Hybrid electric vehicle

Abstract

A hybrid electric vehicle (1) comprising an internal combustion engine (2) and a motor generator (4) as a drive source and being supplied with power by the motor generator (4) in an electric vehicle (EV) mode, the hybrid electric vehicle (1) comprising: a first battery (30); a second battery (31), wherein the first and second batteries (30, 31) differ from each other in their characteristics; a first type of at least one electrical load (37); a second type of at least one electrical load (38); at least one connection switch (40, 41, 42), wherein the connection switch (40, 41, 42) is selectively set to a first state to establish a first connection state in which the first and second batteries (30, 31) are connected in parallel to the first and second types of electrical loads (37, 38), or to a second state to establish a second connection state in which the parallel connection is interrupted, so that the first battery (30) is connected to the first type of electrical load (37) and the second battery (31) is connected to the second type of electrical load (38); and a connection switch control section (10A) configured to control the state of the connection switch (40, 41, 42), wherein the connection switch control section (10A) sets the state of the connection switch (40, 41, 42) so that the second connection state is established during operating times in the EV mode in which power is supplied by the motor generator (4) when a terminal voltage of the first battery (30) is higher than a terminal voltage of the second battery (31).

Description

[Technical field]

[0001] The present invention relates to hybrid electric vehicles. [Previous state of the art]

[0002] Hybrid electric vehicles have an internal combustion engine and a motor generator as a power source, so that they are powered by at least one of the internal combustion engine and the motor generator driven by electrical energy from a battery.

[0003] JP 2014-033571 A discloses a power supply system for electrical loads on a hybrid electric vehicle. The known power supply system includes a power generator, a first power storage device in the form of a lead-acid battery, and a second power storage device in the form of a lithium-ion battery. The lead-acid battery and the lithium-ion battery are connected in parallel with the power generator. An electrical connection between the lead-acid battery and the lithium-ion battery is selectively interrupted by a connection switch located in a connection line between the lead-acid battery and the lithium-ion battery. When an output voltage of the lead-acid battery drops to a charge-execution voltage level, the power generator starts recharging the lead-acid battery.Under the control of a control unit, the charge execution voltage level is raised to a higher level when the connection switch is in the on state to allow the electrical connection between the lead-acid and lithium-ion batteries. [Summary of the invention][Technical problem]

[0004] A hybrid electric vehicle (EV) is powered solely by an electric motor in electric vehicle (EV) mode. In EV mode, the internal combustion engine is shut off, resulting in excellent ride comfort, quietness, and fuel economy due to the absence of engine noise and fuel consumption. Extending the driving range in EV mode is desirable. To extend the driving range in EV mode, it is necessary to maintain a good battery charge level in the vehicle's power supply system for electrical loads.

[0005] JP 2014-033571 A contains no disclosure or teaching regarding extending the range of the EV mode. Consequently, extending the range of the EV mode to improve fuel consumption is not known.

[0006] US 2015 / 0 340 884 A1 discloses a power supply control device for a power supply device including a plurality of storage batteries and a power generator that performs charging of the plurality of storage batteries, wherein the power supply control device controls a parallel connection between the plurality of storage batteries. Voltage adjustment is performed either by charging the storage battery with the lowest output voltage by the power generator or by discharging the storage battery with the highest output voltage, wherein the parallel connection is performed when the output voltage difference between the plurality of storage batteries becomes equal to or less than a predetermined threshold.

[0007] An object of the present invention is to extend the range of the EV mode in order to improve fuel consumption. [Solution to the problem]

[0008] According to one aspect of the present invention, there is provided a hybrid electric vehicle that includes an internal combustion engine and a motor generator as a drive source and that is powered by the motor generator in an electric vehicle (EV) mode, the hybrid electric vehicle comprising: a first battery; a second battery, the first and second batteries being different from each other in characteristics; a first type of at least one electrical load; a second type of at least one electrical load;at least one connection switch, the connection switch being selectively set to a first state to establish a first connection state in which the first and second batteries are connected in parallel to the first and second types of electrical loads, or to a second state to establish a second connection state in which the parallel connection is interrupted, so that the first battery is connected to the first type of electrical load and the second battery is connected to the second type of electrical load;and a connection switch control section for controlling the state of the connection switch, wherein the connection switch control section sets the state of the connection switch so that the second connection state is established during operating times in the EV mode in which power is supplied by the motor generator when a terminal voltage of the first battery is higher than a terminal voltage of the second battery; [Advantageous effect of the invention]

[0009] According to the present invention, the range of the EV mode is extended and the fuel consumption is improved. [Brief description of the drawings] Fig. 1 is a block diagram of a controller for a hybrid electric vehicle; Fig. 2 is a block diagram of a low-voltage system for the hybrid electric vehicle; Fig. 3 is a flowchart; Fig. 4 is a timing diagram. [Description of an embodiment (of embodiments)]

[0010] An embodiment of the present invention will be described with reference to the accompanying drawings. A hybrid electric vehicle to which the present invention is applied will be described below.

