Abstract:
A voltage detection apparatus for detecting a voltage of at least one battery includes: capacitor means to which a voltage is applied from the at least one battery and in which an electric charge accumulates; adjustment means for adjusting potentials at both ends of the capacitor means such that a potential difference between a potential at one end of the capacitor means and a reference potential is less than a potential difference between a potential at one end of the at least one battery and the reference potential, and that a potential difference between a potential at another end of the capacitor means and the reference potential is less than a potential difference between a potential at another end of the at least one battery and the reference potential; and voltage detection means for detecting a voltage of the at least one battery based on a potential difference between the one end of the capacitor means and the other end of the capacitor means after the potentials of both ends of the capacitor means have been adjusted by the adjustment means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for detecting a voltage of batteries used as a power source for an electric motor of a hybrid vehicle which incorporates the electric motor and a combustion engine together as a driving source thereof. 
     2. Description of the Related Art 
     An electric motor used as a driving source of an electric car is driven by a battery power supply in which there are a plurality of battery units in a series connection and each of the plurality of the battery units is composed of several low-voltage batteries in a series connection. In such a battery power supply, all of the batteries in the battery units are required to operate normally and at an equal voltage. Therefore, it is necessary to detect a voltage of each battery unit in which several batteries are combined to confirm that each battery is operating normally. The voltage of the battery unit is usually detected by means of a differential amplifier. 
     However, in the above-described battery power supply, the total voltage may be as high as about 400 V, so that the battery power supply is required to be in a floating state in which the circuitry is not electrically grounded. Therefore, the voltage of each battery unit is measured in the floating state, i.e., without being established against a ground potential, so that there is a possibility that the detected voltages may undergo substantial fluctuation. Particularly, the voltage of a battery power supply used as a driving source of a hybrid vehicle is normally supplied to an electric motor via an inverter so that the voltage of each battery unit may undergo an even greater fluctuation. A battery unit voltage which in itself undergoes a great fluctuation can not be accurately detected. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, there is provided a voltage detection apparatus for detecting a voltage of at least one battery, including: capacitor means to which a voltage is applied from the at least one battery and in which an electric charge accumulates; adjustment means for adjusting potentials at both ends of the capacitor means such that a potential difference between a potential at one end of the capacitor means and a reference potential is less than a potential difference between a potential at one end of the at least one battery and the reference potential, and that a potential difference between a potential at another end of the capacitor means and the reference potential is less than a potential difference between a potential at another end of the at least one battery and the reference potential; and voltage detection means for detecting a voltage of the at least one battery based on a potential difference between the one end of the capacitor means and the other end of the capacitor means after the potentials of both ends of the capacitor means have been adjusted by the adjustment means. 
     In one embodiment of the invention, the capacitor means includes a plurality of capacitors in a series connection. 
     In another embodiment of the invention, the capacitor means is electrically connected with the at least one battery via a first switching means, the adjustment means includes a second switching means for switching a state of the capacitor means so as to be electrically coupled or decoupled to or from a prescribed potential, and the voltage detection means is electrically coupled to the capacitor means via a third switching means. 
     In still another embodiment of the invention, the capacitor means is electrically coupled to the prescribed potential via the second switching means, and the potentials at both ends of the capacitor means are adjusted. 
     In still another embodiment of the invention, a potential difference between the one end of the at least one battery and the other end of the at least one battery is equal to the potential difference between the one end of the capacitor means and the other end of the capacitor means. 
     In still another embodiment of the invention, the capacitor means includes a plurality of capacitors in a series connection and at least one node between the plurality of capacitors is coupled to the prescribed potential via the second switching means. 
     In still another embodiment of the invention, the voltage detection apparatus further includes a fourth switching means for switching a state of a first input terminal of the voltage detection means so as to be electrically coupled or decoupled to or from the reference potential. 
     In still another embodiment of the invention, the first input terminal is electrically coupled to the reference potential via the fourth switching means and a potential of the first input terminal is equal to the reference potential. 
     In still another embodiment of the invention, the voltage detection apparatus further includes conversion means for converting an analog signal from the voltage detection means to a digital signal. 
