Assist device for use in electric vehicle

Abstract: A storage battery charge capacity measuring device for use in an electric vehicle (1) which determines charge capacity based on the terminal voltage of a storage battery (14) measured upon completion of a charging operation, said battery capacity measuring device comprising: means for measuring the terminal voltage across the storage battery (14) at predetermined intervals and determining whether the storage battery (14) is in a surface charge state based on a variation in the measured terminal voltages; means (71) for allowing the storage battery to discharge on a light load when the storage battery (14) is in a surface charge state; and means for determining charge capacity based on the terminal voltage when the surface charge state is eliminated.

Description: The present invention relates to an assist device for use in an electric vehicle, and in particular to the assist device that provides a wide range of ease-of-use design improvements in the electric vehicle. Electric vehicles that use an electric motor as a prime mover recently draw attention as a next generation vehicle that is expected to replace a gasoline-powered vehicle employing an internal combustion engine. A diversity of technical proposals have been made in connection with the electric vehicle. The electric vehicle using a environmentally clean electrical energy solves entirely environmental problems associated with internal combustion engines, such as emissions of toxic exhaust gases and noise. It is estimated that 70% of air pollutants is given off by internal combustion engines. Also, it is believed that the full-scale introduction of electric vehicles effectively slows the current consumption rate of fossil fuels such as petroleum effectively doubling the currently expected remaining availability period of fossil fuels. As in the gasoline-powered vehicle, the electric vehicle is provided with running wheels suspended at shock absorbers by the vehicle body. The running wheels are driven by a power transmission device, the power source of which is an electric motor. The electric motor is power supplied by an electric power device. The electric power device is made up of a battery bank section having a plurality of storage batteries, a power supply circuit section for supplying electric power in a reliable manner, an electric motor for driving, a motor driving circuit, and a control circuit for issuing speed instructions or the like to the motor driving circuit. Driving power generated by the motor is then transmitted via the power transmission device to the running wheels to allow the electric vehicle to run. The battery bank section that feeds electric power to the running motor is constructed of the plurality of storage batteries connected in series to obtain a required voltage. The storage batteries have the following characteristics: the voltage across the battery terminals gradually drops with time elapse when the battery is used or discharged, as plotted by a discharge curve in FIG. 5. As soon as the battery voltage reaches a termination voltage at the rightmost end of the curve in FIG. 5, discharge is terminated to protect the storage battery. As FIG. 6 illustrates, the storage battery follows different discharge curves depending on the current in use. It is known that the operating discharge time of the battery varies as its operating current varies. For this reason, the electric vehicle is provided with a meter, indicative of power remaining unused, employing a diversity of techniques, to correctly know electrical energy remaining unused in the storage battery. The remaining capacity meter estimates electric power remaining and indicates it to a driver. As shown in FIG. 7, electric power from a battery bank section 91 is voltage-regulated by a power supply circuit section 92, and is distributed via unshown power lines to a diversity of devices including a remaining capacity meter 93. The main switch 94 to be operated by the driver is connected to the power supply circuit section 92. By operating the main switch 94, the power supply circuit section 92 is switched on or off. When the vehicle comes to a complete halt to park it, the key switch 94 is switched off cutting battery power to all devices on board including the remaining capacity meter to stop the operation of each device. The main key is detached from its switch and kept in the hands of the driver to prevent the vehicle from being stolen. Electric power regulated by the power supply circuit section 92 is fed to the motor via the motor driving circuit. The motor driving circuit in its chopper controlling mode controls the speed of the motor by raising or lowering the supply voltage to the motor. The chopper control duty ratio that raises or lowers voltage is controlled by the control circuit. The control circuit is electrically connected to an accelerator grip. The control circuit sets a duty ratio in response to the driver's opening setting of the accelerator. In response to the duty factor, the motor driving circuit raises or lowers the RMS voltage fed to the motor. The speed of the motor according to the opening of the accelerator is thus obtained. To charge a consumed storage battery, it is charged with a charging voltage that is typically higher than its nominal voltage. For example, for a 12 V storage battery, approximately 15 V is fed to the battery to charge it. The capacity of the battery is accurately determined based on the fact that the terminal voltage of the battery varies linearly with battery capacity when an open-circuit voltage is measured with the battery set for no load state upon completion of a charging operation. When the charging operation is completed, however, the terminal voltage rises temporarily (this state is hereinafter referred to as "surface charge state") partly because the charging voltage is higher than the nominal voltage and partly because the battery interior remains electrochemically nonuniform at this moment. Terminal voltage measurement provides no accurate measurement of the battery capacity, therefore. For example, a 12 V storage battery is charged with a charging voltage of