[0011] Referring to Fig. 1 comprises a hybrid electric vehicle 1: a motor 2 in the form of an internal combustion engine, a transmission 3, a motor generator 4, a set of drive wheels, only one of which is shown and designated 5, a hybrid control unit (HCU) 10 configured to comprehensively control the hybrid electric vehicle 1, an engine control module (ECM) 11 configured to control the internal combustion engine 2, a transmission control module (TCM) 12, an integrated starter generator control module (ISGCM) 13, an inverter control module (ICM) 14, a low voltage battery management system (LVBMS) 15 and a high voltage battery management system (HVBMS) 16.

[0012] The internal combustion engine 2 is configured with a plurality of cylinders. In the present example, the internal combustion engine 2 is a four-stroke engine, meaning that four piston strokes are required to complete one cycle. The cycle includes four different processes: intake stroke, compression stroke, power stroke, and exhaust stroke.

[0013] An integrated starter generator (ISG) 20 and a starter 21 are operatively connected to the internal combustion engine 2. The ISG 20 is connected to a crankshaft 18 of the internal combustion engine 2 via a belt 22. The ISG 20 functions as an electric motor to start the internal combustion engine 2 upon input of current, and it functions as an electric current generator to generate current upon input of torque from the crankshaft 18.

[0014] In the present example, the ISG 20, under the control of the ISGCM 13, functions as an electric motor to restart the engine 2 after an engine auto-stop function (or an idle stop function) automatically shuts down the engine 2, rather than allowing the engine 2 to idle. The ISG 20 functions as the electric motor to provide power assistance when the engine 2 is driving the hybrid electric vehicle 1.

[0015] The starter 21 includes an electric motor and a gear train, both of which are not shown. The starter 21 provides starting torque for the internal combustion engine 2 by rotating the crankshaft 18 with the electric motor. The internal combustion engine 2 is started by the starter 21 in this way, and is restarted by the ISG 20 after the internal combustion engine's auto-stop function has automatically shut down the internal combustion engine 2.

[0016] The transmission 3 changes the output speed of the engine 2 to drive the set of drive wheels 4 via a drive shaft 23. The transmission 3 includes: a constant-mesh speed change mechanism 25 of a parallel-shaft gear mechanism; a single-disk dry clutch type clutch 26; a differential gear 27; a clutch actuator 51; and a shift actuator 52.

[0017] Under control of the TCM 12, the clutch actuator 51 enables interruption, that is, disengagement and engagement, of the clutch 26. Under control of the TCM, the shift actuator 52 moves a shift sleeve of the speed change mechanism 25, thereby effecting a shift to a new gear. When a shift to a new gear is performed after disengagement of the clutch 26, this is hereinafter referred to simply as a "gear shift."

[0018] As described, the transmission 3 is configured as an automatic transmission referred to as an automated manual transmission (AMT), which enables gear shifting under the control of the TCM 12. The differential gear 27 transmits power from the speed change mechanism 25 to the drive shaft 23.

[0019] The motor generator 4 is connected to the differential gear 27 via a power transmission mechanism 28, such as a chain, etc. The motor generator functions as an electric motor.

[0020] As described, the hybrid electric vehicle 1 is configured for a parallel hybrid system in which both power from the internal combustion engine 2 and that from the motor generator 4 can be used for propulsion. The hybrid electric vehicle 1 is driven using at least one of power from the internal combustion engine 2 and that from the motor generator 4.

[0021] The hybrid electric vehicle 1 can provide an engine-only mode in which only the engine 2 provides driving power, an electric vehicle (EV) mode in which only the motor generator 4 provides driving power, and an assist mode in which the motor generator 4 provides torque assistance to the motor torque of the engine 2 to provide driving power. As described, the hybrid electric vehicle 1 has an EV mode and an assist mode in addition to an engine-only mode.

