hereunder is a description of embodiments of the present invention , with reference to the figures . fig1 is a block diagram showing the configuration of a measuring apparatus for battery voltage in an embodiment of the present invention . in this figure , numeral 30 denotes a secondary battery being the object to be measured , and a plurality of secondary batteries 30 is connected in series to compose a module 10 . a plurality of these modules 10 ( 10 - 1 through 10 - n ) is connected in series , to constitute , for example , a power storage unit . numeral 20 ( 20 - 1 through 20 - n ) denotes a voltage detecting circuit installed in correspondence to each of the modules 10 ( 10 - 1 through 10 - n ), and connected respectively to modules 10 ( 10 - 1 through 10 - n ) in parallel . this voltage detecting circuit 20 ( 20 - 1 through 20 - n ) comprises : a capacitor 101 connected to the module 10 in parallel , a voltage follower 301 connected to the positive electrode of the capacitor 101 , switches 201 and 202 which are inserted between the module 10 and the capacitor 101 and are connected to the positive electrode and the negative electrode of the module 10 respectively , and switches 203 and 204 which are inserted between the voltage follower 301 and the capacitor 101 and are connected to the two electrodes of the capacitor 101 . here , the aforementioned voltage follower 301 is an op - amp ( operational amplifier ) whose amplification factor is “ 1 ”, and whose input voltage is output directly to the output terminal . next is a description of the operation of the voltage detecting circuit of the above construction with reference to fig2 a through 2c . here , all of the switches 201 through 204 are driven by a cpu 700 ( refer to fig3 ) installed externally . the details will be described later . first , as shown in fig2 a , the switches 201 and 202 are turned on , and the switches 203 and 204 are turned off . in this manner , the module 10 and the capacitor 101 becomes a closed circuit connected in parallel , and the voltage of the module 10 charges the capacitor 101 . when the voltage of the capacitor 101 reaches the same voltage as the module 10 and current stops flowing in this closed circuit , then based on a predetermined timing the switches 201 and 202 that had been on are turned off ( refer to fig2 b ). here , at this time the switches 203 and 204 remain off . subsequently , after a predetermined period , the switches 203 and 204 are turned on based on a predetermined timing ( refer to fig2 c ). at this time , the switches 201 and 202 remain off . in this manner , the voltage corresponding to the voltage across the capacitor 101 appears at the output of the voltage follower 301 connected thereafter . this voltage follower 301 , as mentioned above , has infinite input impedance , so that current does not flow in the closed circuit formed by turning the switches 203 and 204 on . consequently , no voltage drop occurs through resistance , which means that the voltage across the capacitor 101 appears at the output of the voltage follower 301 with an amplification factor of “ 1 ”. then , the output of the voltage follower 301 is transformed into a digital signal by an a / d converter etc . connected after the voltage follower 301 . this digital signal is read by a microcomputer ( omitted from the figure ) etc ., thereby detecting the voltage of each module . also by computing the voltage value of each module , the voltage of the battery as a whole can be detected . then , at the next timing , by turning the switches 203 and 204 off , all switches 201 through 204 are turned off . then after a predetermined period the switches 201 and 201 are turned on . here , at this time in the case where the electric potential of the capacitor 101 is higher than the electric potential of the module 10 , the capacitor 101 discharges to the module 10 . accordingly , it is necessary for the switches 201 and 202 to be bidirectional so as to accommodate the charge and discharge of the capacitor 101 . in this manner , the cpu 700 installed externally repeats the series of operations mentioned above at a predetermined timing , so that it is possible to detect the voltage of the module at the predetermined timing . furthermore , by driving the switches mentioned above for all the modules 10 ( 10 - 1 through 10 - n ) at the same timing , it is possible to detect the voltages of all modules 10 ( 10 - 1 through 10 - n ) at the same time . on the other hand , by sequentially driving the switching of each module at different timing , it is also possible to detect the voltages of module 10 - 1 through 10 - n sequentially based on a predetermined timing . furthermore , since there is variation in the speed of response of the switches , then even though the drive signals for the switches are output alternately , there is a case where switching on and off becomes slow due to the speed of response . if the speed of response of the switch to the driving signal of the switch is slow such that all switches 