Charge control method, charge control system, and program

Abstract

When the state of charge of a sub-battery (20) decreases to a lower limit state of charge while a vehicle is in an undrivable state, a control device (100) controls a voltage converter to charge the sub-battery (20) until the state of charge reaches an upper limit state of charge corresponding to full charge. The control device (100) adjusts a charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained while the vehicle is in a drivable state.

Description

Charging Control Method, Charging Control System, and Program

[0001] The present invention relates to a charging control method, a charging control system, and a program.

[0002] In vehicles such as electric vehicles and hybrid vehicles, in addition to a main battery that supplies power to a motor for driving the vehicle, a sub-battery that supplies power to in-vehicle loads such as an ECU (Electronic Control Unit) and sensors is mounted. The sub-battery is charged with the power supplied from the main battery.

[0003] When a large charging current flows from the main battery to the sub-battery while the vehicle is running, the power stored in the main battery significantly decreases, and the electricity cost deteriorates significantly. Therefore, when the vehicle is running, it is desirable to control so that the charging current does not increase. Patent Document 1 describes a technique for suppressing the charging current. The power conversion device described in Patent Document 1 assumes that the state of the sub-battery is a fully charged state when the charging current falls below a predetermined value, and reduces the charging voltage to further reduce the charging current.

[0004] Japanese Patent Application Laid-Open No. 2019-22273

[0005] However, generally, when the sub-battery deteriorates, the charging current flowing from the main battery to the sub-battery becomes small. For this reason, in the power conversion device described in Patent Document 1, when the sub-battery is deteriorated, the charging voltage drops before the sub-battery reaches the fully charged state, and the sub-battery is not fully charged. Here, when the sub-battery is a lead-acid battery, if the state of low charging rate is maintained, the sub-battery further deteriorates. Therefore, a technique for suppressing the deterioration of the sub-battery while suppressing the charging current is desired.

[0006] The present invention has been made in view of the above problems, and an object thereof is to provide a charging control method, a charging control system, and a program that suppress the deterioration of the sub-battery while suppressing the charging current.

[0007] To achieve the above objective, the charging control method according to the present invention performs the following processes. First, it controls a voltage converter that transforms the power supplied from the main battery, which supplies power to the motor that drives the vehicle, and supplies it to the sub-battery, including a lead-acid battery, in order to charge the sub-battery. It also measures the charging voltage applied to the sub-battery by the voltage converter when the sub-battery is being charged. It also measures the charging current flowing from the voltage converter to the sub-battery when the sub-battery is being charged. It also calculates the charge level of the sub-battery. Furthermore, if the charge level drops to the lower limit charge level while the vehicle is in a state where it cannot be driven, it charges the sub-battery until the charge level reaches the upper limit charge level corresponding to full charge. Furthermore, while the vehicle is in a state where it can be driven, it adjusts the charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained.

[0008] According to the present invention, it is possible to suppress the degradation of the sub-battery while suppressing the charging current.

[0009] This is a diagram of the in-vehicle system according to Embodiment 1. This is a diagram of the functional configuration of the control device according to Embodiment 1. This is a graph showing the correspondence between the charging voltage and charging current in the sub-battery according to Embodiment 1. This is a flowchart showing the charging control process when the vehicle is unable to drive, executed by the control device according to Embodiment 1. This is a flowchart showing the charging control process when the vehicle is able to drive, executed by the control device according to Embodiment 1. This is a flowchart showing the reference voltage determination process shown in Figure 5. This is a flowchart showing the reference voltage re-determination process executed by the control device according to Embodiment 2. This is a flowchart showing the simplified reference voltage determination process executed by the control device according to Embodiment 3.

[0010] Embodiments of the present invention will be described in detail below with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

[0011] (Embodiment 1) Figure 1 is a diagram showing the configuration of an in-vehicle system 2000 according to Embodiment 1. The in-vehicle system 2000 is installed in vehicles such as electric vehicles and hybrid vehicles. The in-vehicle system 2000 comprises a main battery 10, a sub-battery 20, a DC (Direct Current) / DC converter 30, a motor 40, a load 50, and a charging control system 1000.

[0012] The main battery 10 is a secondary battery installed in the vehicle and supplies power to loads with relatively high power consumption. The main battery 10 stores power supplied from the motor 40, generator (not shown), etc. The main battery 10 supplies the stored power to the motor 40, sub-battery 20, etc. The main battery 10 is, for example, a lithium-ion battery. The output voltage of the main battery 10 is higher than the output voltage of the sub-battery 20, for example, several hundred volts.