[0022] The motor generator 4 can function as a power generator, enabling the generation of power under certain circumstances when the hybrid electric vehicle 1 is in operation. It is not necessary to couple the motor generator 4 to the differential gear 27. The motor generator 4 can be coupled to any point on an axis between the transmission 3 and the drive wheel 5.

[0023] The hybrid electric vehicle 1 includes: a first power storage device 30; a low-voltage battery pack 32; a high-voltage battery pack 34; a high-voltage cable 35; and a low-voltage cable 36. The low-voltage battery pack 32 includes a second power device 31. The high-voltage battery pack 34 includes a third power storage device 33.

[0024] The first power storage device 30, the second power storage device 31, and the third power storage device 33 are secondary batteries and rechargeable batteries, respectively. In the present example, the first power storage device 30 is in the form of a lead-acid battery. The output power and energy density of the second power storage device 31 are higher than those of the first power storage device 30.

[0025] The second power storage device 31 can be recharged in a shorter time than the first power storage device 30. In the present example, the second power storage device 31 is in the form of a lithium-ion battery. The second power storage device 31 may be in the form of a nickel-metal hydride battery.

[0026] Each of the first and second power storage devices 30 and 31 is a low-voltage battery whose number of cells is set to generate an output voltage of approximately 12 volts. The third power storage device 33 is in the form of, for example, a nickel-metal hydride battery or a lithium-ion battery.

[0027] The third power storage device 33 is a high-voltage battery whose number of cells is set such that it can generate a higher output voltage, for example, 100 volts, which is higher than an output voltage generated by the first and second power storage devices 30 and 31. The state of the third power storage device 33, such as a remaining capacity, is managed by the HVBMS 16.

[0028] The hybrid electric vehicle 1 is equipped with two different types of electrical load groups, i.e., a general load group 37 and a critical load group 38. The general loads 37 and the critical loads 38 are electrical loads except for the starter 21 and the ISG 20.

[0029] The critical loads 38 are electrical loads that constantly require a stable power supply. The critical loads 38 include an electronic stability control system 38A that improves vehicle stability by detecting and reducing loss of traction to prevent the hybrid electric vehicle 1 from sliding sideways; an electronic steering control system 38B that assists a driver in steering by improving steering effort; and vehicle headlights 38C. In addition, the critical loads 38 include lamps and gauges within an instrument panel (not shown), as well as a vehicle navigation system.

[0030] General loads 37 are electrical loads that are used temporarily and therefore do not require a constant, stable power supply, unlike the critical loads 38. General loads 37 include, for example, windshield wipers and an electric cooling fan that supplies cooling air to the combustion engine 2.

[0031] The low-voltage battery pack 32 includes, in addition to the second power storage device 31, switches 40 and 41, and the LVBMS 15. The first power storage device 30 and the second power storage device 31 are connected to the low-voltage cable 36, to which the stator 21, the ISG 20, the general loads 37, and the important loads 38 are connected such that, under certain circumstances, the first power storage device 30 provides power to the general loads 37 and the second power storage device 31 provides power to the important loads 38, while the ISG 20 charges the first and second power storage devices 30 and 31 under other circumstances. With respect to the important loads 38, the first and second power storage devices 30 and 31 are connected in parallel.

[0032] With respect to the low-voltage cable 36, the switch 40 is provided such that, under certain circumstances, an electrical connection between the second power storage device 31 and the important loads is selectively interrupted when it is in the off state. The switch 41 is provided in the low-voltage cable 36 between the first power storage device 30 and the second power storage device 31 such that, when it is in the off state, an electrical connection between the first power storage device 30 and the second power storage device 31 is selectively interrupted.

[0033] The LVBMS 15 controls the charging (i.e., recharging) of the second power storage device 31 with power output from the ISG 20 and power supply to the important loads 38 from the second power storage device 31 by selectively setting the switches 40 and 41 to an on and / or off state. The vehicle 1 is a stop / start vehicle equipped with an auto-stop function of an internal combustion engine. This function automatically shuts off the internal combustion engine 2 during certain operating times, for example, when the vehicle 1 is stopped, rather than allowing the internal combustion engine 2 to idle.When this function turns off the internal combustion engine 2, the LVBMS 15 opens the switch 41 with the switch 40 closed, disconnecting the second power storage device 31 from the ISG 20 and connecting the second power storage device 31 to the important loads 38 to provide a stable power supply from the storage device 31 having a high output power and a high energy density.