201 through 204 are turned on , the voltage of the module 10 ( 10 - 1 through 10 - n ) cannot be detected . to avoid this situation , a step is provided as shown in fig2 b , so that a situation where switches 201 through 204 are all on at the same time due to variations in the speed of response is avoided . accordingly , it is necessary to set the period when all switches 201 through 204 are turned off , to longer than the speed of response of the switches 201 through 204 used . next is a description of a construction and a drive method for a photo mosfet used for the aforementioned switches 201 through 204 . the aforementioned switching elements comprise mosfets which turn on and off in response to light , and light emitting elements such as leds ( light emitting diodes ) which emit light . these light emitting elements such as leds are driven by the cpu 700 installed externally . to be specific , the cpu 700 installed externally outputs , for example , a rectangular wave signal of 0v through 5v , and current flows through the light emitting elements in response to this voltage . consequently the light emitting elements emit light , and the mosfets are driven in response to this light . the aforementioned photo mosfets are bi - directional and the light emitting elements are power sources . the mosfets , unlike photo couples , are electrically insulated . since current flows bi - directionally , the switching elements used for the aforementioned switches 201 through 204 need to be bi - directional . furthermore , the power supply system that drives the switches , and the mosfets being the switches in the apparatus for measuring battery voltage , must be insulated so that they can be adapted even in the case where the voltage of the battery to be measured is high . accordingly , photo mosfets which can insulate from the power supply system and which are bidirectional switching elements , are suitable for the switches 201 through 204 of the present embodiment . furthermore , instead of photo mosfets , any switching elements that meet the aforementioned conditions could be substituted . moreover , the switches 203 and 204 that connect the capacitor 101 and the voltage follower 301 in fig1 since there is no current flow , do not need to be bidirectional as long as they are insulated from the power supply system that drives the switches . therefore , photo couplers etc . can also be used . in this case , since there is a possibility that the speed of response of switches is different , it is necessary to set a long period when all switches are turned off . furthermore , photo mosfets have a characteristic in that the voltage drop that occurs when the current flows is small . accordingly , a problem such as vcesat ( collector - emitter saturation voltage ) which occurs when using phototransistors or the like can be solved . as a result , it is possible to detect the voltage of a battery accurately . here , in the aforementioned embodiment , a voltage follower is used as the voltage acquisition circuit . however , without using a voltage follower , it is possible to directly connect to an a / d converter that detects voltage , or instead of the voltage follower , a differential amplifier , an inverting amplifier , a non - inverting amplifier and the like may be used . here , in the aforementioned embodiment , a module in which a plurality of battery cells is connected is mentioned . however , this could be a single battery cell . moreover , in the aforementioned embodiment , the capacitor 101 may be an electric double layer capacitor . furthermore , a small capacitance capacitor or a power storage element comprised of electric double layer capacitors connected in series may be used . thus any construction , as long as it can store enough voltage corresponding to a block , may be used . furthermore , the present invention is not limited to this embodiment , and the embodiment can be implemented with appropriate modifications within a scope that does not change the gist of the present invention . next is a description of a case where , as an application example of the apparatus for measuring battery voltage of the present invention , this is applied to detect the voltage of a battery in a parallel hybrid vehicle , with reference to figures . fig4 shows a block schematic diagram of a parallel hybrid vehicle . in this figure , numeral 1 denotes a high voltage system battery . this is constructed from individual modules 10 ( 10 - 1 , 10 - 2 . . . ) where a plurality of cells is connected in series , with a plurality of these modules connected in series . a voltage detecting circuit 20 ( 20 - 1 , 20 - 2 . . . ) for detecting voltages is installed in each module ( 10 - 1 , 10 - 2 . . . ), and the voltage value of each module detected by this voltage detecting circuit is output to a battery control apparatus 5 . furthermore , a current sensor for detecting the current that flows in the battery 1 , and a temperature sensor for detecting the temperature of the battery 1 , according to a