approximately 15 V. Immediately after the charging operation, the open-circuit voltage stays in the vicinity of 15 V regardless of actual capacity of the battery. Even when the charging operation is stopped before being fully charged, the battery may be mistakenly considered as fully charged because the measured terminal voltage is high. It is known that as time elapses, such a surface charge state indicating a high voltage is eliminated and the terminal voltage reaches to its equilibrium one reflecting actual charged battery capacity. In one of known methods of determining charged battery capacity immediately after charging, a regression equation that relates elapsed time after the completion of charging to open-circuit voltage is determined, the regression equation is used to estimate the open-circuit voltage at the equilibrium state after a sufficient duration of time has elapsed, and charged capacity is then calculated from the estimated open-circuit voltage (for example, Japanese Patent Application Hei-4-363679). In the above described method in which charged battery capacity immediately after charging is determined using the regression equation from the terminal voltage, charge and discharge characteristics are different from battery to battery in terms of capacity and degree of ageing. Therefore, tests should be beforehand conducted to collect characteristics data. Such extra job is time consuming. US 4520353 discloses a state of charge indicator for a battery selectively connected and disconnected from an external load. When the battery is disconnected from the external load, a test load is connected to the battery for a predetermined time to draw a current pulse therefrom. The voltage of the battery is measured at the same time. The measured voltage is then converted to a digital signal, which is stored and subsequently transmitted to a visual display device. In view of the above problems, the present invention has been developed. It is an object of the present invention to provide a battery charge capacity measuring device which determines accurately charge capacity by eliminating surface charge state upon a charging operation. According to the present invention there is provided a storage battery charge capacity measuring device for use in an electric vehicle which determines charge capacity based on the terminal voltage of a storage battery measured upon completion of a charging operation, said battery capacity measuring device comprising means for measuring the terminal voltage across the storage battery at predetermined intervals, characterised in that: said means for measuring is adapted for determining whether the storage battery is in a surface charge state based on a variation in the measured terminal voltages, and by further comprising: means for allowing the storage battery to discharge on a light load when the storage battery is in a surface charge state; and means for determining charge capacity based on the terminal voltage when the surface charge state is eliminated. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a side view showing generally the electric motorcycle according to one embodiment of the electric vehicle of the present invention. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1. FIG. 3 is a flow chart of the control program of one embodiment of battery charge capacity measuring device according to the electric vehicle of the present invention. FIG. 4 is a schematic diagram of a discharge circuit used in the battery charge capacity measuring device in the above embodiment. FIG. 5 is a graph showing the discharge characteristics at constant current of a typical storage battery. FIG. 6 is a graph showing the relationship between battery discharge current and discharge capacity. FIG. 7 is a block diagram of the prior art battery bank and power supply circuit. Referring now to FIGS. 1 through 4, an embodiment of the present invention is now discussed. The electric vehicle in the embodiment is illustrated using a two-wheel electric motorcycle (including an electric scooter) as an example. The embodiment uses the identical basic construction of the electric motorcycle. As in the known internal-combustion-engine driven motorcycle, an electric scooter 1 has on the front and rear sides of a vehicle body 2 running wheels 4, 5 suspended by a body main frame 3 and a driver's seat 2a on the approximately middle position of the vehicle body 2. The front wheel 4 is steered by a handle 6. The rear wheel 5 is driven by a running electric motor rather than by an engine. A display panel 8 disposed in the middle of the handle 6 has an indicator device 9 for displaying a cruising speed, battery remaining capacity and the like and a plurality of switches for controlling on-board devices. Disposed below the display panel 8 on the main body 2 is a main switch 10 for turning on or off all on-board devices. The main frame 3 carries an electric power device 11 for supplying electric power required for running, a motor that converts electric power from the electric power device 11 into mechanical force, and a power transmission device 12 for transmitting the mechanical force to the rear wheel 5. Accelerator grip 6a, a brake lever, a brake mechanism, suspension systems and some other unshown components remain unchanged from those used in the prior are two-wheel motorcycle. A diversity of switches including the accelerator grip 6a, the brake lever, the main switch 10 and the like are electrically connected to the electric power device 11. The electric power device 11 is made up of a battery bank section 14 disposed in an approximately middle lower position of the main frame 3 of the vehicle body 2, a remaining capacity meter 15, a power supply circuit 16, a control circuit 17, a charger 18 and a motor driving circuit 19 disposed beside the battery bank section 14, and a diversity of sensors disposed in the on-board devices. The battery bank section 14 comprises four batteries 14a secured onto the main frame by brackets. These batteries 14a are connected in series by heavy wire gauge cables to avoid voltage drop and to obtain a required voltage. Electric power from the battery bank section 14 is