[0034] When the starter 21 starts the engine 2 or the ISG 20 restarts the engine 2, the LVBMS 15 opens the switch 41, with the switch 40 closed, causing the first power storage device 30 to provide power to the stator 21 or the ISG 20. When the switch 41 is open, with the switch 40 closed, the first power storage device 30 also provides power to the general loads 37.

[0035] As described, the first power storage device 30 provides at least one power for the starter 21 and the ISG 20, which serve as starting means for the internal combustion engine 2. The second power storage device 31 provides at least one power for the general loads 37 and the important loads 38.

[0036] The second power storage device 31 is connected in such a way that it can provide power to both the general loads 37 and the important loads 38, but the LVBMS 15 controls the switches 40 and 41 in such a way that the second power storage device 31 preferentially supplies power to the important loads 38 that constantly require a stable power supply.

[0037] Taking into account the state of charge (SOC) of each of the first and second power storage devices 30 and 31, as well as operating requirements for the general loads 37 and the important loads 38, the LVBMS 15 sometimes controls the switches 40 and 41 in ways different from what is described above.

[0038] The high-voltage battery pack 34 includes, in addition to the third power storage device 33, an inverter 45, the INVCM 14, and the HVBMS 16. The high-voltage battery pack 34 is connected to the motor generator 4 via the high-voltage cable 35 in such a way that it can provide power to the motor generator 4.

[0039] Under the control of the INVCM 14, the inverter 45 converts, under certain circumstances, an input direct current output from the third power storage device 33 into an output alternating current flowing through the high-voltage cable 35, and under other circumstances, converts an input alternating current flowing through the high-voltage cable 35 into an output direct current supplied to the third power storage device 33. For example, the INVCM 14 converts a direct current output from the third power storage device 33 into an alternating current supplied to the motor generator 4 in response to a power request for operating the motor generator 4 in the power mode.

[0040] The INVCM 14 converts an alternating current generated by the motor generator 4 into a direct current using the inverter 45 and supplies the direct current to the third power storage device 33 to recharge it in response to a regeneration request for operating the motor generator 4 in a regenerative mode.

[0041] In the present example, each of the HCU 10, the ECM 11, the TCM 12, the ISGCM 13, the INVCM 14, the LVBMS 15, and the HVBMS 16 consists of a computer unit including a central processing unit (CPU), a random access memory (RAM), a read-only memory (ROM), a flash memory for data backup, input channels, and output channels.

[0042] The ROMs of these computer units store programs in addition to various constants and various programs to cause each of the computer units to function as the corresponding one of the HCU 10, ECM 11, TCM 12, ISGCM 13, INVCM 14, LVBMS 15 and HVBMS 16.

[0043] In other words, each of these computer units is caused to function as an HCU 10, ECM 11, TCM 12, ISGCM 13, INVCM 14, LVBMS 15, and HVBMS 16 by allowing the CPU to execute programs stored in the ROM using the RAM as the work area.

[0044] In the present example, the ECM 11 includes an internal combustion engine auto-stop function. This function automatically shuts down the internal combustion engine during certain operating times to save fuel. For example, the internal combustion engine auto-stop function may be active when the vehicle 1 is stopped, rather than allowing the internal combustion engine 2 to idle. The internal combustion engine 2 may be restarted under the control of the ISGCM 13 via the ISG 20 when the driver releases the brake or depresses the accelerator or gas pedal.

[0045] In the present example, the ECM 11 turns off the engine 2 in response to the determination of a vehicle stop, which can be determined when the vehicle speed is zero. As described, the hybrid electric vehicle 1 has an engine auto-stop function that automatically turns off the engine 2 instead of allowing the engine to idle in response to the determination of a vehicle stop. In response to the determination of automatic engine stop when the vehicle 1 stops on an uphill road whose road surface is inclined, the motor generator 4 functions as an electric motor to maintain the stopped state of the vehicle 1. This function of maintaining the stopped state of the vehicle 1 is performed by means of a power from the third power storage device 33.

[0046] The hybrid electric vehicle 1 has a local area network (LAN) that meets Controller Area Network (CAN) standards and CAN communication lines 48 and 49.

