1.  Field of the Invention
The present invention relates to a technology for detecting a charge/discharge current flowing through a secondary battery mounted on an electrically-driven vehicle such as an electric vehicle (PEV) and a hybrid vehicle (HEV), an uninterruptible power supply system, a backup power source device, and the like. 
2.  Related Background Art
Recently, nickel-metal hydride (Nixe2x80x94MH) batteries mainly have been used as a main power source for driving a motor in an electric vehicle (PEV) and a so-called hybrid vehicle (HEV) provided with an engine and a motor, because of their high energy density (i.e., capable of storing energy compactly) and high output density.  In the PEV and HEV, an assembled battery made up of a combination of a plurality of single cells or unit cells is employed so as to supply a sufficient output to the motor. 
As for HEVs, in a case where an output from an engine is larger than the power required for driving the vehicle, then the surplus power is used for driving a generator so as to charge a secondary battery.  Conversely, in a case where an output from the engine is smaller, then the electrical power from the secondary battery is used for driving the motor so as to compensate a shortage of the power.  In the latter case, the secondary battery is discharged.  When mounting a secondary battery on a hybrid vehicle or the like, it is required to control such charge/discharge operations so as to maintain the appropriate operating conditions.  To this end, it becomes necessary that a residual battery capacity (i.e., State of Charge (SOC)) is estimated and an SOC control is conducted so as to optimize the fuel efficiency of the vehicle. 
One of parameters for estimating the SOC of a battery is a charge/discharge current in the battery.  Therefore, in order to perform the SOC control securely, it is required to use a current sensor that can detect a charge/discharge current in the battery accurately. 
Conventionally, as the current sensor used in the HEV or the like, an insulated type current sensor employing a Hall element has been known generally for the purpose of preventing electric leakage.  For example, JP 5(1993)-297026A discloses a current sensor including a Hall element, a core provided with a winding, and an electronic circuit.  However, this current sensor has the following problems:
(a) the core is provided with a winding, and many windings are required, which makes miniaturization of the current sensor difficult.  Especially in the case of the HEV where a principal current is large and a ratio of a detected current at the side of the electronic circuit should be made relatively small, the number of turns of the wiring becomes large, which further makes the miniaturization difficult and leads to an increase in the cost.  On the other hand, a wiring with a smaller wire diameter is used for the purpose of miniaturization, which would lead to a temperature rise and fraying of the wiring.  Consequently, the reliability of the current sensor would deteriorate;
(b) an offset error in the detected current due to the temperature characteristics of the Hall element would occur;
(c) a circuit is required for not only at the side of receiving the detected current but also at the side of the current sensor, which doubles the number of the components such as a circuit board and therefore is uneconomical; and
(d) when an electrical wiring to the current sensor has failed, a judgment cannot be made as to whether the situation applies to the case where a current does not flow through the wiring or the case where the wiring is broken. 
To cope with the above problem (b), JP10(1998)-177926A discloses a current sensor including not a Hall element but a detection coil, an exciting coil, and a core.  However, this current sensor does not solve the above problems (a) and (d). 
Then, as an alternative for the insulated type current sensor that requires a core provided with a wiring, a current sensor in a shunt resistor method is known, which detects a current flowing through the shunt resistor as a voltage. 
For example, JP11(1999)-308701A discloses a battery indicator for electrically-driven vehicles, including a shunt resistor type current sensor within it, and JP5(1993)-66250A discloses an electricity quantity totaling device that detects a charge/discharge electricity quantity of the secondary battery as a pulse using a current detection resistor (shunt resistor), totals this pulse with a counter to store the result, and indicates a residual battery capacity. 
The battery indicator described in the above JP11(1999)-308701A is equipped with a shunt resistor within it, as illustrated in this publication.  Generally, a driving output of the HEV or the like is large, and therefore a current flowing through the battery becomes large.  With the increase in the current, the amount of heat generated in the shunt resistor increases.  Therefore, a design to make the permissible loss acceptable makes the shunt resistor large and makes a wire diameter of an electric wiring to the shunt resistor also large.  As a consequence, the battery indicator including the shunt resistor and the electric wiring within it becomes large, which would increase the cost and would cause deterioration in the fuel efficiency because of the increase in the weight of the vehicle. 
In addition, since the detected current value is large, the resistance value of the shunt resistor should be rather small in order to detect such a large amount of current as a voltage suitable for processing it in a circuit.  Therefore, without an expensive high-precision shunt resistor with superior temperature characteristics, an offset error would occur in the detected current. 
Further, in the electrically-driven vehicles, a switching current of an inverter flows through the shunt resistor.  This switching current has a large amount of high frequency component, which would become a noise source.  Therefore, if the shunt resistor is arranged adjacent to a microcomputer and other electronic circuits, then there is the fear of generating a malfunction in the circuit due to the noise, and the reliability would deteriorate. 
The electricity quantity totaling device described in the above JP5(1993)-66250A is mounted on mobile equipment such as a camera-integrated video, a mobile phone, a personal computer, and a word processor, and realizes a function of indicating a residual capacity of a secondary battery.  Therefore, in contrast to the secondary batteries mounted on the HEV or the like, the detected current is small, and naturally the shunt resistor is included within the device together with other electronic components.  Consequently, this publication does not address the problem in detecting a large amount of current flowing through a secondary battery mounted on the HEV or the like as stated above. 
Therefore, with the foregoing in mind, it is an object of the present invention to provide a battery power source device provided with a system capable of detecting a charge/discharge current flowing through a secondary battery mounted on an electric vehicle, an uninterruptible power supply system, a backup power source device, and the like at low cost, with high reliability, and with high precision. 
