Differential voltage measuring device

Abstract

Differential voltage measuring device (100; 200) for detecting the voltage of a battery (B), comprising: a first capacitor (113; C1) and a second capacitor (114; C2), each formed by a ceramic capacitor, a differential amplifier (111; 240) for outputting a voltage corresponding to a difference voltage between a voltage held by the first capacitor (113; C1) and a voltage held by the second capacitor (114; C2), and a control unit (140; 210) configured to apply a first voltage to the first capacitor (113; C1) and a second voltage to the second capacitor (114; C2), the first capacitor (113; C1) holding the first voltage, and to detect an output of the differential amplifier (111; 240) corresponding to the difference voltage of the first voltage and the second voltage while the first capacitor (113; C1) holds the first voltage and the second capacitor (114; C2) holds the second voltage, wherein in a first time period, the control unit (140; 210) applies a third voltage to the first capacitor (113; C1) and, after in a second time period following the first time period, the control unit (140; 210) stops applying the third voltage to the first capacitor (113; C1) and applies the third voltage to the second capacitor (114; C2), and the first capacitor (113; C1) is discharged, in a third time period following the second time period, the control unit (140; 210) stops applying the third voltage to the second capacitor (114; C2) and applies the first voltage to the first capacitor (113; C1); and in a fourth time period following the third time period, the control unit (140; 210) stops applying the first voltage to the first capacitor (113; C1) and applies the second voltage to the second capacitor (114; C2).

Description

Technical field

[0001] The present invention relates to a technique for improving the measurement accuracy of a voltage difference between a first voltage and a second voltage obtained successively. State of the art

[0002] For example, in various vehicles such as an electric vehicle (EV) that travels using an electric motor, a hybrid vehicle (HEV) that travels using a combination of an internal combustion engine and an electric motor, or a similar vehicle, secondary batteries such as rechargeable lithium-ion batteries or rechargeable nickel-hydrogen batteries are installed as a power source for the electric motor.

[0003] It is known that such a secondary battery deteriorates due to repeated charging and discharging, and its charging capacity (current capacity, electric power capacity, etc.) gradually decreases. Therefore, in the electric vehicle using the secondary battery, the charging capacity is measured and the degree of deterioration of the secondary battery is recorded to calculate the travel distance of the secondary battery, the service life of the secondary battery, and the like.

[0004] An indicator of the deterioration of the secondary battery is the SOH (State of Health), which is the ratio of the current charge capacity to the initial charge capacity. It is known that the SOH correlates with the internal resistance of the secondary battery. Therefore, by measuring the internal resistance of the secondary battery, the SOH can be obtained based on this internal resistance.

[0005] While it is generally difficult to achieve sufficient detection accuracy because the internal resistance is very small, Patent Literature 1 discloses a battery state detecting device with improved detection accuracy of the internal resistance.

[0006] Fig. 6 is a schematic view showing a configuration of a battery state detection device 500 described in Patent Document 1. The secondary battery B to be detected includes an electromotive force part e that generates a voltage and an internal resistance r. By detecting the internal resistance r, the SOH of the secondary battery B can be obtained.

[0007] The secondary battery B generates a voltage V between both electrodes (a positive electrode Bp and a negative electrode Bn), where the voltage V is determined by a voltage Ve generated by the electromotive force at the electromotive force part e and by a voltage Vr generated by the current flowing through the internal resistance r (V = Ve + Vr). The negative electrode Bn of the secondary battery B is connected to the reference potential G.

[0008] The battery condition detecting device 500 includes a differential amplification circuit 511, a changeover switch 512, a first capacitor 513, a second capacitor 514, a charging unit 515, a first analog-to-digital converter (ADC) 521, a second analog-to-digital converter (ADC) 522, and a microcomputer (µCOM) 540.

[0009] In the configuration shown, when the µCOM 540 sends a control signal for a charging start to the charging unit 515 via the output terminal PO2, the charging unit 515 starts to flow a predetermined constant current Ic to the secondary battery B. As a result, the charging of the secondary battery B begins.

[0010] When charging begins, the µCOM 540 controls the changeover switch 512 via the output terminal PO2, so that the positive electrode Bp of the secondary battery B and the first capacitor 513 are connected. As a result, the voltage V1 = Ve + r × Ic z between the two electrodes of the secondary battery B is maintained in the first capacitor 513 during charging.