[0047] The HCU 10 is connected to the INVCM 14 and the HVBMS 16 via the CAN communication line 48. The HCU 10, the INVCM 14, and the HVBMS 16 mutually transmit and receive signals, such as control signals, via this CAN communication line 48.

[0048] The HCU 10 is connected to the ECM 11, the TCM 12, the ISGCM 13, and the LVBMS 15 via the CAN communication line 49. Via this CAN communication line 49, the HCU 10, the ECM 11, the TCM 12, the ISGCM 13, and the LVBMS 15 mutually transmit and receive signals, such as control signals, from one another.

[0049] In the present example, the hybrid electric vehicle 1 has a torque gap fill function. The torque gap fill function provides torque assistance to the drive wheels 5 by supplying power to the motor generator 4 during gear shifting in the transmission 3 to cause the motor generator 4 to generate torque.

[0050] The HCU 10 implements the gap-filling function by executing torque-gap-filling control actions. During gear shifting in the transmission 3, the clutch 26 is disengaged so that engine torque from the engine 2 is not transmitted to the drive wheels 5. In the torque-gap-filling function, the HCU 10 provides torque assist to the drive wheels 5 by operating the motor generator 4 in a power mode during gear shifting in the transmission to cause the motor generator 4 to generate torque. This torque-gap-filling function reduces a feeling of deceleration when the clutch 26 is disengaged during gear shifting, improving the operability of the vehicle 1.

[0051] Now referring to Fig. 2, the hybrid electric vehicle 1 includes a first state of charge (SOC) detection unit 61. This first SOC detection unit 61 detects the SOC of the first power storage device 30 and supplies the HCU 10 with a detection signal indicative of the detected SOC. The first SOC detection unit 61 is provided in the vicinity of the first power storage device 30. This detection of the SOC of the first power storage device 30 is performed by detecting a terminal voltage of the first power storage device 30, as well as input currents to and output currents from the first power storage device 30.

[0052] Furthermore, the hybrid electric vehicle 1 includes a second SOC detection unit 62. This second SOC detection unit 62 detects the SOC of the second power storage device 31 and supplies the HCU 10 with a detection signal indicative of the detected SOC. The second SOC detection unit 62 is provided in the vicinity of the second power storage device 31. This detection of the SOC of the second power storage device 31 is performed by detecting a terminal voltage of the second power storage device 31, as well as input currents to and output currents from the second power storage device 31. The second SOC detection unit 62 supplies the detection signal to the HCU 10 via the LVBMS 15.

[0053] The low-voltage cable 36 is branched into two branches at a point between the first power storage device 30 and the second power storage device 31. The aforementioned switch 41 and a fuse are provided in one of the two branches of the low-voltage cable 36. A switch 42 and a fuse are provided in the other of the branches of the low-voltage cable 36. As described later, the switches 41 and 42 are opened or closed at the same time. For example, the switch 42 is in the on state when the switch 41 is in the on state. Thus, the switch 42 is in Fig. 1 omitted.

[0054] The general loads 37 include, in addition to the aforementioned windshield wiper and electric cooling fan, a blower fan, a radiator fan, an electric water pump, an electric vacuum pump, an interior light, and so on.

[0055] The important loads 38 include, in addition to the electronic stability control system 38A, the electronic steering control system 38B, and vehicle headlights 38C, a navigation or vehicle navigation system, an audio system, gauges, a vehicle air conditioning control panel, a steering angle sensor, a stereo camera, and so on.

[0056] When all of the aforementioned switches 40, 41, and 42 are in the on state (or closed state), a first connection state is established. In this first connection state, the first power storage device 30 and the second power storage device 31 are connected in parallel with the general loads 37 and the important loads 38.

[0057] When switch 40 is in the on state (or closed state) and switches 41 and 42 are in the off state (or open state), a second connection state is established. In this second connection state, first power storage device 30 is connected to general loads 37, and second power storage device 31 is connected to important loads 38.

[0058] In this second connection state, the parallel configuration of the first power storage device 30 and the second power storage device 31 is interrupted because the switches 41 and 42 are in the off state (or open state). The switches 40, 41, and 42 form connection switches as claimed. The first power storage device 30 forms a first battery as claimed, and the second power storage device 31 forms a second battery as claimed.

[0059] The following describes in detail the differences in battery characteristics between the first power storage device 30 in the form of a lead-acid battery and the second power storage device 31 in the form of a lithium-ion battery.

[0060] The differences in battery characteristics between the first power storage device 30 and the second power storage device 31 are as follows.