     In still another embodiment of the invention, the voltage detection means is a differential amplifier. 
     In another aspect of the invention, there is provided a method for detecting a voltage of at least one battery by means of the voltage detection apparatus, including the steps of: electrically coupling the at least one battery to the capacitor means via the first switching means so as to allow an electric charge to accumulate in the capacitor means; electrically isolating the at least one battery and the capacitor means from each other after the electric charge is accumulated in the capacitor means; electrically coupling the capacitor means to the reference potential via the second switching means, thereby adjusting the potentials at both ends of the capacitor means; electrically coupling the capacitor means to the voltage detection means via the third switching means; and detecting the potential difference between the one end of the capacitor means and the other end of the capacitor means by means of the voltage detection means. 
     In one embodiment of the invention, the voltage detection apparatus includes a fourth switching means for switching a state of a first input terminal of the voltage detection means so as to be electrically coupled or decoupled to or from the reference potential, the method further includes the steps of: electrically coupling the first input terminal to the reference potential via the fourth switching means; and switching a state of the first input terminal so as not to be electrically coupled to the reference potential via the fourth switching means after the step of electrically coupling the capacitor means to the voltage detection means. 
     In another embodiment of the invention, the step of adjusting the potentials at both ends of the capacitor means and the step of electrically coupling the first input terminal to the reference potential are performed simultaneously. 
     Thus, the invention described herein makes possible the advantages of providing a battery voltage detection apparatus which is capable of accurately measuring a voltage of a battery, and a method of detecting a voltage of a battery by means of the apparatus. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating an example of a voltage detection apparatus according to an embodiment of the present invention. 
     FIG. 2 is a timing chart describing an operation of the voltage detection apparatus according to an embodiment of the present invention. 
     FIG. 3 is a circuit diagram illustrating an example of a voltage detection apparatus according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
     FIG. 1 is a circuit diagram illustrating an example of a voltage detection apparatus  20  according to an embodiment of the present invention. The voltage detection apparatus  20  is used for detecting a voltage of a battery power supply  11  used as a power source for an electric motor of a hybrid vehicle which incorporates the electric motor and a combustion engine together as a driving source thereof. In the battery power supply  11 , for example, twenty four battery units  11   a  (three shown in FIG. 1 for simplicity), each including a prescribed number (e.g., 10) of batteries (nickel metal hydride storage batteries) in a series connection. One or more voltage detection apparatuses  20  are used for detecting the voltages of the respective battery units  11   a.    
     An output from the battery power supply  11  is supplied to a three-phase motor  13  via an inverter  12 . The rotating operation of the motor  13  provides a driving force to the vehicle. 
     Each of the voltage detection apparatuses  20  has a pair of capacitors  28  and  29  in a series connection, across which the voltage of the battery unit  11   a  can be applied via a first switch  21 . Hereinafter, the capacitors  28  and  29  may be collectively referred to as “capacitor means”. However, as in the embodiment described later with reference to FIG. 3, the capacitor means may include only one capacitor. The first switch  21  includes a pair of normally-open contacts  21   a  and  21   b  interlocked to each other. The normally-open contact  21   a  of the first switch  21  is inserted between a resistance  41  coupled to a positive electrode of the battery unit  11   a  and one terminal of a series circuit composed of the first capacitor  28  and the second capacitor  29 . The normally-open contact  21   b  of the first switch  21  is inserted between a resistance  42  coupled to a negative electrode of the battery unit  11   a  and another terminal of the series circuit composed of the first capacitor  28  and the second capacitor  29 . 
     A node between the first capacitor  28  and the second capacitor  29  can be body-grounded via a second switch  22 . In other words, the node between the first capacitor  28  and the second capacitor  29  has a potential equal to that of the body ground when the second switch  22  is turned on. As described above, the second switch  22  functions as adjusting means for the potential of the node between the first capacitor  28  and the second capacitor  29 . 