[0013] The sub-battery 20 is a secondary battery installed in the vehicle and supplies power to loads with relatively low power consumption. The sub-battery 20 stores power supplied from the main battery 10 or motor 40 via the DC / DC converter 30. The sub-battery 20 supplies the stored power to the load 50. The sub-battery 20 is, for example, a lead-acid battery in which six lead-acid batteries with a single-cell voltage of 2.1V are connected in series. The output voltage of the sub-battery 20 is, for example, 12.6V. The sub-battery 20 is equipped with a temperature sensor 21. The temperature sensor 21 measures the temperature of the electrolyte contained in the sub-battery 20. Hereinafter, the temperature of the electrolyte in the sub-battery 20 will be referred to as the battery temperature. The temperature sensor 21 outputs temperature information indicating the measured battery temperature.

[0014] The DC / DC converter 30 is a device that converts DC power, transforming the DC voltage applied between the input terminals and applying the transformed DC voltage between the output terminals. In this embodiment, the DC / DC converter 30 transforms the power supplied from the main battery 10, motor 40, etc., and supplies it to the sub-battery 20. The transformation ratio and output terminal state of the DC / DC converter 30 are controlled by the charge control system 1000. The output terminal state can be either a conductive state or an open state. The DC / DC converter 30 is an example of a voltage converter.

[0015] The motor 40 is mounted on the vehicle and functions as both an electric motor and a generator. When functioning as an electric motor, the motor 40 generates torque using power supplied from the main battery 10, and drives the vehicle with the generated torque. The torque generated by the motor 40 is transmitted to a rotating mechanism connected to the wheels of the vehicle. When functioning as a generator, the motor 40 converts the rotational force supplied from the rotating mechanism into electricity and supplies the resulting electricity to the main battery 10.

[0016] Load 50 is a low-power load mounted on the vehicle. Load 50 includes the ECU (Electronic Control Unit), sensors, electrical components, etc. Load 50 operates on power supplied from the sub-battery 20.

[0017] The charging control system 1000 controls the charging of the sub-battery 20 so that the charging current to the sub-battery 20 is reduced and the deterioration of the sub-battery 20 is suppressed. The charging control system 1000 comprises a control device 100, a voltage sensor 200, and a current sensor 300.

[0018] The control device 100 controls the charging of the sub-battery 20. The control device 100 controls the DC / DC converter 30 based on information obtained from the temperature sensor 21, voltage sensor 200, current sensor 300, etc. The control device 100 comprises a control unit 110, a storage unit 120, and an input / output unit 130. The control unit 110 functions as a central processing unit that executes processing and calculations related to the control of the control device 100. The control unit 110 includes, for example, a CPU (Central Processing Unit).

[0019] The memory unit 120 stores programs and data used by the control unit 110. The memory unit 120 is composed of, for example, ROM (Read Only Memory), RAM (Random Access Memory), etc. The memory unit 120 stores information for calculating the charge rate, map information, etc., which will be described later.

[0020] The input / output unit 130 receives data from outside the control device 100 and outputs data to outside the control device 100. The input / output unit 130 includes, for example, a digital input port, a digital output port, an analog input port, an analog output port, etc. The input / output unit 130 may also function as a communication circuit that communicates with the ECU via an in-vehicle LAN (Local Area Network). The control device 100 is, for example, a microcomputer. The control device 100 is an example of a control device.

[0021] The voltage sensor 200 detects the voltage between the terminals of the sub-battery 20. The voltage sensor 200 detects the charging voltage and the open-circuit voltage. The charging voltage is the voltage between the terminals of the sub-battery 20 when the sub-battery 20 is being charged by the DC / DC converter 30. The open-circuit voltage is the voltage between the terminals of the sub-battery 20 when each terminal of the sub-battery 20 is open. Basically, the open-circuit voltage is the voltage between the terminals of the sub-battery 20 when each output terminal of the DC / DC converter 30 is open. The voltage sensor 200 supplies charging voltage information indicating the charging voltage and open-circuit voltage information indicating the open-circuit voltage to the control device 100. The voltage sensor 200 is an example of a voltage sensor.

[0022] The current sensor 300 detects the current flowing into or out of the sub-battery 20. In this embodiment, the current value corresponding to the magnitude of the current is sometimes simply referred to as "current." The current sensor 300 detects the charging current and the discharge current. The charging current is the current that flows from the DC / DC converter 30 to the sub-battery 20 when the sub-battery 20 is being charged. The discharge current is the current that flows from the sub-battery 20 to the load 50 when the load 50 consumes power. The current sensor 300 supplies charging current information indicating the charging current and discharge current information indicating the discharge current to the control device 100. The current sensor 300 may also include a shunt resistor provided on the path connecting the output terminal of the DC / DC converter 30 and the terminal of the sub-battery 20, and a voltage sensor that detects the voltage across the shunt resistor. The current sensor 300 is an example of a current sensor.