[0061] With respect to a terminal voltage (or a battery voltage) at full charge, the voltage of the second power storage device 31 in the form of the lithium-ion battery is lower than the voltage of the first power storage device 30 in the form of the lead-acid battery.

[0062] With respect to an internal resistance for a battery, the internal resistance of the second power storage device 31 in the form of the lithium-ion battery is lower than that of the first power storage device 30 in the form of the lead-acid battery.

[0063] With respect to a time required for full charging, the time required for the second power storage device 31 in the form of the lithium-ion battery is shorter (or the second power storage device 31 is faster) than that for the first power storage device 30 in the form of the lead-acid battery.

[0064] It follows that the second power storage device 31 in the form of the lithium-ion battery is fully charged before the first power storage device 30 in the form of the lead-acid battery is fully charged when the charging of the first power storage device 30 and the second power storage device 31 is started simultaneously, the first and second power storage devices 30 and 31 being connected in parallel with the ISG 20.

[0065] When the lithium-ion battery is made from a battery pack consisting of four (4) cells, it has a voltage of 9.2 volts, which is closest to or within a supply voltage range of 12 volts (i.e., a range of 6 volts to 14 volts), since a unit cell has a voltage of 2.3 volts (a lower limit voltage is 1.5 volts and an upper limit voltage is 3.3 volts). Moreover, when the lithium-ion battery is made from the battery pack consisting of four cells, the upper limit voltage is 13.5 volts and the lower limit voltage is 9.2 volts. As described, the first and second power storage devices 30 and 31 differ in characteristics such as the voltage at full charge.

[0066] During operation in EV mode, the first power storage device 30 and the second power storage device 31 must supply power to the general loads 37 and the important loads 38 because the engine 2 is turned off and the ISG 20 cannot generate power.

[0067] Therefore, it is necessary for the first power storage device 30 and the second power storage device 31 to be maintained at a good SOC in order to perform operation in EV mode.

[0068] As described above, the first power storage device 30 and the second power storage device 31 differ in their characteristics, so that the required SOC for allowing operation in the EV mode is set to different values ​​for the first power storage device 30 and the second power storage device 31.

[0069] In the present example, the HCU 10 includes a connection switch control section 10A. This connection switch control section 10A controls the on / off states of the switches 40, 41, and 42.

[0070] Furthermore, the connection switch control section 10A sets the on / off states of the switches 40, 41, and 42 during operation times in the EV mode when the motor generator 4 functions as an electric motor, so that the second connection state is established. In the second connection state, the first power storage device 30 is connected to the general loads 37, and the second power storage device 31 is connected to the important loads 38.

[0071] The actual switching actions of these switches 40, 41, and 42 are performed by the LVBMS 15 in response to a switching request sent from the interconnect switch control section 10A of the HCU 10 to the LVBMS 15. In the present example, the interconnect switch control section 10A is provided in the HCU 10, but it may be provided in the ECM 30.

[0072] Referring to the flowchart in Fig. 3 describes switching actions performed in the hybrid electric vehicle. It is now assumed that switch 40 is constantly in the on state to select the connection state, and that switches 41 and 42 are in the off state to interrupt the electrical connection between the two batteries, i.e., between the first power storage device in the form of lead-acid battery 30 and the second power storage device in the form of lithium-ion battery 31, or that they are in the on state to enable the electrical connection between the two batteries 30 and 31.

[0073] Referring to Fig. 4, it is determined whether or not the engine 2 is turned off in the EV mode at step S1. This determination can be performed by the HCU 10.

[0074] If it is not determined at step S1 that the internal combustion engine 2 is switched off, then the algorithm returns to step S1.

[0075] If the internal combustion engine 2 is switched off at step S1, then it is determined at step S2 whether V Pb > V Li is or not, where V Pb a terminal voltage of the lead-acid battery is 30 and V Li a terminal voltage of the lithium-ion battery 31. This determination may be made by the HCU 10 in response to signals from the first and second SOC detection units 61 and 62 (see Fig. 2).

[0076] If at step S2 V Pb > V Liis, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is interrupted by setting the connection switches 41 and 42 to the off state at step S3. The algorithm then returns to step S1. The connection switches 41 and 42 can be controlled by the HCU 10.