     A voltage across the first and second capacitors  28  and  29  in a series connection is supplied to a differential amplifier (voltage detection means)  25  via a third switch  23 . The third switch  23  includes a pair of normally-open contacts  23   a  and  23   b  interlocked to each other. The normally-open contact  23   a  of the third switch  23  is inserted between the first capacitor  28  and a resistance  43  coupled to a negative terminal of the differential amplifier  25 . The normally-open contact  23   b  of the third switch  23  is inserted between these condcapacitor  29   a  and a resistance  44  coupled to a positive terminal of the differential amplifier  25 . 
     Input voltages to the negative terminal and the positive terminal of the differential amplifier  25  can be respectively fixed by an interlocking pair of normally-open contacts  24   a  and  24   b  of a fourth switch  24 . The normally-open contact  24   a  of the fourth switch  24  is inserted between the negative terminal of the differential amplifier  25  and an output terminal of the differential amplifier  25 . A resistance  46  is provided in parallel connection with the normally-open contact  24   a.  The normally-open contact  24   b  of the fourth switch  24  is inserted between the positive terminal of the differential amplifier  25  and the body ground. A resistance  45  is provided in parallel connection with the normally-open contact  24   b.    
     An output from the differential amplifier  25  is supplied to an A/D converter  26 . An output from the A/D converter  26  is supplied to a control device (CPU)  30  as an output from the entire voltage detection apparatus  20 . 
     Now, operation of the voltage detection apparatus  20  having the above-described structure for detecting a voltage of each of the battery units  11   a  of the battery power supply  11  will be described based on a timing chart shown in FIG.  2 . When detecting a voltage of a battery unit  11   a , all of the first through fourth switches  21 - 24  are placed in an off-state (non-conducting state). Then, firstly, the first switch  21  is turned on (conducting) so that normally-open contacts  21   a  and  21   b  will conduct. This allows the voltage of the battery unit  11   a  to be applied to the first and second capacitors  28  and  29  in a series connection, thereby permitting an electric charge to be accumulated in the first and second capacitors  28  and  29 . 
     After the first switch  21  has been kept ON for a prescribed period of time, the first switch  21  is turned OFF so that the normally-open contacts  21   a  and  21   b  will not conduct. This allows an electric charge corresponding to the voltage of the battery unit  11 a to be stored in the capacitors  28  and  29  in a series connection. 
     After the first switch  21  is turned OFF, the second switch  22  and the fourth switch  24  are turned ON after a lapse of a prescribed period of time. In this case, it is preferable that the second switch  22  and the fourth switch  24  are turned on simultaneously, but they may be turned on at separate times. When the second switch  22  is turned on, the node between the first capacitor  28  and the second capacitor  29  is body-grounded and a potential of the node between the capacitors  28  and  29  is fixed to the body ground potential (preferably 0 V). In this case, a potential at one end of the capacitor  29  may be equal to or less than that of the body ground. Moreover, when the fourth switch  24  is turned on so that each of the normally-open contacts  24   a  and  24   b  will conduct, the potential of the negative terminal of the differential amplifier  25  is fixed to a potential equal to that of the output node of the differential amplifier  25  (0 V), and the potential of the positive terminal thereof is fixed to the body ground potential (0 V). The potential of the node between the capacitors  28  and  29 , which is equal to the potential of the positive terminal of the differential amplifier  25  at this point, serves as a reference potential of the differential amplifier  25 . 
     After a lapse of a prescribed period of time since the second switch  22  and the fourth switch  24  are turned on, the third switch  23  is turned on so that the normally-open contacts  23   a  and  23   b  will conduct. This allows the first and second capacitors  28  and  29  to be respectively coupled to the negative terminal and the positive terminal of the differential amplifier  25 . Since the fourth switch  24  is in an on-state, the voltage of each terminal of the differential amplifier  25  is fixed. Therefore, the voltage across the first and second capacitors  28  and  29  is not applied to the respective terminals of the differential amplifier  25 . 
     After a lapse of a prescribed period of time since the third switch  23  is turned on, the fourth switch  24  is turned off. As a result, the voltage of each terminal of the differential amplifier  25  is no longer fixed, so that a voltage corresponding to the electric charge stored in the first and second capacitors  28  and  29  is applied to the differential amplifier  25 . 