[0023] Next, the functions of the control device 100 will be described with reference to Figure 2. Functionally, the control device 100 comprises a charge control unit 101, a voltage information acquisition unit 102, a current information acquisition unit 103, a charge rate calculation unit 104, and a temperature information acquisition unit 105. Each of these functions is realized by software, firmware, or a combination of software and firmware. The software and firmware are written as programs and stored in the storage unit 120. The CPU in the control unit 110 then realizes each of these functions by executing the programs stored in the storage unit 120.

[0024] The charging control unit 101 controls the DC / DC converter 30 to charge the sub-battery 20. The charging control unit 101 controls the output of the DC / DC converter 30 by transmitting a control signal to the DC / DC converter 30. For example, the charging control unit 101 changes the voltage transformation ratio of the DC / DC converter 30 to output an arbitrary voltage between the output terminals of the DC / DC converter 30. The voltage between the output terminals of the DC / DC converter 30 is the output voltage of the DC / DC converter 30. The charging control unit 101 also switches the output terminals of the DC / DC converter 30 between a conductive state and an open state. When the sub-battery 20 is being charged, a charging voltage slightly lower than the output voltage of the DC / DC converter 30 is applied to the sub-battery 20. The reason why the charging voltage applied to the sub-battery 20 is slightly lower than the output voltage of the DC / DC converter 30 is due to voltage drops in elements, wiring, etc., in the path connecting the DC / DC converter 30 and the sub-battery 20. The charging control unit 101 is an example of a charging control means.

[0025] The voltage information acquisition unit 102 acquires charging voltage information from the voltage sensor 200, which indicates the charging voltage applied to the sub-battery 20 by the DC / DC converter 30 when the sub-battery 20 is being charged. The voltage information acquisition unit 102 also acquires open-circuit voltage information from the voltage sensor 200, which indicates the open-circuit voltage of the sub-battery 20. The voltage information acquisition unit 102 is an example of a voltage information acquisition means.

[0026] The current information acquisition unit 103 acquires charging current information from the current sensor 300, which indicates the charging current that flows from the DC / DC converter 30 to the sub-battery 20 when the sub-battery 20 is being charged. The current information acquisition unit 103 also acquires discharge current information from the current sensor 300, which indicates the discharge current that flows from the sub-battery 20 to the load 50. The current information acquisition unit 103 is an example of a current information acquisition means.

[0027] The charge rate calculation unit 104 calculates the charge rate of the sub-battery 20 based on the open-circuit voltage of the sub-battery 20. For example, the charge rate calculation unit 104 determines the charge rate from the open-circuit voltage based on the charge rate calculation information stored in the memory unit 120. The charge rate calculation information is information for calculating the charge rate of the sub-battery 20 from the open-circuit voltage of the sub-battery 20. The charge rate calculation information may be in the form of a table showing the correspondence between the open-circuit voltage and the charge rate, or it may be information showing a calculation formula for calculating the charge rate from the open-circuit voltage. The charge rate is also called SOC (State Of Charge). Note that the higher the charge rate, the higher the open-circuit voltage. For example, if the open-circuit voltage when the charge rate is 100% is 12.6V, then the open-circuit voltage when the charge rate is less than 100% will be lower than 12.6V.

[0028] The charge rate calculation unit 104 can calculate the current charge rate based on past charge rates, the integrated value of charging current, and the integrated value of discharge current. For example, the current charge rate can be determined by adding the increase in charge rate to the past charge rate and subtracting the decrease in charge rate. The increase in charge rate is the ratio of the total amount of charge that has flowed into the sub-battery 20 as charging current from the past to the present to the total amount of charge that can be charged. The decrease in charge rate is the ratio of the total amount of charge that has been discharged from the sub-battery 20 as discharge current from the past to the present to the total amount of charge that can be charged. The charge rate calculation unit 104 is an example of a charge rate calculation means.

[0029] The temperature information acquisition unit 105 acquires temperature information indicating the battery temperature from the temperature sensor 21. The lower the battery temperature, the less likely the electrolyte in the sub-battery 20 is to undergo a chemical reaction, and the higher the charging voltage required for charging. Therefore, it is preferable that a higher charging voltage be applied to the sub-battery 20 when the battery temperature is low.

[0030] The charging control unit 101 maintains the charge level of the sub-battery 20 above the lower limit charge level while the vehicle is in a non-drivable state. In other words, if the charge level of the sub-battery 20 drops to the lower limit charge level, the charging control unit 101 controls the DC / DC converter 30 to charge the sub-battery 20 until the charge level reaches the upper limit charge level. A non-drivable state is a state in which the vehicle does not accelerate when the accelerator is pressed, and is also called the Ready OFF state or standby state. A drivable state is a state in which the vehicle accelerates when the accelerator is pressed, and is also called the Ready ON state. A drivable state includes a drivable stopped state and a driving state. The charging control unit 101 can acquire vehicle status information indicating the state of the vehicle from the ECU via the in-vehicle LAN.