[0077] When the vehicle 1 is operating in the EV mode, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is interrupted at step S3, so that the lead-acid battery 30 supplies current to the general load(s) 37 and the lithium-ion battery 31 supplies current to the important load(s) 38. This prevents current from flowing from the lead-acid battery 30 to the lithium-ion battery 31. Consequently, the driving range of the EV mode is further increased by preventing propulsion in the EV mode from being inhibited due to a drop in the SOC of the lead-acid battery 30.

[0078] If referring to step S4 in Fig. 3 at step S2 V Pb not larger than V Li then it is determined whether V Pb ≥ TH VPb and V Li ≥ TH VLi is or not, where TH VPb and TH VLiThreshold voltages are indicative of lower limits for operation in EV mode. This determination can be performed by the HCU 10.

[0079] If at step S4 V Pb ≥ TH VPb and V Li ≥ TH VLi are, then at step S5, it is determined whether or not the event of an automatic engine restart of the internal combustion engine 2 in response to detecting that the vehicle air conditioning system is operating or detecting that the accelerator pedal or the accelerator pedal is depressed is expected. This determination can be performed by the HCU 10.

[0080] If the automatic engine restart of the internal combustion engine 2 is expected at step S5, the internal combustion engine 2 is restarted at step S6. The internal combustion engine restart can be performed by the HCU 10.

[0081] If the automatic engine restart of the internal combustion engine 2 is not expected at step S5, then the algorithm returns to step S1.

[0082] Following step S6, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by setting the connection switches 41 and 42 to the on state at step S7. The HCU 10 may perform the operation of setting the connection switches 41 and 42 to the on state.

[0083] If it is not determined at step S4 that V Pb ≥ TH VPb and V Li ≥ TH VLi , then the internal combustion engine 2 is restarted in step S8. The internal combustion engine restart can be performed by the HCU 10.

[0084] Following step S8, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by setting the connection switches 41 and 42 to the on state at step S9, and the algorithm returns to step S1. The HCU 10 may perform setting the connection switches 41 and 42 to the on state.

[0085] As described, in a state where the lead-acid battery 30 and the lithium-ion battery 31 can be charged, when the internal combustion engine 2 is restarted, the electrical connection between the two batteries 30 and 31 is enabled at step S7 or step S9 to connect the two batteries 30 and 31 in parallel with the ISG 20. This enables charging of both the lead-acid battery 30 and the lithium-ion battery 31.

[0086] Referring to Fig.4, the internal combustion engine 2 is operating during a period before a time t1, and the connection switches 41 and 42 are in the on state to enable electrical connection between the lead-acid battery 30 and the lithium-ion battery 31. Then, the ISG 20, using the internal combustion engine 2 as a power source, generates current to charge both the lead-acid battery 30 and the lithium-ion battery 31.

[0087] At time t1, the connection switches 41 and 42 are set to the off state to interrupt the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 according to the fact that the terminal voltage V Li the lithium-ion battery 31 during the period before time t1 a voltage V Li(Vollladung)at full charge. This breaks an electrical connection between the lithium-ion battery 31 and the ISG 20, stopping charging of the lithium-ion battery 31 and allowing charging of the lead-acid battery 30 to resume.

[0088] Subsequently, the combustion engine 2 is switched off at a time t2 according to the fact that the terminal voltage V Pb during a period from time t1 to time t2 a voltage V Pb(Vollladung) at full charge. This stops charging of the lead-acid battery 30. At time t2, the EV mode is executed.

[0089] After the time t2, the terminal voltage V Pb the lead-acid battery 30 and the terminal voltage V Li of the lithium-ion battery 31. At a time t3, the terminal voltage V Pb of the lead-acid battery 30 to the voltage V Li(Vollladung)when the lithium-ion battery 31 is fully charged. At a time t4, the terminal voltage V Li of the lithium-ion battery 31 to the lower limit of the threshold voltage TH VLi the lithium-ion battery 31.

[0090] In order to charge the lead-acid battery 30 to meet a charging requirement for charging the lead-acid battery 30 while preventing the lithium-ion battery 31 from being overcharged, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is kept interrupted for a period from time t2 to time t3 by keeping the switch 41 in the off state, even if the engine 2 is restarted to start power generation. During this period, charging of only the lead-acid battery 30 is performed by the ISG 20 because the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is interrupted by setting the switch 41 in the off state.