[0011] Then, when the voltage between both electrodes of the secondary battery B obtained through the input terminal PI1 reaches the predetermined state detection voltage, the µCOM 540 controls the changeover switch 512 through the output terminal PO1 so that the positive electrode Bp of the secondary battery B and the second capacitor 514 are connected, and sends a control signal for a charging stop to the charging unit 515 through the output terminal PO2.

[0012] Therefore, when the charging current Ic to the secondary battery B is stopped and the storage state of the second capacitor 514 becomes stable, the voltage V2 = Ve between both electrodes of the secondary battery B is maintained in the second capacitor 514.

[0013] In this state, the µCOM 540 detects the amplified voltage Vm output by the differential amplifier 511 through the input terminal PI2. Then, the detected amplified voltage Vm is divided by the rate Av of the differential amplifier 511 and further divided by the charging current Ic, so that the internal resistance r (= (Vm / Av) / Ic) is detected.

[0014] Finally, the µCOM 540 sends the control signal for charging start to the charging unit 515 via the output terminal PO2. In accordance with this control signal, the charging unit 515 begins to flow the predetermined constant charging current Ic into the secondary battery B again. As a result, charging resumes and the battery state detection process is completed.

[0015] Patent Document 2 describes a charge balancing device for optimally charging a battery by measuring an internal voltage drop as a voltage difference between the battery terminals during charging.

[0016] Patent Document 3 describes a capacitive humidity detector. The current charge state of a capacitor is determined by detecting a voltage difference between a current capacitor voltage and a predetermined supply voltage.

[0017] Patent Document 4 describes a capacitance measuring device for a touch controller. A voltage across the capacitor is compared with various reference voltages. Reference list[Patent document] Patent Document 1: Unexamined Japanese Patent Application Publication No. JP 2014-219311 A Patent document 2: US 2015 / 0 188 329 A1 Patent document 3: US 2014 / 0 046 611 A1 Patent document 4: US 2011 / 0 273 192 A1 Description of the inventionProblem of the invention

[0018] With the battery state detecting device 500 described in Patent Document 1, the detection accuracy of the internal resistance of the secondary battery can be improved and deterioration of the detection accuracy of the battery state can be suppressed.

[0019] And by applying the technique of the battery state detecting device 500, a differential voltage measuring device can be configured that can not only measure the internal resistance of the secondary battery, but also measure a slight voltage change between the first state and the second state in a power source such as a battery, or a potential difference of two points in the circuit, etc., with high accuracy.

[0020] In this case, the voltage of the voltage source in the first state or the voltage of a point in the circuit is sampled and held as the first voltage with the switch 512 and the capacitor 513, and then the voltage of the voltage source in the second state or the voltage at another point in the circuit is sampled and held as the second voltage with the switch 512 and the second capacitor 514, before the difference between the first voltage and the second voltage is amplified by the differential amplifier 511 and the gain voltage Vm output by the gain unit 511 is divided by the gain factor Av of the differential amplifier unit 511, so that a slight voltage change between the first state and the second state in the voltage source or between one point and another point in the circuit can be measured with great accuracy.

[0021] In the example of Patent Document 1, the state in which the constant current Ic flows through the secondary battery B corresponds to the first state, and the state in which no current flows corresponds to the second state. Note that the voltage source as the voltage change detection target is not limited to a secondary battery, and may also be a cell of a primary battery or a composite battery, a power supply circuit, or the like.

[0022] In an actual capacitor, the accumulated charge may be released due to a slight leakage current under certain circumstances. Therefore, after the first capacitor 513 samples and holds the battery voltage in the first state until the second capacitor 514 completes sampling and holds the battery voltage in the second state, the charge accumulated in the first capacitor 513 slightly leaks due to the leakage current. When the charge is released from the first capacitor 513, the first voltage is lower than the actual voltage, thereby reducing the measurement accuracy.

[0023] And in order to reduce the cost and size of this type of device, a ceramic capacitor is preferably used for the first capacitor 513 or the second capacitor 514. The ceramic capacitor has a DC bias characteristic in which the electrostatic capacitance decreases as the voltage increases. With this DC bias characteristic, when the sensed and held charge leaks due to the leakage current, the first voltage becomes smaller and the electrostatic capacitance increases, so that a reduction in the first voltage continues, raising the problem of further deteriorating the measurement accuracy. Therefore, it is difficult to use a ceramic capacitor in the conventional method.