To fulfill the above-stated object, a battery power source device according to the present invention is a battery power source device to which electrical power is supplied from an assembled battery including a combination of a plurality of single cells or unit cells, each of which is a secondary battery, and includes: a current detection resistor connected to the assembled battery in series; transmission means for transmitting a voltage signal generated across the current detection resistor; and a control device separated from the assembled battery, the current detection resistor, and the transmission means.  The control device includes: a discharge current detection unit that receives the voltage signal transmitted via the transmission means and outputs a current signal in proportion to a discharge current flowing through the current detection resistor at the time of discharge from the assembled battery; a first capacitor that totals the current signals output from the discharge current detection unit; a pulse generation unit during discharge that, when a voltage of the first capacitor increases to a first threshold value, reverses an output signal and makes the first capacitor discharge, and when the voltage of the first capacitor decreases to a second threshold value, reverses the output signal again to generate a pulse and stops the first capacitor discharging; a charge current detection unit that receives the voltage signal transmitted via the transmission means and outputs a current signal in proportion to a charge current flowing through the current detection resistor at the time of charge of the assembled battery; a second capacitor that totals current signals output from the charge current detection unit; a pulse generation unit during charge that, when a voltage of the second capacitor increases to a third threshold value, reverses an output signal and makes the second capacitor discharge, and when the voltage of the second capacitor decreases to a fourth threshold value, reverses the output signal again to generate a pulse and stops the second capacitor discharging; and a current detection unit that counts the number of pulses output from the pulse generation unit during discharge and the pulse generation unit during charge and calculates a current flowing through the assembled battery at the time of discharge and charge using the counted number of pulses. 
With this configuration, since the current detection resistor through which a switching current of a inverter, which includes a high frequency component that might become a noise source, flows is provided outside of the control device, malfunction of a microcomputer and other electronic circuits mounted inside of the control device can be prevented, whereby the reliability of the system can be improved. 
In the battery power source device according to the present invention, it is preferable that the current detection unit included in the control device counts in advance the number of pulses output from the pulse generation unit during discharge and the pulse generation unit during charge immediately after power-up of the battery power source device, the counted number of pulses in advance is subtracted from the number of pulses counted during operation of the battery power source device, and a current flowing through the assembled battery is calculated using a result of the subtraction. 
In the battery power source device according to the present invention, it is preferable that each of the discharge current detection unit and the charge current detection unit further includes: a differential operation circuit having a first input terminal and a second input terminal; a first resistor having one end connected to the first input terminal of the differential operation circuit; a second resistor having one end connected to the second input terminal of the differential operation circuit; a first constant current circuit having an output end connected to the first input terminal of the differential operation circuit; a second constant circuit having an output end connected to the second input terminal of the differential operation circuit; a first current source that operates in accordance with an output signal from the differential operation circuit and functions so that a voltage level at the first input terminal of the differential operation circuit becomes equal to a voltage level at the second input terminal; and a second current source that operates in accordance with the output signal from the differential operation circuit, has a current mirror relationship with the first current source, and outputs a current signal in proportion to a current flowing through the current detection resistor to the outside.  Here, the control device further includes: a first switch having one end connected to the other end of the first resistor and the other end electrically connected to one end of the current detection resistor via the transmission means; a second switch having one end connected to the other end of the second resistor and the other end electrically connected to the other end of the current detection resistor via the transmission means; and a third switch connected between the other ends of the first resistor and the second resistor. 
In the above device, it is preferable that the control device further includes an offset correction unit that makes a correction to the current in accordance with a result obtained by subtracting a first counted number of pulses from a second counted number of pulses, where the first counted number of pulses is output from the pulse generation unit during discharge and the pulse generation unit during charge for a constant time period in a state where the first switch and the second switch are open and the third switch is closed during operation of the battery power source device, and the second counted number of pulses is output from the pulse generation unit during discharge and the pulse generation unit during charge in a state where the first switch and the second switch are closed and the third switch is opened during operation of the battery power source device.  With this configuration, while the vehicle is in motion, even when components characteristics vary in accordance with a fluctuation in the environmental temperature, an offset error in the current can be eliminated at any time. 
In the battery power source device according to the present invention, it is preferable that a side of the pulse generation unit during discharge and the pulse generation unit during charge and a side of the current detection unit are isolated electrically.  With this configuration, the circuit at the side of the assembled battery and the circuit at the side of an auxiliary battery that supplies electrical power to the control device (ECU) can be isolated electrically (insulated), whereby malfunction due to electric leakage and noises can be prevented securely. 
In the battery power source device according to the present invention, it is preferable that the current detection resistor includes at least: an alloy plate subjected to a resistance value trimming; a pair of metal plates on which both ends of the alloy plate are fixed and mounted and to which one end of the transmission means is fixed; and molded resin for sealing the alloy plate.  With this configuration, a current detection resistor having a low resistance value and high precision can be obtained with a simple configuration. 
In the battery power source device according to the present invention, it is preferable that the assembled battery is configured to be separated into a plurality of blocks, the battery power source device further includes a current control unit that controls a charge/discharge current with respect to the assembled battery; and the control device further includes a voltage detection unit that detects a battery voltage at each of the plurality of blocks; and a voltage comparison unit that compares battery voltages obtained from the voltage detection unit.  Here, as a result of the comparison by the voltage comparison unit, if a difference in voltage is generated among the battery voltages at the plurality of blocks, the current control unit controls so as to decrease a current flowing through the current detection resistor.  With this configuration, a variation in battery voltages among blocks can be reduced, and a battery performance as a whole of the assembled battery can be achieved sufficiently.