch 21 on the basis of the charge-discharge balance that is the difference between the amount of charge and the amount of discharge.

It should be appreciated that, in the period from time t3 to time t6, the vehicle speed has not yet decreased below the turn-off vehicle speed. Accordingly, the MOS switch 21 is not turned off according to the vehicle-speed condition expressed by a relation "(Vehicle speed) < (Turn-off vehicle speed)". However, if this vehicle-speed condition is satisfied before the current integrated value reaches the discharge permission value, the MOS switch 21 will be turned off accordingly.

The embodiment described above has the following advantages.

In the configuration described above, the discharging state of the lead battery 12 is monitored during regenerative generation, and the MOS switch 21 is electrically disconnected based on the discharging condition. Accordingly, the amount of charge (PbSOC) of the lead battery 12 is prevented from being unintentionally decreased. Thus, electrical charge comes to be efficiently applied to the batteries 12 and 13. This can prevent unintentional decrease of PbSOC during regenerative generation. Also, this can resultantly prevent the alternator 11 from being forced to apply electrical charge to the lead battery 12 in a period other than the period of regenerative generation. Thus, fuel efficiency is hardly impaired in the generation performed by the alternator 11.

The time point when the batteries 12 and 13 are electrically disconnected from each other during regenerative generation (the time point when the state of the MOS switch 21 is changed from an on state to an off state) relies on the amount of charge and the amount of discharge to/from the lead battery 12 after the start of regenerative generation. For example, when the amount of charge to the lead battery 12 is comparatively large after the start of regenerative generation, discharge from the lead battery 12 is permitted accordingly. In this regard, according to the configuration described above, the MOS switch 21 is electrically disconnected (brought into an off state) based on the balance between the amount of charge and the amount of discharge to/from the lead battery 12 (current integrated value). Accordingly, the MOS switch 21 can be electrically disconnected at a more appropriate timing.

According to the configuration described above, a discharge permission value as a discharge reference value is ensured to be set based on the amount of charge (PbSOC) of the lead battery 12 at the start of regenerative generation. In essence, the amount of permitted discharge of the lead battery 12 during regenerative generation depends on whether PbSOC (the amount of charge of the lead battery 12) is comparatively large or comparatively small at the start of regenerative generation. In this regard, since a discharge permission value (discharge; reference value) is set based on PbSOC in the configuration described above, the MOS switch 21 can be turned off at more appropriate timing.

According to the above configuration, the : value set as a discharge permission value on the basis of PbSOC is either 0 (that satisfies a relation "(Amount of charge) = (Amount of discharge)") or a positive value (that satisfies a relation "(Amount of charge) < (Amount of discharge)") (see Fig. 4). Accordingly, when PbSOC is small, the amount of discharge of the lead battery 12 is prevented from exceeding the amount of charge thereof during regenerative generation.

Further, according to the above configuration, during regenerative generation associated with deceleration of the vehicle, the MOS switch 21 is brought into an off state from an on state at a time point when the vehicle speed decreases to the turn-off vehicle speed (connection prohibition vehicle speed) . Accordingly, the engine load caused by the rotation of the alternator 11 is reduced in a predetermined low-speed range. Thus, drivability will not be impaired right before the stop of the vehicle.

Smaller PbSOC at the start of regenerative generation leads to more necessity of applying electrical charge to the lead battery 12. In this regard, according to the above configuration, the turn-off vehicle speed is ensured to be high when PbSOC is small at the start of regenerative generation. Accordingly, discharge from the lead battery 12 is reduced during regenerative generation to thereby prevent PbSOC from being decreased. Further, when PbSOC at the start of regenerative generation is large, the turn-off vehicle speed is ensured to be low. Accordingly, the lithium ion battery 13 can be preferentially charged.