[0024] In view of the above circumstances, it is an object of the present invention to provide a differential voltage measuring apparatus which can suppress deterioration in the measurement accuracy of the voltage difference between a first voltage and a second voltage obtained sequentially, while reducing the cost and size of the apparatus. Problem solving

[0025] This problem is solved by the features of patent claim 1. Advantages of the invention

[0026] According to the present invention, the third voltage is applied to at least one of the first capacitor and the second capacitor, and the first voltage or the second voltage is applied to the first capacitor or the second capacitor after the predetermined time has elapsed after the application of the third voltage is stopped. In this way, the first voltage and the second voltage can be discharged after the DC bias characteristic of the ceramic capacitor has changed. This suppresses an increase in the electrostatic capacitance after the voltage drop due to the influence of the DC bias characteristic of the ceramic capacitor and further reduces a decrease in the voltage across the capacitor, so that a reduction in the measurement accuracy of the voltage difference between the first voltage and the second voltage can be suppressed.And because the ceramic capacitor is used, cost reduction and miniaturization of the device can be realized. Brief description of the drawings Fig. 1 is a schematic view showing a configuration of a battery state detecting device according to a first embodiment. Fig. 2 is a flowchart showing an example of a battery state detecting process in the battery state detecting device. Fig. 3 is a graph showing an example of a DC bias characteristic of a ceramic capacitor. Fig. 4 is a schematic view showing a configuration of a differential voltage measuring apparatus according to a second embodiment. Fig. Figure 5 is a circuit diagram showing an example of a differential amplifier. Fig. 6 is a schematic view showing a configuration of a conventional battery condition detecting device. Embodiments of the invention

[0027] Embodiments of the present invention will be described in detail below with reference to the drawings. A first embodiment is an example in which the differential voltage measuring device is applied to a battery condition detecting device. Fig. 1 is a schematic view showing a configuration of a battery condition detecting device according to the first embodiment of the present invention.

[0028] The battery state detection device 100 of this embodiment is mounted on an electric vehicle and connected between the electrodes of a secondary battery included in the vehicle, for example, and detects the internal resistance of the secondary battery as a state of the secondary battery. Of course, it may also be a device or system equipped with the secondary battery other than an electric vehicle.

[0029] The secondary battery B to be detected has an electromotive force part e that generates a voltage and an internal resistance r. By detecting the internal resistance r, the SOH of the secondary battery B can be obtained.

[0030] The secondary battery B generates a voltage V between both electrodes (a positive electrode Bp and a negative electrode Bn), which is determined by the voltage Ve generated by the electromotive force generated by the electromotive force part e and the voltage Vr generated by a current flowing through the internal resistance r (V = Ve + Vr). The negative electrode Bn of the secondary battery B is connected to the reference potential G.

[0031] As shown in the figure, the battery condition detecting device 100 includes a differential amplifier 111, a changeover switch 112, a first capacitor 113, a second capacitor 114, a charging unit 115, a first analog-to-digital converter (ADC) 121, a second analog-to-digital converter (ADC) 122, and a microcomputer (µCOM) 140.

[0032] The differential amplifier 111 is constituted by, for example, an operational amplifier or the like, has two input terminals (a first input terminal In1 and a second input terminal In2) and one output terminal (output terminal Out), and outputs from the output terminal the voltage Vm obtained by amplifying the differential value of the voltage input to these two input terminals by a predetermined gain Av.

[0033] The changeover switch 112 is, for example, a two-contact SPDT (Single Pole Double Throw) switch formed by an analog switch or the like. One of the two changeover switches a and b in the changeover switch 112 is connected to the first input terminal In1 of the differential amplifier 111, and the changeover switch b is connected to the second input terminal In2 of the differential amplifier 111. Furthermore, the common terminal c in the changeover switch 112 is connected to the positive electrode Bp of the secondary battery B.

[0034] The changeover switch 112 is connected to an output terminal PO1 of the µCOM 140 described below, the connection between the two changeover switches a, b and the common switch c is switched in accordance with the control signal from the µCOM 140, and the positive electrode Bp of the secondary battery B is exclusively connected to either the first input terminal In1 or the second input terminal In2.

[0035] The first capacitor 113 is connected between the first input terminal In1 of the differential amplifier 111 and the reference potential G. That is, the first capacitor 113 is arranged between the first input terminal In1 and the negative electrode Bn of the secondary battery B. As a result, the first capacitor 113 holds the voltage between the power terminal In1 and the negative electrode Bn of the secondary battery B.

[0036] The second capacitor 114 is connected between the second input terminal In2 of the differential amplifier 111 and the reference potential G. That is, the second capacitor 114 is arranged between the second input terminal In2 and the negative electrode Bn of the secondary battery B. As a result, the second capacitor 114 holds the voltage between the second terminal In2 and the negative electrode Bn of the secondary battery B.