[0031] The lower limit of the charge rate corresponds to the lower limit of the charge rate and is close to 100%. For example, the lower limit of the charge rate is 98.8%. The lower limit of the charge rate is predetermined. The upper limit of the charge rate corresponds to a full charge and is basically 100%. The charge control unit 101 maintains the charge rate of the sub-battery 20 close to 100% while the vehicle is in a state where it cannot be driven. Basically, while the vehicle is in a state where it cannot be driven, even if power is supplied from the main battery 10 to the sub-battery 20 to charge it, the fuel efficiency does not deteriorate.

[0032] The charging control unit 101 adjusts the charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained while the vehicle is in a drivable state. In other words, the charging control unit 101 continuously charges the sub-battery 20 while the vehicle is in a drivable state. During this time, the charging control unit 101 adjusts the charging voltage within a reference voltage range so that the charging current is minimized. That is, the charging control unit 101 searches for the charging voltage at which the charging current is minimized while changing the charging voltage within the reference voltage range, and changes the charging voltage so that the charging current is continuously minimized. Note that the charging voltage at which the charging current is minimized depends on the charge rate, battery temperature, etc., but these charge rate, battery temperature, etc. change while the sub-battery 20 is being charged. Therefore, the charging control unit 101 needs to change the charging voltage in order to maintain the minimum charging current.

[0033] The method by which the charging control unit 101 applies the target charging voltage to the sub-battery 20 can be adjusted as appropriate. For example, the charging control unit 101 may cause the DC / DC converter 30 to output an output voltage slightly higher than the target charging voltage. If the charging voltage detected by the voltage sensor 200 is lower than the target charging voltage, the charging control unit 101 slightly increases the output voltage of the DC / DC converter 30. On the other hand, if the detected charging voltage is higher than the target charging voltage, the charging control unit 101 slightly decreases the output voltage of the DC / DC converter 30. The charging control unit 101 repeats these processes until the detected charging voltage matches the target charging voltage.

[0034] The following describes how to determine the reference voltage and reference voltage range based on the measurement results of the relationship between charging voltage and charging current. Figure 3 is a graph showing the measurement results of the relationship between charging voltage and charging current when the sub-battery 20 is charged while gradually increasing the charging voltage until the sub-battery 20 is fully charged. In Figure 3, the charging current decreases from I1 to Imin while the charging voltage decreases from 13.8V to 14.1V, and increases from Imin to I2 while the charging current increases from 14.1V to 14.4V. In other words, in Figure 3, the charging current decreases until the charging voltage rises to 14.1V, and increases when the charging voltage exceeds 14.1V. In this example, the minimum point of the charging current is when the charging voltage is 14.1V, and the charging current is at its minimum when the charging voltage is 14.1V.

[0035] Thus, the reason why the charging current is minimal at a charging voltage of 14.1V when the charge level is close to 100% is thought to be as follows: In other words, in the range where the charging voltage exceeds 14.1V, the effect of the sub-battery 20 behaving like a resistor is significant, and the higher the charging voltage, the larger the charging current. On the other hand, in the range where the charging voltage is less than 14.1V, the effect of chemical reactions being more likely to occur at lower charging voltages is significant, and the lower the charging voltage, the larger the charging current.

[0036] When the measurement results shown in Figure 3 are obtained, the reference voltage is set to 14.1V, and the reference voltage range is set to a range of 13.8V to 14.4V, which is 0.3V above and below the reference voltage. Note that the width of the reference voltage range is not limited to 0.3V above and below the reference voltage. In other words, the width of the reference voltage range may be less than 0.6V or greater than 0.6V.

[0037] Next, the charging control process performed by the control device 100 when the vehicle is unable to drive will be described with reference to the flowchart in Figure 4. The charging control process when the vehicle is unable to drive is a charging control process that is performed when the vehicle is unable to drive. For example, the charging control process when the vehicle is unable to drive is performed when the vehicle's state changes from a drivable state to an immobile state.

[0038] First, the control unit 110 of the control device 100 calculates the charge rate (step S101). For example, the control unit 110 stops the output of the DC / DC converter 30 and waits until the sub-battery 20 stabilizes. When the output of the DC / DC converter 30 is stopped, each terminal of the sub-battery 20 becomes open. For example, the control unit 110 considers the sub-battery 20 to be stable when a sufficient amount of time has elapsed since the sub-battery 20 was charged and the battery temperature has dropped to around room temperature. When the control unit 110 detects that the sub-battery 20 has stabilized, it detects the open-circuit voltage of the sub-battery 20 and determines the charge rate from the open-circuit voltage and the charge rate calculation information. The control unit 110 may also determine the current charge rate by adding the increase in the charge rate to the past charge rate and subtracting the decrease in the charge rate.