On the other hand, the higher the vehicle speed is, the larger the kinetic energy becomes in the vehicle. Accordingly, the higher the vehicle speed is at the starttof regenerative generation, the larger the amount of generation becomes in regenerative generation. According to the configuration described above, when the vehicle speed at the start of regenerative generation is higher than the turn-on vehicle speed (connection permission vehicle speed), the MOS switch 21 is controlled to be electrically connected, so that both of the batteries 12 and 13 can be charged. Also, when the vehicle speed is equal to or less than the turn-on vehicle speed and thus the amount of generation in regenerative generation is small, only the lead battery 12 is charged. In this case, electrical charge is favorably applied to both of the batteries 12 and 13.

Smaller PbSOC at the start of regenerative generation leads to more necessity of applying charge to the lead battery 12. In this regard, according to the above configuration, the turn-on vehicle speed is ensured to be high when PbSOC is small at the start of regenerative generation. Accordingly, the lead battery 12 can be preferentially charged during regenerative generation. Also, when PbSOC is large at the start of regenerative generation, the turn-on vehicle speed is ensured to be low to thereby apply charge to both of the batteries 12 and 13. It will be appreciated that the present invention is not limited to the configurations described above, but any and all modifications, variations or equivalents, which may occur to those who are skilled in the art, should be considered to fall within the scope of the present invention.

(Modifications)

The embodiment described above may be modified as set forth below.

The following configuration may be used to perform the step of setting a turn-off vehicle speed (connection prohibition vehicle speed) (step S14 of Fig. 2). In this configuration, a turn-off vehicle speed is set based on not only the amount of charge of the lead battery 12 (PbSOC) at the start of regenerative generation, but also the amount of charge of the lithium ion battery 13 (LiSOC) at the start of regenerative generation. Specifically, using a map shown in Fig. 6, a turn-off vehicle speed is set based on PbSOC and LiSOC at every cycle. In Fig. 6, when PbSOC is small, the turn-off vehicle speed is set to a high level compared with the case where the PbSOC is large. Further, when LiSOC is small, the turn-off vehicle speed is set to a low level compared with the case where the LiSOC is large.

Thus, the timing of turning off the MOS switch 21 can be delayed by setting a low turn-off vehicle speed when LiSOC is small at the start of regenerative generation. Accordingly, the lithium ion battery 13 is preferentially charged during regenerative generation. Further, the timing of turning off the MOS switch 21 can be advanced by setting a high turn-off vehicle speed when LiSOC is large at the start of regenerative generation. Accordingly, discharge from the lead battery 12 is reduced during regenerative generation to thereby prevent decrease of PbSOC. The ratio of PbSOC to LiSOC ( = PbSOC/LiSOC) may be calculated. When PbSOC/LiSOC is large, the turn-off vehicle speed may be set to a low level compared with the case where the PbSOC/LiSOC is small.

The following configuration may be used to perform the step of setting a turn-on vehicle speed (connection permission vehicle speed) (step S14 of Fig. 2). In this configuration, a turn-on vehicle speed is set based on not only the amount of charge (PbSOC) of the lead battery 12 at the start of regenerative generation, but also the amount of charge (LiSOC) of the lithium ion battery 13 at the start of regenerative generation. Specifically, using a map similar to the one shown in Fig. 6, a turn-on vehicle speed is set based on PbSOC and LiSOC at every cycle. In this case, numerical values of +a (comparatively higher vehicle speeds) may be used with respect to the numerical values shown in Fig. 6. When PbSOC is small, the turn-on vehicle speed is set to a high level compared with the case where the PbSOC is large. Further, when LiSOC is small, the turn-on vehicle speed is set to a low level compared with the case where the LiSOC is large.

Thus, the lithium ion battery 13 will have more opportunities of being charged, by setting a low turn-on vehicle speed when LiSOC is small at the start of regenerative generation. Further, the lead battery 12 can be preferentially charged during regenerative generation, by setting a high turn-on vehicle speed when LiSOC is large at the start of regenerative generation.

The ratio of PbSOC to LiSOC (=PbSOC/LiSOC) may be calculated. When PbSOC/LiSOC is large, the turn-on vehicle speed may be set to a low level compared with the case where PbSOC/LiSOC is small.