[0037] In the battery state detection device 100 of this embodiment, the first capacitor 113 and the second capacitor 114 are each formed by a ceramic capacitor. Furthermore, the capacitances of the first capacitor 113 and the second capacitor 114 are different, and the capacitance of the first capacitor 113, which accumulates an electric charge, is set larger than the capacitance of the second capacitor 114. In general, with a larger capacitance, the effect of a voltage drop due to a leakage current is smaller, and with a smaller capacitance, the time required for sampling and holding is shorter.

[0038] In this embodiment, the capacitance of the first capacitor 113 and the capacitance of the second capacitor 114 are different, but the capacitances may also be the same.

[0039] The charging unit 115 is connected between the positive electrode Bp of the secondary battery B and the reference potential G to allow a predetermined charging current Ic to flow in the secondary battery B for charging the secondary battery B. Thus, it can function as a current output unit. The charging unit 115 is connected to an output terminal PO2 of the µCOM 140 described below. The charging current Ic flows through the secondary battery B in accordance with the control signal, and stopping the flow of the charging current I through the secondary battery B stops the charging.

[0040] The first analog-to-digital converter (ADC) 121 quantizes the voltage between both electrodes of the secondary battery B and outputs a signal indicating a digital value corresponding to this voltage. The analog-to-digital converter (ADC) 122 quantizes the amplified voltage Vm output from the differential amplifier 111 and outputs a signal indicating a digital value corresponding to the amplified voltage Vm.

[0041] The µCOM 140 includes a CPU, a ROM, a RAM, and the like, functions as a controller, and controls the entire battery condition detecting device 100. The µCOM 140 is provided with a first output terminal PO1 connected to the changeover switch 112 and a second output terminal PO2 connected to the charging circuit 115, and sends a control signal to the changeover switch 112 via the first output terminal PO1, controls the changeover switch 112 to connect the positive electrode Bp of the secondary battery B to the first input terminal In1 while the secondary battery B is being charged, and to connect the positive electrode Bp of the secondary battery B and the second input terminal In2 while the charging of the secondary battery B is stopped.

[0042] Furthermore, a control signal is sent to the charging unit 115 via the second output terminal PO2, and the charging unit 115 is controlled to stop charging the secondary battery B when the voltage V between both electrodes of the secondary battery B reaches the predetermined state detection voltage Vt during charging of the secondary battery B by the charging unit 115.

[0043] The µCOM 140 has a first input terminal PI1 to which a signal output from the first ADC 121 is input, and a second input terminal PI2 to which a signal output from the second ADC 122 is input. Based on these signals, the µCOM 140 detects the voltage V between the two electrodes of the secondary battery B and the amplified voltage Vm output from the differential amplifier 111. Then, the internal resistance r of the secondary battery B is detected based on the amplified voltage Vm and the charging current Ic.

[0044] In the following, with reference to the flowchart of Fig. 2, an example of the battery state detection process in the µCOM 140 in the battery state detection device 100 according to the first embodiment will be described.

[0045] For example, after receiving the charge start command for the secondary battery B from an electronic control unit on a vehicle via a communication port, the µCOM 140 sends a charge start control signal to the charging unit 115 via the second output port PO2. In response to this control signal, the charging unit 115 starts supplying a predetermined constant charging current Ic to the secondary battery B. As a result, charging of the secondary battery B starts.

[0046] When the charging current Ic flows through the secondary battery B and the battery is charged, the µCOM 140 sends a control signal for connecting the changeover terminal a and the common terminal c to the changeover switch 112 via the control terminal PO1 (S110).

[0047] The changeover switch 112 connects the changeover terminal a and the common terminal c in accordance with this control signal, so that the positive electrode Bp of the secondary battery B and the first input terminal In1 of the differential amplifier section 111 are connected.

[0048] As a result, the first capacitor 113 is connected between the positive electrode Bp and the negative electrode Bn of the secondary battery B, and a charge flows into the first capacitor 113 from the secondary battery B and the charging unit 115. When a certain period of time elapses, the first capacitor 113 accumulates charges up to an upper capacity, and also the voltage (third voltage) between both electrodes of the secondary battery B is maintained in the first capacitor 113 during charging.

[0049] Then, the µCOM 140 waits until the voltage between the two electrodes of the secondary battery B reaches the state detection voltage Vth (S120). When the voltage between the two electrodes of the secondary battery B reaches the predetermined state detection voltage Vth, the µCOM 140 sends a control signal to the changeover switch 112 for connecting the changeover terminal b and the terminal c via the first output terminal PO1 (S130).