predetermined timing , detect the current and the temperature respectively , and output these values to the battery control apparatus 5 . numeral 2 denotes a power drive unit , which is constructed such that three pairs of serially connected two switching components are connected in parallel . numeral 3 denotes an engine , which is driven by fuel combustion energy , and numeral 4 denotes a motor , which is used together with engine 3 and is driven by electric energy . the driving force from both the engine 3 and the motor 4 is transmitted to driving wheels ( not shown in the figure ), via an automatic transmission or a manual transmission ( not shown in the figure ). furthermore , at the time of deceleration of the hybrid vehicle , a driving force is transmitted to the motor 4 from the driving wheels . the motor 4 functions as a generator to produce so called regenerative braking force , and charges the battery 1 . here a construction may also be such that , besides the motor 4 for driving , a generator for charging the battery 1 is provided . the drive and regeneration of the motor 4 are performed by the power drive unit 2 , which receives control instructions from a motor control apparatus . to be specific , the switching elements in the power drive unit 2 are turned on and off by the motor control apparatus 6 , so that electric power from the battery i is supplied to the motor 4 via a three - phase line , or regenerative electric power from the motor 4 is supplied to the battery 1 . numeral 5 denotes the battery control apparatus , which receives the voltage v ( v 1 through vn ) of each module comprising the battery 1 from the voltage detecting circuit 20 ( 20 - 1 through 20 - n ), also receives , from the current sensor , battery current ibatt flowing from the battery 1 , and also receives temperature tbatt of the battery 1 at a predetermined timing , and computes the remaining battery charge soc of the battery 1 from these output values . numeral 7 denotes an engine control apparatus , which monitors engine speed ne , vehicle speed and the like every predetermined period to determine modes such as motor regeneration , assistance and deceleration . at the same time , the engine control apparatus 7 determines the assistance / regeneration amount from the result of the aforementioned mode determination and from the remaining battery charge soc transmitted from the battery control apparatus 5 . here , the battery control apparatus 5 , the motor control apparatus 6 and the engine control apparatus 7 , which are each composed of a cpu ( central processing unit ) and memory , realize their functions by running a program to realize the function of the control apparatuses . in the hybrid vehicle with the aforementioned construction , the voltage detection of the battery 1 will be described . first , based on a predetermined timing , the battery control apparatus 5 outputs a signal for turning the switches 201 and 202 on , to the voltage detecting circuit 20 . when the switches 201 and 202 are turned on by this signal , the module 10 and the capacitor 101 become a closed circuit connected in parallel , and the voltage of the module 10 charges the capacitor 101 . then , after the voltage of the capacitor 101 reaches the same voltage as the voltage of the module 10 and no current flows in this closed circuit , the battery control apparatus 5 , based on a predetermined timing , outputs a signal for turning the switches 201 and 201 which had been on , off , to the voltage detecting circuit 20 . based on this signal , all switches 201 through 204 are turned off . subsequently , after a predetermined period , based on a predetermined timing , the battery control apparatus 5 outputs a signal for turning the switches 203 and 204 on , to the voltage detecting circuit 20 . at this time , the switches 201 and 202 remain off . in this manner the electric charge stored in the capacitor 101 , via the switch 203 which is conducting , is input to the input terminal of the voltage follower 301 connected thereafter . then , by the voltage follower 301 , the input voltage is output with an amplification factor of “ 1 ”. that is to say , the same voltage as the input voltage is output to the output terminal of the voltage follower 301 . then , the output from the voltage follower 301 is input to the battery control apparatus 5 . the battery control apparatus 5 , by reading this output , detects the voltage of the battery 1 , and also detects the remaining charge of the battery 1 from the values of the battery current ibatt and the battery temperature tbatt received at the same time . in this manner , with a high accuracy voltage detecting circuit , by detecting the voltage of the battery , it is possible to control each part of the hybrid vehicle with high accuracy . furthermore , the apparatus for measuring battery voltage of the present invention is small sized and inexpensive so that it is also possible to achieve cost reduction by using this apparatus for measuring the voltage of the batteries .