In the embodiment described above, a discharge permission value is set based on the relationship shown in Fig. 4. Also, in this case, the value set as a discharge permission value on the basis of PbSOC is either 0 (that satisfies a relation "(Amount of charge) = (Amount of discharge)") or a positive value (that satisfies a relation "(Amount of charge) < (Amount of discharge)"). Alternatively to this, for example, a discharge permission value may be set to a positive value in any case. In this case, the larger the PbSOC is at the start of regenerative generation, the larger the value may be that is set as the discharge permission value.

The value set as a discharge permission value may be a negative value "(Amount of charge) > (Amount of discharge)". A negative discharge permission value may be set when PbSOC is small at the start of regenerative generation. Setting a negative value as a discharge permission value, the lead battery 12 is more preferentially charged during regenerative generation. Accordingly, PbSOC is reliably prevented from being decreased. It may also be so configured that any one of a negative value, a value 0 and a positive value is set as a discharge permission value on the basis of PbSOC.

In the embodiment described above, the discharging state of the lead battery 12 is monitored during regenerative generation. In this case, a charge-discharge balance (current integrated value) is calculated as monitoring information. Further, in this case, the charge-discharge balance is the difference between the amount of charge of the lead battery 12 in a charging state and the amount of discharge of the lead battery 12 after having transitioned from the charging state to a discharging state. Alternatively to this, for example, the monitoring information obtained through calculation may be only the amount of discharge of the lead battery 12 in a discharging state. Based on the amount of discharge, the MOS switch 21 may be electrically disconnected. With this configuration as well, the lead battery 12 (first battery) is prevented from being overdischarged during regenerative generation. In the embodiment described above, the lead battery 12 is used as the first battery and the lithium ion battery 13 is used as the second battery. Alternatively to this, for example, the second battery may be a different secondary battery, such as a nickel-cadmium battery or a nickel-hydrogen battery. Alternatively, the first and second batteries may both be a lead battery, or the first and second batteries may both be a lithium ion battery. In any case, the terminal voltage of the first and second batteries may only have to be different.

Hereinafter, aspects of the above-described embodiments will be summarized.

As an aspect of the embodiment, a power system for a vehicle is provided which includes: a generator (11); a first battery (12) and a second battery (13) which are connected in parallel with the generator; land a connection switch (21) which is provided on a connection line (18) electrically connecting the first and second batteries, and which electrically connects and electrically disconnects the first battery and the generator to/from the second battery. The power system performs regenerative generation by the generator when the vehicle decelerates. A terminal voltage of the first battery is made larger than a terminal voltage of the second battery. The power system further includes: a first control means (30) which makes the connection switch an electrically connected state to apply electrical charge to both of the first battery and the second battery during regenerative generation by the generator; a discharge monitor means (30) which monitors a discharging state of the first battery during regenerative generation; and a second control means (30) which disconnects the connection switch based on the discharging state of the first battery monitored by the discharge monitor means during regenerative generation.

During regenerative generation, the connection switch is brought into an electrically connected state to apply electrical charge to both of the first battery and the second battery. However, the first and second batteries have a differently set terminal voltage. Specifically, the terminal voltage of the first battery is set to a higher level than that of the second battery. Accordingly, even during regenerative generation, the state of the first battery may transition from a charging state to a discharging state. For example, in regenerative generation associated with deceleration of the vehicle, the vehicle speed gradually decreases to decrease the amount of regenerative generation performed by the generator. When the amount of regenerative generation decreases in this way, the state of the first battery may transition from a charging state to a discharging state. Thus, the amount of charge (residual capacity) of the first battery may unintentionally decrease during regenerative generation. As a result, the generator can be forced to apply electrical charge to the first battery in a period other than the period of regenerative generation.

In this regard, according to the configuration set forth above, the discharging state of the first battery is monitored during regenerative generation. Based on the discharging state, the connection switch is electrically disconnected. Thus, the amount of charge of the first battery is prevented from being unintentionally decreased. In this way, electrical charge is efficiently applied to the first and second batteries.