[0050] The changeover switch 112 connects the changeover terminal b and the common terminal c in accordance with this control signal, so that the positive electrode Bp of the secondary battery B and the second input terminal In2 of the differential amplifying section 111 are connected.

[0051] As a result, the second capacitor 114 is connected between the positive electrode Bp and the negative electrode Bn of the secondary battery B, and charges flow into the second capacitor 114 from the secondary battery B and the charging unit 115. When a certain period of time elapses, the second capacitor 114 accumulates charges up to its upper capacity limit, and the voltage (third voltage) between both electrodes of the secondary battery B is also maintained in the second capacitor 114 during charging.

[0052] Then, the µCOM 140 waits until the voltage between both electrodes of the secondary battery B reaches the state detection voltage Vth (S140). Until then, a preliminary sample and hold operation is performed for the first capacitor 113 and the second capacitor 114. The effect of this preliminary sample and hold operation will be described below.

[0053] When the voltage between the two electrodes of the secondary battery B reaches the predetermined state detection voltage Vth, the µCOM 140 sends a control signal for connecting the changeover terminal a and the common terminal c to the changeover switch 112 via the control terminal PO1 (S150).

[0054] The changeover switch 112 connects the changeover terminal a and the common terminal c in accordance with this control signal, so that the positive electrode Bp of the secondary battery B and the first input terminal In1 of the differential amplification circuit 111 are connected.

[0055] As a result, the first capacitor 113 is connected between the positive electrode Bp and the negative electrode Bn of the secondary battery B, and charges flow into the first capacitor 113 from the secondary battery B and the charging unit 115. When a certain period of time elapses, the first capacitor 113 accumulates charges up to its upper capacity limit, and further, the voltage between both electrodes of the secondary battery B during charging is maintained in the first capacitor 113 as the first voltage.

[0056] Then, the µCOM 140 waits until the voltage between the two electrodes of the secondary battery B reaches the state detection voltage Vth (S160). When the voltage between the two electrodes of the secondary battery B reaches the predetermined state detection voltage Vth, the µCOM 140 sends a control signal to the changeover switch 112 for connecting the changeover terminal b and the terminal c via the first output terminal PO1 (S170), and almost simultaneously sends a control signal for stopping charging to the charging unit 115 via the second output terminal PO2.

[0057] The changeover switch 112 connects the changeover terminal b and the common terminal c in accordance with this control signal, so that the positive electrode Bp of the secondary battery B and the second input terminal In2 of the differential amplifying section 111 are connected.

[0058] As a result, the second capacitor 114 is connected between the positive electrode Bp and the negative electrode Bn of the secondary battery B, and charges flow into the first capacitor 114 from the secondary battery B. Furthermore, the charging unit 115 stops the charging current Ic to the secondary battery B in accordance with the control signal from the µCOM 140.

[0059] While the second capacitor 114 and the secondary battery B are connected by the changeover switch 112, although the first capacitor 113 is disconnected from the secondary battery B, the amount of charge leaking due to a leakage current can be reduced because the size of the first capacitor 113 is made large.

[0060] Then, the second capacitor 114 waits until the power storage period set for accumulating charge up to the upper capacity limit expires (S190). Because the capacitance is set small, the second capacitor 114 can store electric charge up to the upper capacity limit in a short time. This can further reduce the amount of charge leaking through the capacitor 113.

[0061] When this short storage period has elapsed, the second capacitor 114 stores electric charge up to the upper limit of its capacity, the voltage held by the secondary battery B is stabilized, and the voltage between both electrodes of the secondary battery B is held as the second voltage in the second capacitor 114.

[0062] Then, when the voltage held by the second capacitor has been stabilized (ie, when the storage period has elapsed), the µCOM 140 detects the amplified voltage Vm output from the differential amplifier 111 based on the information obtained from the signal input to the second input terminal PI2 (S200).

[0063] The µCOM 140 divides the detected amplified voltage Vm by the gain factor Av of the differential amplifier 111 and further by the charging current Ic, so that the internal resistance r of the secondary battery B is detected (r = (Vm / Av) / Ic) (S210).

[0064] Finally, the µCOM 140 sends a control signal for starting charging to the charging unit 115 via the second output terminal PO2 (S220). In response to this control signal, the charging unit 115 again starts flowing a predetermined constant charging current Ic to the secondary battery B. As a result, charging resumes and the battery state detection process is terminated.