     In this case, the voltage of each of the capacitors  28  and  29  is applied to each input terminal of the differential amplifier  25 , while the voltage of each input terminal of the differential amplifier  25  is fixed to 0 V. Therefore, it is ensured that the voltage of each input terminal will be kept within a tolerable range. Accordingly, an output from the differential amplifier  25  is prevented from being saturated, and the voltage applied from each of the capacitors  28  and  29  can be accurately detected. Moreover, since the voltage input to the differential amplifier  25  will be kept within a tolerable range, deterioration and breakdown of the differential amplifier  25  can be prevented. 
     As described above, after an electric charge corresponding to the voltage of the battery unit  11   a  is accumulated in the first capacitor  28  and the second capacitor  29 , a stabilized voltage across the first capacitor  28  and the second capacitor  29  is applied to the differential amplifier  25 , so that the differential amplifier  25  can stably detect the voltage across the capacitors  28  and  29 . 
     The differential amplifier  25  outputs to the A/D converter  26  a signal corresponding to the voltage applied from the capacitors  28  and  29 . The A/D converter  26  converts the output from the differential amplifier  25  to a digital signal and outputs the resultant signal to the control device  30 . The control device  30  determines whether the voltage of the battery unit  11   a  as detected by the voltage detection apparatus  20  has a normal value based on the digital signal output from the A/D converter  26 . 
     As described above, the voltage of each battery unit  11   a  is separately detected by the respective voltage detection apparatus  20 , whereby it can be determined whether the voltage of each battery unit  11   a  is normal. 
     FIG. 3 illustrates a voltage detection apparatus  20 ′ according to an embodiment of the present invention. The structure of the voltage detection apparatus  20 ′ is the same as that of the voltage detection apparatus  20  (FIG. 1) except that the capacitor  29 , the normally-open contact  23   b  and the resistance  44  are omitted. Operating procedures are essentially the same as those of the voltage detection apparatus  20 . Only one capacitor  28  is provided in the voltage detection apparatus  20 ′ as a capacitor to which a voltage of the battery unit  11   a  is applied, whereby the structure of the voltage detection apparatus  20 ′ can be simplified. 
     In each of the above-described embodiments of the invention, as each of the first through third switches  21 - 23  (but not the fourth switch  24  used for stabilizing the voltage of each input terminal of the differential amplifier  25 ), a transistor such as an FET (field-effect transistor) or an SSR (solid-state relay) incorporating such a transistor so as to have a high voltage breakdown value, e.g.,. 400 V, may be used. The differential amplifier  25  used herein may be of any type commonly used in the art. 
     Moreover, the voltage detection apparatus may be structured so that the on/off timing of each of the switches  21 - 24  is controlled by the control device  30  or another control device. 
     The above-described embodiments have a structure in which the voltage detection apparatus  20  or  20 ′ is provided corresponding to each battery unit  11   a,  but the present invention is not limited to such a structure. For example, only one voltage detection apparatus  20  or  20 ′ may be used so as to be selectively coupled to each of the battery units  11   a  in turn. 
     Moreover, three or more capacitors may be provided for accumulating therein an electric charge corresponding to a voltage of a battery unit  11   a.  In this case, at least one preselected capacitor may be body-grounded via the second switch  22  while the other capacitors may be electrically coupled to the second switch  22  via the preselected capacitor. 
     Moreover, the present invention is operable even if the fourth switch  24  is omitted from the voltage detection apparatus  20  or  20 ′. However, it is more preferable that the voltage detection apparatus  20  or  20 ′ includes the fourth switch  24 . 
     Moreover, the A/D converter  26  may be provided externally to the voltage detection apparatus  20  or  20 ′. 
     The present invention provides an apparatus and a method for detecting a voltage which is capable of accurately measuring a voltage of a battery, where a differential amplifier is used to detect a voltage across a capacitor(s) to which a voltage of the battery is applied, the voltage across the capacitor(s) being stabilized in the manner described above. Moreover, a voltage of each input terminal of the differential amplifier is fixed to a prescribed voltage value so that the voltage applied by each capacitor to the differential amplifier is stabilized. Therefore, the voltage of a battery can be detected more accurately. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.