regulated by the power supply circuit 16 before it is distributed to each of the on-board devices. The motor driving circuit 19 is a circuit chiefly constructed of large-sized MOS FETs as high-power switching elements. Switching operation of the FET circuit in the form of chopper control raises or lowers the RMS voltage supplied to the running motor to control the motor speed. Switching operation is performed according to speed instruction from the control circuit 17 in response to accelerator setting. The control circuit 17 comprises a microcomputer that receives signals from the accelerator grip 6a and sensors mounted in the on-board devices and outputs signals to the motor driving circuit 19 and the indicator device 9 on the display panel 8. The microcomputer is provided with an A/D converter for converting an input signal into a digital signal, an I/O port, a CPU, and memory. Under the control of the program stored in the memory, the microcomputer processes sensor signals including the one regarding accelerator setting set by the operation of the accelerator grip 6a, and signals from other sensors, and issues required instructions such as a duty setting signal to the motor driving circuit 19. The power transmission device 12 is made up of the running motor and a continuously variable transmission 20. The power transmission device 12 converts a rotary motion the motor generates into an appropriate torque by means of the continuously variable transmission 20 and transmits the torque to the rear wheel 5. When the driver turns on the main switch 10, the power supply circuit 16 starts to operate, distributing electric power from the battery bank section 14 to the on-board devices and making the vehicle ready to start. Next, when the driver operates the accelerator grip 6a, the control circuit 17 in response issues a speed instruction to the motor driving circuit 19. In response to the speed instruction, the motor driving circuit 19 raises or lowers the driving power fed to the running motor to control the speed of the running motor. The rotary motion generated by the running motor is converted into an appropriate torque which is then transmitted to the rear wheel 5. The two-wheel electric scooter 1 thus starts to run at a speed the driver desires. Next, the battery charge capacity measuring device of the electric vehicle according to the present invention is discussed referring to FIGS. 3 and 4. The charge capacity measuring device eliminates the surface charge that indicates a voltage different from the normal terminal voltage, for a predetermined duration of time after a charging operation is completed, and allows the correct capacity of the batteries to be measured. Namely, the batteries are charged with a charging voltage that is higher than the terminal voltage, the terminal voltage is measured at predetermined intervals after the charging operation is completed, and, if any variation between measured voltages is detected, a light load discharging is performed on the assumption that the batteries are in a surface charge state. This cycle is repeated until a stable voltage free from surface charge state is measured. When the surface charge state is eliminated, charge capacity is determined based on the measured voltage. The charge capacity measuring device comprises the battery bank section having the plurality of storage batteries, the remaining capacity meter for determining capacity from the open-circuit voltage of the battery bank section, and the control circuit connected to the remaining capacity meter. The charge capacity measuring device further comprises a discharge circuit 71 shown in FIG. 4 and a charge capacity measuring program that controls the operation of the discharge circuit 71 and the remaining capacity meter. The charge capacity measuring program uses the remaining capacity meter to determine whether or not the batteries are in a surface charge state. The program uses the discharge circuit 71 to eliminate the surface charge state, and uses the remaining capacity meter to determine the current charge capacity. The operation program of the charge capacity measuring device is discussed referring to the flow chart in FIG.3. In the flow chart in FIG. 3, the program for measuring charge capacity is organized in three main blocks: a first block (steps S1 through S4) for determining whether or not the batteries are in a state of surface charge, a second block (steps S5 through S7), in succession to the first block, for eliminating the surface charge by discharging the batteries on a light load when the batteries are found to be in a surface charge state, and a third block (step S8) for determining the charge capacity based on a stable terminal voltage after the surface charge is eliminated. At step S1, the terminal voltage is measured with the battery bank section in an open-circuit, no-load state. The measured value E1 is stored in memory means. At step S2, a predetermined duration Δt1 is time counted. When the predetermined duration Δt1 has elapsed, the process goes to step S3. At step S3, the terminal voltage is measured with no load on the batteries. The measured value E2 is stored in the memory means. At step S4, a determination is made of whether or not the batteries are in a surface charge state by comparing E1 and E2. Specifically, measured value E2 is subtracted from measured value E1, and the difference therebetween is then compared with ΔE that is error-compensated. When the difference is greater than ΔE, namely, when the terminal voltage suffers a variation greater than a predetermined rate of reduction even after the predetermined duration of Δt1, the batteries are found to be still in a surface charge state and not in equilibrium. On the other hand, when the difference is smaller, namely, when the terminal voltage indicates a marginal variation after the predetermined duration of Δt1, the batteries are thought to be in equilibrium with the surface charge state eliminated. When step S4 determines that the surface charge state presents, the process starts over with step S1 after a sequence of steps (steps S5 through S7) for eliminating the surface charge. A cycle of S1 through S7 is