[0065] As described above, in the battery state detection device 100 according to this embodiment, as in step S120 or S140, once a voltage is applied to the first capacitor 113 and the second capacitor 114 (as preliminary sample and hold), the first voltage and the second voltage are held (sample and hold) for detecting the internal resistance r. In this way, the risk of the detection accuracy of the internal resistance r deteriorating due to the influence of the DC bias characteristic of the ceramic capacitor can be reduced.

[0066] Fig. Figure 3 shows an example of the DC bias characteristic of the ceramic capacitor. In Fig. 3, the vertical axis represents the rate of change of electrostatic capacity and the horizontal axis represents the DC voltage. The solid line in Fig. 3 represents a characteristic curve when no preliminary sampling and holding is performed, and the dashed line represents a characteristic curve when preliminary sampling and holding is performed.

[0067] As in Fig. As shown in Figure 3, it is known that the effective electrostatic capacitance decreases when a DC voltage is applied to the ceramic capacitor. Furthermore, it is known that the ceramic capacitor changes to a characteristic based on the decreased capacitance as indicated by the dashed line when a voltage is applied to the ceramic capacitor and discharge occurs after charging.

[0068] Therefore, once the DC voltage is applied to the ceramic capacitor for charging, a discharge after stopping charging can change the characteristic from the solid line to the dashed line of Fig. 3 Change. Off Fig. 3 clearly shows that the characteristic curve of the dashed line is more moderate than that of the solid line when the rate of change of the static capacitance changes due to the voltage change. Therefore, changing the characteristic curve through preliminary sample and hold and performing sample and hold for the actual measurement can reduce the influence of the capacitor voltage drop due to leakage current.

[0069] In the configuration of Fig. 1, the preliminary sampling and holding of the second capacitor 114 is performed (steps S130 and S140), while the preliminary sampling and holding of the first capacitor 113 is performed (steps S110, S120), and the sampling and holding for actual measurement of the first capacitor 113 is performed (steps S150, S160). During the preliminary sampling and holding of the second capacitor 114, the charging of the first capacitor 113 is stopped. The preliminary sampling and holding of the second capacitor 114 causes the first capacitor 113 to leave an interval between the preliminary sampling and holding and the sampling and holding for the actual measurement. Thus, the application of the third voltage is stopped, and a predetermined time elapses, and during this interval, discharge is caused by the leakage current.Therefore, the first capacitor 113 changes the DC bias characteristic from the solid line to the dashed line in . Fig. 3 during sampling and holding for the actual measurement.

[0070] The same applies to the second capacitor 114. During the preliminary sampling and holding of the second capacitor 114 (steps S130 and S140) and the sampling and holding for actual measurement (steps S170 to S190), the sampling and holding for actual measurement of the first capacitor 113 is performed (steps S150, S160). As a result, during the sampling and holding for actual measurement of the first capacitor 113, the charging of the second capacitor 114 is stopped. The second capacitor leaves an interval between the preliminary sampling and holding and the sampling and holding for actual measurement. That is, the application of the third voltage is stopped and a predetermined time elapses. Therefore, the first capacitor 114 changes the DC bias characteristic from the solid line to the dashed line in Fig. 3 when scanning and holding for an actual measurement.

[0071] The interval (the predetermined time) between the preliminary sample and hold and the sample and hold for an actual measurement is preferably such that the converted DC bias characteristic does not return to the original characteristic if, for example, the capacitor is not fully discharged. Furthermore, during the preliminary sample and hold, the capacitor does not necessarily have to be charged to the upper limit of the capacity. In short, it is sufficient, as in Fig. 3 that the DC bias characteristic can be changed by the preliminary sample and hold, wherein the predetermined time, the capacity to be charged and the like can be obtained by, for example, an experiment.

[0072] According to this embodiment, the µCOM 140 controls the changeover switch 112 so that the first capacitor 113 has a voltage equal to the first voltage, and then controls the changeover switch 112 so that the third capacitor 114 has a voltage equal to the first voltage and stops the application of the first voltage to the capacitor 113. Then, the changeover switch 112 is controlled so that the first voltage is applied to the first capacitor 113. In this way, the first voltage or the second voltage can be derived after changing the DC bias characteristic of the ceramic capacitor.Therefore, an increase in electrostatic capacitance associated with a voltage drop due to the leakage current can be suppressed under the influence of the DC bias characteristic of the ceramic capacitor, and a voltage drop across the capacitor can be further reduced, thereby suppressing a decrease in the measurement accuracy of the voltage difference between the first voltage and the second voltage. Furthermore, because ceramic capacitors are used, cost reduction and device downsizing can be achieved.