[0039] Next, the control unit 110 determines whether the charge level is below the lower limit charge level (step S102). If the control unit 110 determines that the charge level is not below the lower limit charge level (step S102: NO), it returns to step S101. If the control unit 110 determines that the charge level is below the lower limit charge level (step S102: YES), it starts the output of the DC / DC converter 30 (step S103). For example, the control unit 110 sends a control signal to the DC / DC converter 30 and makes the output terminal of the DC / DC converter 30 conductive.

[0040] Next, the control unit 110 adjusts the charging voltage to a specified voltage (step S104). For example, the control unit 110 sends a control signal to the DC / DC converter 30 to adjust the voltage transformation ratio and sets the output voltage of the DC / DC converter 30 to a voltage slightly higher than the specified voltage. The control unit 110 adjusts the output voltage of the DC / DC converter 30 so that the detected charging voltage matches the specified voltage. The specified voltage is the charging voltage when the vehicle is in a non-running state, and may be, for example, a previously determined reference voltage.

[0041] Next, the control unit 110 calculates the charge rate (step S105). For example, the control unit 110 obtains the current charge rate by adding the increase in the charge rate and subtracting the decrease in the charge rate to the charge rate calculated in step S101. Next, the control unit 110 determines whether the charge rate has reached the upper limit charge rate (step S106). If the control unit 110 determines that the charge rate has not reached the upper limit charge rate (step S106: NO), it returns to step S105.

[0042] When the control unit 110 determines that the charge rate has reached the upper limit charge rate (step S106: YES), it stops the output of the DC / DC converter 30 (step S107). For example, the control unit 110 sends a control signal to the DC / DC converter 30, opening the output terminal of the DC / DC converter 30. After completing the process in step S107, the control unit 110 returns to step S101. In this way, in the charging control process when the vehicle is unable to drive, the sub-battery 20 is charged when its charge rate falls below the lower limit charge rate.

[0043] Next, the charging control process performed by the control device 100 when the vehicle is ready to drive will be explained with reference to the flowchart in Figure 5. The charging control process when the vehicle is ready to drive is a charging control process that is performed when the vehicle is in a state where it is ready to drive. For example, the charging control process when the vehicle is ready to drive is performed when the vehicle's state switches from a state where it is not ready to drive to a state where it is ready to drive.

[0044] First, the control unit 110 executes a reference voltage determination process (step S201). The reference voltage determination process will be described with reference to FIG. 6. The reference voltage determination process is a process of determining a reference voltage that is an estimated value of the charging voltage that minimizes the charging current.

[0045] First, the control unit 110 starts the output of the DC / DC converter 30 (step S301). Next, the control unit 110 adjusts the charging voltage to the start voltage (step S302). For example, the control unit 110 transmits a control signal to the DC / DC converter 30 to adjust the transformation ratio, and sets the output voltage of the DC / DC converter 30 to a voltage slightly higher than the start voltage. The control unit 110 adjusts the output voltage of the DC / DC converter 30 so that the detected charging voltage matches the start voltage. The start voltage is, for example, a voltage that is several volts lower than the estimated value of the reference voltage.

[0046] Next, the control unit 110 identifies the charging current (step S303). For example, the control unit 110 acquires charging current information from the current sensor 300 and identifies the charging current indicated by the charging current information. Next, the control unit 110 associates and records the charging voltage and the charging current (step S304). For example, the control unit 110 adds information indicating a set of the current charging voltage and the current charging current to the map information stored in the storage unit 120. The map information includes a plurality of pieces of information indicating sets of charging voltages and charging currents.

[0047] Next, the control unit 110 determines whether the charging voltage has reached the end voltage (step S305). The end voltage is, for example, a voltage that is several volts higher than the estimated value of the reference voltage. When the control unit 110 determines that the charging voltage has not reached the end voltage (step S305: NO), the control unit 110 raises the charging voltage by one step (step S306). For example, the control unit 110 raises the output voltage of the DC / DC converter 30 by about 0.1 V so that the charging voltage rises by 0.1 V. When the control unit 110 completes the process of step S306, the control unit 110 returns the process to step S303.

[0048] When the control unit 110 determines that the charging voltage has reached the end voltage (step S305: YES), it determines the charging voltage corresponding to the minimum charging current as the reference voltage (step S307). For example, the control unit 110 identifies the set including the minimum charging current in the map information stored in the storage unit 120, and determines the charging voltage included in the identified set as the reference voltage.

[0049] Next, the control unit 110 determines a reference voltage range based on the reference voltage (step S308). For example, the control unit 110 determines the range from a voltage 0.3 V lower than the reference voltage to a voltage 0.3 V higher than the reference voltage as the reference voltage range. When the control unit 110 completes the process of step S308, it completes the reference voltage determination process. Thus, in the reference voltage determination process, the charging current is specified for each charging voltage, and the charging voltage corresponding to the minimum charging current is determined as the reference voltage.