repeated until the surface charge is eliminated. When the surface charge is eliminated, the process goes to step S8, where the charge capacity of the batteries is determined. The block for eliminating the surface charge includes discharging the batteries on a light load for a duration to remove charge in the vicinity of the terminals of the batteries in order to eliminate the surface charge state. A discharge process is performed using the discharge circuit 71 in FIG. 4. The discharge circuit 71 comprises a semiconductor switching device 72 and a resistor 73 having a predetermined high resistance connected in a series connection. This series connection is connected in parallel with the output terminals of the battery bank section 14 having a plurality of batteries 14a. The switching operation of the switching device 72 allows the resistor 73 to discharge the batteries for a predetermined duration of time of Δt2. At step S5, the switching device 72 is turned on, and the battery bank section 14 starts discharging through the high resistor 73 by a small current I. At step S6, the predetermined duration of Δt2 is counted. When the predetermined duration of Δt2 has elapsed, the switching device 72 is turned off at step S7. The discharging of the storage batteries 14a is terminated. The process starts over with step S1 again. A sequence of steps for the determination of the surface charge and elimination of the surface charge (steps S1 through S7) is repeated until the surface charge state is eliminated. When the surface charge state is eliminated through the process of the determination and then elimination of the surface charge, the charge capacity of the batteries is finally determined at step S8. The measured open-circuit voltage E2 is thought of as a stable voltage with the batteries at equilibrium. The remaining capacity meter thus determines the charge capacity of the batteries from the measured value E2. As described above, according to the battery charge capacity measuring device of this embodiment, the post-charging terminal voltage, different from its normal terminal voltage, is measured at predetermined intervals for a predetermined duration of time after the completion of a charging operation, the presence of a surface charge state is determined from a variation in the measured terminal voltages, the surface charge state is eliminated by allowing the battery to discharge on a light load, and this determination/elimination process is repeated until the surface charge state is completely eliminated and reliable measured values are obtained. Thus, the surface charge state is eliminated in a short time and in a reliable manner. Furthermore, charge capacity is accurately determined. The terminal voltage is measured with the surface charge state eliminated, and then the charge capacity of the batteries is determined. Thus, any regression equation that expresses rough estimate of the characteristics of the batteries is not required. The principle of the invention can be applied to a diversity of storage batteries that suffer surface charge. The principle of this aspect of the present invention permits easy measurement and finds a wide range of applications. Even if the characteristics of the batteries are varied due to operating and ambient conditions, an accurate measurement of the charge capacity is performed because the capacity of the batteries is determined after the surface charge is eliminated. According to the present invention in connection with a battery charge capacity measuring device for use in an electric vehicle, the post-charging terminal voltage, different from its normal terminal voltage, is measured at predetermined intervals for a predetermined duration of time after the completion of a charging operation, the presence of a surface charge state is determined from a variation in the measured terminal voltages, the surface charge state is eliminated by allowing the battery to discharge on a light load, and this determination/elimination process is repeated until the surface charge state is completely eliminated and reliable measured values are obtained. Thus, the surface charge state is eliminated in a short time and in a reliable manner. Furthermore, charge capacity is accurately determined. According to the electric vehicle of the present invention, there is provided a battery charge capacity measuring device which determines accurately charge capacity by eliminating surface charge state upon a charging operation. As described above, the present invention provides an improved easy-to-operate feature in a wide range of operation of an electric vehicle. The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention as defined by the appended claims.

Claims: A storage battery charge capacity measuring device for use in an electric vehicle (1) which determines charge capacity based on the terminal voltage of a storage battery (14) measured upon completion of a charging operation, said battery capacity measuring device comprising means for measuring the terminal voltage across the storage battery (14) at predetermined intervals, characterised in that : said means for measuring is adapted for determining whether the storage battery (14) is in a surface charge state based on a variation in the measured terminal voltages, and by further comprising: means (71) for allowing the storage battery to discharge on a light load when the storage battery (14) is in a surface charge state; and means for determining charge capacity based on the terminal voltage when the surface charge state is eliminated. An electric vehicle (1) comprising one or more of the devices claimed in claim 1.

Inventors: KOIKE YOSHIKAZU, KOZAKI MINORU, TADOKORO KOHICHI, TAKAGI KUNIHIKO, KOIKE, YOSHIKAZU, KOZAKI, MINORU, TADOKORO, KOHICHI, TAKAGI, KUNIHIKO, Koike, Yoshikazu, c/o Seiko Epson Corporation, Kozaki, Minoru, c/o Seiko Epson Corporation, Tadokoro, Kohichi, c/o Tokyo R&amp;D Co. Ltd, Takagi, Kunihiko c/o Seiko Epson Corporation
Applicants: SEIKO EPSON CORP, TOKYO R & D KK, SEIKO EPSON CORPORATION, TOKYO R & D CO. LTD.
Application year: 1996
Main classification: B60L11
IPC codes: G01R31, B60L11, B62M23, B62M7, B60L15, B60L3