[0073] Furthermore, increasing the capacitance of the first capacitor 113 can reduce the voltage drop due to charge leakage during charge accumulation by the second capacitor 114, and reducing the capacitance of the capacitor 114 can shorten the storage period of the second capacitor 114. Therefore, the differential voltage can be detected in a state where the first voltage of the first capacitor 113 does not increase, thereby improving the measurement accuracy.

[0074] In the embodiment described above, the voltage (third voltage) held by the first capacitor and the second capacitor during the preliminary sampling and holding is equal to the first voltage, but it may also be equal to the second voltage or a voltage different from these voltages. Therefore, by Fig. 3, the preliminary sample and hold is performed, the DC bias characteristic of the ceramic capacitor can be changed.

[0075] Furthermore, in the above-described embodiment, preliminary sampling and holding is performed on both the first capacitor 113 and the second capacitor 114, but it may be performed on only one of them. For example, it may be performed on only the first capacitor 114, which has a longer standby time. While preliminary sampling and holding on both sides preferably changes the DC bias characteristics of the two capacitors, performing it on only one of them can also reduce a decrease in the voltage across the capacitor if the influence of the DC bias characteristics is not so great as to perform preliminary sampling and holding on both of them.

[0076] A second embodiment of the present invention will be described below. Fig. 4 is a schematic view showing a specific differential voltage measuring device of a second embodiment of the present invention. A differential voltage measuring device 200 of the second embodiment is applied to the technique of the battery condition measuring device 100 and includes a voltage source for measuring an assembled battery BS in which a plurality of battery cells (Ce1 to Ce4) are combined.

[0077] As shown in the drawing, the differential voltage measuring device 200 includes a first capacitor C1, a second capacitor C2, a µCOM 210, a current output unit 220, a changeover switch 230, a differential amplification unit 240, an ADC 250, a detection object selection switch 261, a reference potential setting switch 262, and a protection switch 270.

[0078] The current output unit 220 supplies a constant current to the assembled battery BS based on a command from the μCOM 210. Alternating the constant current flowing to the battery BS creates the first state and the second state of the assembled battery BS. Either the first state or the second state may be a state in which no current flows.

[0079] The first capacitor C1 maintains the voltage of the part to be measured in the assembled battery BS in the first state as the first voltage. The second capacitor C2 maintains the voltage of the part to be measured in the assembled battery B2 in the second state as the second voltage. The capacitance of the first capacitor C1 is intended to be larger than the capacitance of the second capacitor C2. However, the capacitances of the first capacitor C1 and the second capacitor C2 can also be equal.

[0080] The changeover switch 230 is provided with a switch SW31 for supplying the voltage (first voltage) of the measurement target part in the assembled battery BS in the first state to the first capacitor C1 and with a switch SW32 for supplying the voltage (second voltage) of the measurement target part in the assembled battery BS in the second state to the second capacitor C2.

[0081] The detection object selection switch 261 is provided between each battery cell (Ce1 to Ce4) of the assembled battery BS and the changeover switch 230. Specifically, an SW11 is provided between the end of the battery cell Ce1 corresponding to the positive electrode side of the assembled battery BS and the changeover switch 230, an SW12 is provided between the connection point of the battery cell Ce1 and the battery cell Ce2 and the changeover switch 230, an SW13 is provided between the connection point of the battery cell Ce2 and the battery cell Ce3 and the changeover switch 230, and an SW14 is provided between the connection point of the battery cell Ce3 and the battery cell Ce4 and the changeover switch 230.

[0082] The reference potential setting switch 262 is a switch for setting the reference potential of the first capacitor C1 and the second capacitor C2. Specifically, there are provided an SW 24 for setting the reference potential of the first capacitor C1 and the second capacitor C2 to the reference potential G, an SW 23 for setting the reference potential of the first capacitor C1 and the second capacitor C2 to the voltage of the battery cell Ce4, an SW 22 for setting the reference potential of the first capacitor C1 and the second capacitor C2 to the voltage of the battery cell Ce4 plus the battery cell Ce3, and an SW 21 for setting the reference potential of the first capacitor C1 and the second capacitor C2 to the battery cell Ce4 plus the battery cell Ce3 plus the battery cell Ce2.