[0050] When the control unit 110 completes the reference voltage determination process of step S201, it adjusts the charging voltage to the reference voltage (step S202). For example, the control unit 110 transmits a control signal to the DC / DC converter 30 to adjust the transformation ratio, and sets the output voltage of the DC / DC converter 30 to a voltage slightly higher than the reference voltage. The control unit 110 adjusts the output voltage of the DC / DC converter 30 so that the detected charging voltage matches the reference voltage.

[0051] Next, the control unit 110 raises the charging voltage by one step (step S203). For example, the control unit 110 raises the output voltage of the DC / DC converter 30 by about 0. V so that the charging voltage rises by IV. Next, the control unit 110 specifies the charging current (step S204). Next, the control unit 110 determines whether the charging voltage has reached the upper limit voltage (step S205). The upper limit voltage is the voltage at the upper limit of the reference voltage range.

[0052] If the control unit 110 determines that the charging voltage has not reached the upper limit voltage (step S205: NO), it determines whether the charging current has decreased (step S206). In other words, the control unit 110 determines whether the charging current has decreased due to the increase in the charging voltage. If the control unit 110 determines that the charging current has decreased (step S206: YES), it returns to step S203 and further increases the charging voltage.

[0053] If the control unit 110 determines that the charging voltage has reached the upper limit voltage (step S205: YES), or if it determines that the charging current has not decreased (step S206: NO), it lowers the charging voltage by one step (step S207). For example, the control unit 110 lowers the output voltage of the DC / DC converter 30 by about 0.1V so that the charging voltage decreases by 0.1V. Next, the control unit 110 determines the charging current (step S208). Next, the control unit 110 determines whether or not the charging voltage has reached the lower limit voltage (step S209). The lower limit voltage is the lower limit voltage of the reference voltage range.

[0054] If the control unit 110 determines that the charging voltage has not reached the lower limit voltage (step S209: NO), it determines whether the charging current has decreased (step S210). In other words, the control unit 110 determines whether the charging current has decreased as a result of lowering the charging voltage. If the control unit 110 determines that the charging current has decreased (step S210: YES), it returns to step S207 and further lowers the charging voltage.

[0055] If the control unit 110 determines that the charging voltage has reached the lower limit voltage (step S209: YES), or if it determines that the charging current has not decreased (step S210: NO), it returns to step S203. In this way, in the charging control process when the vehicle is ready to drive, the charging voltage is adjusted within the reference voltage range so that the charging current is minimized, and the sub-battery 20 continues to be charged with the minimum charging current.

[0056] In this embodiment, while the vehicle is in a non-running state, the sub-battery 20 is maintained in a near-fully charged state, and while the vehicle is in a drivable state, charging is maintained at the minimum charging current. In this embodiment, since the sub-battery 20 is maintained in a near-fully charged state regardless of the vehicle's state, deterioration of the sub-battery 20 is suppressed. Also, in this embodiment, since the sub-battery 20 is charged in a near-fully charged state, excessively large charging currents are suppressed. For example, even if an excessively high charging voltage is applied to the sub-battery 20, only a small charging current will flow because the sub-battery 20 has little charging capacity. Furthermore, by suppressing the charging current while the vehicle is in a drivable state, the deterioration of energy efficiency is reduced. In addition, by suppressing the charging current, deterioration of the sub-battery 20 due to Joule heat generated by the charging current is also suppressed.

[0057] Furthermore, in this embodiment, the reference voltage determination process is executed when the vehicle's state changes from a non-drivable state to a drivable state. Therefore, according to this embodiment, the minimum charging current is searched for within an appropriate reference voltage range corresponding to the charge level of the sub-battery 20, battery temperature, etc., and the charging current is appropriately suppressed.

[0058] (Embodiment 2) Embodiment 1 described an example in which the reference voltage determination process is executed when the state of the vehicle changes from a non-running state to a drivable state. This embodiment describes an example in which the reference voltage determination process is executed again when a specified time has elapsed since the first execution of the reference voltage determination process. Note that the same configurations and functions as in Embodiment 1 will be omitted or simplified as appropriate.

[0059] Referring to Figure 7, the reference voltage re-determination process will be explained. The reference voltage re-determination process is a process that executes the reference voltage determination process described in Embodiment 1 again. The reference voltage re-determination process is executed, for example, immediately before the process of step S203 in the charging control process when the vehicle is ready to drive shown in Figure 5 of Embodiment 1 (that is, immediately after steps 202, S206: YES, S209: YES, and S210: NO).