[0091] During a period from time t3 to time t4, the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by placing the switch 41 in the on state after the internal combustion engine 2 is restarted to meet a power demand, such as when the vehicle air conditioning system is detected to be operating or when the accelerator pedal is detected to be depressed. During this period, charging of both the lead-acid battery 40 and the lithium-ion battery is performed by the ISG 20 because the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by placing the switch 41 in the on state.

[0092] In order to prevent over-discharge of at least one of the lead-acid battery 30 and the lithium-ion battery 31, at time t4 and immediately thereafter, the internal combustion engine 2 is restarted, and then the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by setting the switch 41 to the on state to charge at least those of the lead-acid battery 30 and the lithium-ion battery 31 whose terminal voltage V Pb or V Li to the lower limit of their threshold voltage TH VPb and TH VLi drops. During this period, charging of both the lead-acid battery 30 and the lithium-ion battery 31 is carried out by the ISG 20 because the electrical connection between the lead-acid battery 30 and the lithium-ion battery 31 is enabled by setting the switch 41 to the on state.

[0093] As described, the hybrid electric vehicle 1 includes the first power storage device 30 and the second power storage device 31, which differ from each other in their characteristics.

[0094] Furthermore, the hybrid electric vehicle 1 includes switches 40, 41, and 42, which are selectively set in a first scheme for establishing the first connection state or in a second scheme for establishing the second connection state. In the first connection state, the first power storage device 30 and the second power storage device 31 are connected in parallel to the general loads 37 and the important loads 38. In the second connection state, this parallel connection is broken, so that the first power storage device 30 is connected to the general loads 37 and functions as an exclusive power supply for the general loads 37, and the second power storage device 31 is connected to the important loads 38 and functions as an exclusive power supply for the important loads 38.

[0095] In addition, the hybrid electric vehicle 1 includes the connection switch control section 10A, which controls the on / off states of the switches 40, 41 and 42.

[0096] Furthermore, the connection switch control section 10A sets the on / off states of the switches 40, 41, and 42 during operation times in the EV mode when the motor generator 4 functions as an electric motor, so that the second connection state is established. In the second connection state, the first power storage device 30 is connected to the general loads 37 and functions as an exclusive power supply for the general loads 37, and the second power storage device 31 is connected to the important loads 38 and functions as an exclusive power supply for the important loads 38.

[0097] According to this configuration, during operation in EV mode, switches 40, 41, and 42 are set in the second scheme to establish the second connection state. In the second connection state, the first power storage device 30 and the second power storage device 31 can supply electric power to the electrical loads (ie, the general loads 37 and the important loads 38), with the electrical connection between the first power storage device 30 and the second power storage device 31 being interrupted.

[0098] Electric current is prevented from flowing from the first power storage device 30 to the second power storage device 31 because the electrical connection between them is interrupted. This prevents any of the terminal voltages of the first and second power storage devices 30 and 31 from dropping to the lower limit voltage level and failing to meet the conditions for realizing EV mode operation. This extends the driving range of the EV mode and improves fuel consumption. [Description of reference symbols] 1 hybrid electric vehicle 2 combustion engine 4 Motor generator 10A connection switch control section 30 first power storage device (first battery) 31 second power storage device (second battery) 37 general load (electrical load, a portion of the electrical load) 38 important load (electrical load, the other part of the electrical load) 40, 41, 42 switches (connection switches)

Claims

[1] A hybrid electric vehicle (1) comprising an internal combustion engine (2) and a motor generator (4) as a drive source and being powered by the motor generator (4) in an electric vehicle (EV) mode, the hybrid electric vehicle (1) comprising: a first battery (30); a second battery (31), wherein the first and second batteries (30, 31) differ from each other in their characteristics; a first type of at least one electrical load (37); a second type of at least one electrical load (38); at least one connection switch (40, 41, 42), wherein the connection switch (40, 41, 42) is selectively set to a first state to establish a first connection state in which the first and second batteries (30, 31) are connected in parallel to the first and second types of electrical loads (37, 38), or to a second state to establish a second connection state in which the parallel connection is interrupted, so that the first battery (30) is connected to the first type of electrical load (37) and the second battery (31) is connected to the second type of electrical load (38); and a connection switch control section (10A) configured to control the state of the connection switch (40, 41, 42), wherein the connection switch control section (10A) sets the state of the connection switch (40, 41, 42) so that the second connection state is established during operating times in the EV mode in which power is supplied by the motor generator (4) when a terminal voltage of the first battery (30) is higher than a terminal voltage of the second battery (31).

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