[0060] First, the control unit 110 determines whether a specified time has elapsed since the previous reference voltage determination process was executed (step S401). If the control unit 110 determines that a specified time has elapsed since the previous reference voltage determination process was executed (step S401: YES), it executes the reference voltage determination process (step S402). This reference voltage determination process is as described in Embodiment 1.

[0061] Next, the control unit 110 adjusts the charging voltage to the reference voltage (step S403). For example, the control unit 110 sends a control signal to the DC / DC converter 30 to adjust the voltage transformation ratio and sets the output voltage of the DC / DC converter 30 to a voltage slightly higher than the newly determined reference voltage. The control unit 110 adjusts the output voltage of the DC / DC converter 30 so that the detected charging voltage matches the newly determined reference voltage.

[0062] If the control unit 110 determines that a specified time has not elapsed since the previous execution of the reference voltage determination process (step S401: NO), or if it has completed the process in step S403, it completes the reference voltage re-determination process and executes the processes from step S203 onward in the drivable charging control process. In this way, the reference voltage re-determination process is executed every specified time while the vehicle is in a drivable state.

[0063] In this embodiment, the reference voltage determination process is executed again when a specified time has elapsed since the initial execution of the reference voltage determination process. While the vehicle is in a drivable state, the battery temperature may change, and the optimal voltage for the reference voltage may change. According to this embodiment, even if the battery temperature changes, the minimum charging current is searched for within an appropriate reference voltage range corresponding to the battery temperature, thus appropriately suppressing the charging current.

[0064] (Embodiment 3) Embodiment 1 described an example in which the reference voltage is determined based on the measurement results of the charging current. This embodiment describes an example in which a new reference voltage is determined based on a previously determined reference voltage. Note that the same configurations and functions as in Embodiments 1 and 2 will be omitted or simplified as appropriate.

[0065] In this embodiment, when the charge control unit 101 performs a reference voltage determination process at the first timing, it determines the reference voltage at the second timing based on the reference voltage determined in the reference voltage determination process, the battery temperature at the first timing, and the battery temperature at the second timing, which is later than the first timing. In other words, the charge control unit 101 determines the current reference voltage based on past reference voltages, past battery temperatures, and the current battery temperature. The reason for determining the reference voltage while considering the battery temperature is that the optimal voltage as a reference voltage depends on the battery temperature. More specifically, the lower the battery temperature, the less likely the electrolyte in the sub-battery 20 is to undergo a chemical reaction, so a higher voltage is preferable as a reference voltage.

[0066] Referring to Figure 8, the simplified reference voltage determination process will be explained. The simplified reference voltage determination process is a process that simply determines a new reference voltage from past reference voltages without measuring the charging current. The simplified reference voltage determination process is performed, for example, in place of the reference voltage determination process shown in Figure 6 of Embodiment 1.

[0067] First, the control unit 110 identifies a past reference voltage (step S501). For example, the control unit 110 identifies the reference voltage determined in the last executed reference voltage determination process. In this embodiment, it is assumed that information indicating past reference voltages is stored in the storage unit 120. The past reference voltage is, for example, a reference voltage determined in the past by the reference voltage determination process shown in Figure 6. Next, the control unit 110 identifies a past battery temperature (step S502). For example, the control unit 110 identifies the battery temperature when the last reference voltage determination process was executed. In this embodiment, it is assumed that information indicating past battery temperatures is stored in the storage unit 120.

[0068] Next, the control unit 110 determines the current battery temperature (step S503). For example, the control unit 110 acquires temperature information from the temperature sensor 21 and determines the battery temperature indicated by the temperature information. Next, the control unit 110 determines the current reference voltage from the determined reference temperature and battery temperature (step S504). In other words, the control unit 110 determines the current reference voltage from past reference voltages, past battery temperatures, and the current battery temperature. For example, if the optimal voltage as the reference voltage increases by 0.1V for every 1°C decrease in battery temperature, and the current battery temperature is 2°C lower than the past temperature of the sub-battery 20, then the control unit 110 determines a voltage 0.2V higher than the past reference voltage as the current reference voltage.

[0069] Next, the control unit 110 determines the current reference voltage range based on the current reference voltage (step S505). For example, the control unit 110 determines the current reference voltage range to be a voltage from 0.3V lower than the current reference voltage to a voltage 0.3V higher than the current reference voltage. Once the control unit 110 completes the process in step S505, it completes the simplified reference voltage determination process.

[0070] In this embodiment, the current reference voltage is determined based on past reference voltage, past battery temperature, and current battery temperature. In this embodiment, an appropriate reference voltage is determined without performing the process of measuring the charging current while adjusting the charging voltage. Therefore, according to this embodiment, an appropriate reference voltage can be easily determined.

[0071] (Modifications) Although embodiments have been described above, various modifications and applications are possible. It is arbitrary which parts of the configuration, function, and operation described in the above embodiments are adopted. In addition to the configuration, function, and operation described above, further configurations, functions, and operations may be adopted. Furthermore, the configurations, functions, and operations described in the above embodiments can be freely combined.

[0072] In the embodiment, the control unit 110 functioned as the respective parts shown in Figure 2 by the CPU executing a program stored in the ROM or storage unit 120. However, in this disclosure, the control unit 110 may be dedicated hardware. Dedicated hardware includes, for example, a single circuit, a composite circuit, a programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof. If the control unit 110 is dedicated hardware, each function of each part may be realized by separate hardware, or the functions of each part may be realized together by a single piece of hardware. Furthermore, some of the functions of each part may be realized by dedicated hardware, and other parts by software or firmware. In this way, the control unit 110 can realize the above-mentioned functions by hardware, software, firmware, or a combination thereof.

[0073] This invention allows for various embodiments and modifications without departing from the broad spirit and scope of the invention. Furthermore, the embodiments described above are for illustrative purposes only and do not limit the scope of the invention. In other words, the scope of the invention is indicated not by the embodiments, but by the claims. Various modifications made within the scope of the claims and the equivalent scope of the meaning of the invention are considered to be within the scope of the invention.

[0074] 10 Main battery, 20 Sub-battery, 21 Temperature sensor, 30 DC / DC converter, 40 Motor, 50 Load, 100 Control device, 101 Charging control unit, 102 Voltage information acquisition unit, 103 Current information acquisition unit, 104 Charge rate calculation unit, 105 Temperature information acquisition unit, 110 Control unit, 120 Memory unit, 130 Input / Output unit, 200 Voltage sensor, 300 Current sensor, 1000 Charging control system, 2000 In-vehicle system.

Claims

1. A charging control method comprising: controlling a voltage converter that transforms the power supplied from a main battery that supplies power to a motor that drives a vehicle and supplies it to a sub-battery including a lead-acid battery to charge the sub-battery; measuring the charging voltage applied to the sub-battery by the voltage converter when the sub-battery is being charged; measuring the charging current flowing from the voltage converter to the sub-battery when the sub-battery is being charged; calculating the charge level of the sub-battery; charging the sub-battery until the charge level reaches the upper limit charge level corresponding to full charge when the vehicle is in a state where it cannot be driven; and adjusting the charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained when the vehicle is in a state where it can be driven.

2. The charging control method according to claim 1, wherein when the state of the vehicle changes from the inoperable state to the drivable state, the charging voltage is gradually increased while measuring the charging current, and a reference voltage determination process is performed in which the charging voltage when the minimum charging current is measured is determined to be the reference voltage.

3. The charging control method according to claim 2, wherein the reference voltage determination process is executed again when a specified time has elapsed since the execution of the reference voltage determination process.

4. The charging control method according to claim 2 or 3, wherein, when the reference voltage determination process is performed at the first timing, the reference voltage at the second timing is determined based on the reference voltage determined in the reference voltage determination process, the temperature of the sub-battery at the first timing, and the temperature of the sub-battery at the second timing which is later than the first timing.

5. A charging control system comprising: a control device that controls a voltage converter that transforms power supplied from a main battery that supplies power to a motor that drives a vehicle and supplies it to a sub-battery including a lead-acid battery to charge the sub-battery; a voltage sensor that measures the charging voltage applied to the sub-battery by the voltage converter when the sub-battery is being charged; and a current sensor that measures the charging current flowing from the voltage converter to the sub-battery when the sub-battery is being charged, wherein the control device calculates the charge level of the sub-battery; the control device charges the sub-battery until the charge level reaches the upper limit charge level corresponding to full charge when the vehicle is in a state where it cannot be driven and the charge level has fallen to the lower limit charge level; and the control device adjusts the charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained when the vehicle is in a state where it can be driven.

6. A program that causes a computer to function as a charging control means for charging a sub-battery, including a lead-acid battery, by controlling a voltage converter that transforms power supplied from a main battery that supplies power to a motor that drives a vehicle and supplies it to a sub-battery. The voltage information acquisition means acquires charging voltage information indicating the charging voltage applied to the sub-battery by the voltage converter when the sub-battery is being charged. The current information acquisition means acquires charging current information indicating the charging current flowing from the voltage converter to the sub-battery when the sub-battery is being charged. The charge rate calculation means calculates the charge rate of the sub-battery. The charging control means controls the voltage converter to charge the sub-battery until the charge rate reaches the upper limit charge rate corresponding to full charge, when the vehicle is in a state where it cannot be driven, if the charge rate of the sub-battery falls to the lower limit charge rate. The charging control means adjusts the charging voltage within a reference voltage range based on a reference voltage so that charging with the minimum charging current is maintained when the vehicle is in a state where it can be driven.

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