[0083] The protection switch 270 is a switch that protects the differential amplification unit 240 and also reduces a leakage current to the differential amplifier 240 connected to the first capacitor C1 and the second capacitor C2. The protection switch 270 is a switch that supplies the first voltage / second voltage to the differential amplifier 240 after the sample and hold for the first capacitor C1 and the second capacitor C2 is completed. It is provided with a switch SW41 arranged between the first capacitor C1 and the first input terminal In1 and a switch SW42 arranged between the second capacitor C2 and the second input terminal In2. Both switches SW41 and SW42 are turned off during the sample and hold, and supply the first voltage and the second voltage to the differential amplifier 240 when the sample and hold is completed.

[0084] The differential amplifier 240 has two input terminals (a first input terminal In1 and a second input terminal In2) and one output terminal (output terminal Out), and outputs the amplified voltage Vm from the output terminal, wherein the voltage input to these two terminals is amplified by a predetermined gain Av. The differential amplification unit 240 can be implemented by, for example, an operational amplifier or a Fig. The circuit shown in Figure 5 can be formed.

[0085] The ADC 250 quantizes the amplified voltage Vm output from the differential amplifier 240 and outputs a signal indicating a digital value corresponding to the amplified voltage Vm.

[0086] The µCOM 210 has a built-in CPU, ROM, RAM, and the like, and functions as a control unit for controlling the entire differential voltage measuring device 200. The µCOM 210 is provided with a first output terminal PO1 connected to the input terminal 20, a first input terminal PI1 to which a signal output from the ADC 250 is input, and a switch control unit 211 for controlling each switch.

[0087] For example, the differential voltage measuring device 200 can obtain the SOH for each battery cell by measuring the internal resistance of each battery cell. The following describes a case where the internal resistance of the battery cell Ce1 is measured. To perform preliminary sample and hold, SW11 of the detection object selection switch 261 is turned on, and only SW21 of the reference potential setting switch 262 is turned on. Accordingly, the voltage between the two terminals of Ce1 is supplied to the capacitor C1 and the capacitor C2.

[0088] Then, a predetermined first constant current I1 is caused to flow from the current output section 220 as the first state, and only the changeover switch SW31 of the changeover switch 230 is turned on. As a result, the preliminary sample and hold is performed for the capacitor C1, and the voltage of the battery cell Ce1 is held in the first capacitor C1.

[0089] Then, SW31 of the changeover switch 230 is turned off, and SW32 is turned on. Thus, the preliminary sampling and holding of the capacitor C2 is performed, and the voltage of Ce1 is held in the second capacitor C2.

[0090] Then, sample and hold are performed for actual measurement. SW32 of changeover switch 230 is turned off, and SW31 is turned off. As a result, the voltage of battery cell Ce1 is held in the first capacitor C1 when the first voltage is charged to the cell.

[0091] Then, a predetermined second constant current I2 is caused to flow from the current output unit 220 as the second state, and the changeover switch SW31 of the SW230 is turned off and the SW32 is turned on. As a result, the voltage of the battery cell Ce1 in the second state is held in the second capacitor C2 as the second voltage.

[0092] When the protection switch 270 is turned on to supply the first voltage and the second voltage to the differential amplifier 240, the voltage difference is input to the µCOM 210. The µCOM 210 can obtain the internal resistance r1 of the battery cell Ce1 using the expression r1 = (Vm / Av) / (I1 - I2) according to the same principle as in the first embodiment. The internal resistance of other battery cells can be obtained in a similar manner.

[0093] When the differential voltage is detected with respect to the assembled battery BS as in this embodiment, a voltage drop caused by a contact resistance and a wiring resistance of connecting members such as bus bars for connecting the cells can be detected. Therefore, the contact resistance and wiring resistance of the connecting member are measured in advance, and the voltage drop caused by the influence of the connecting member can be subtracted from the detected differential voltage. For example, assuming that ΔV is the actual differential voltage, Vm / Av is the detected differential voltage, and R is the contact resistance of the connecting member and the wiring resistance, ΔV = (Vm / Av) - R × (11 - 12) is obtained. Therefore, by obtaining the internal resistance r1 based on this true differential voltage ΔV, the detection accuracy can be improved.

[0094] And because the preliminary sample and hold is performed in the second embodiment, the change rate of the electrostatic capacitance due to the voltage change is smoothly changed with respect to the DC bias characteristic of the ceramic capacitor, thereby reducing the voltage drop across the capacitor due to the influence of the DC bias characteristic, thereby reducing the influence of the voltage drop of the capacitor due to the leakage current.

[0095] And because the capacitance of the first capacitor C1 is larger and the capacitance of the second capacitor C1 is smaller, the voltage drop of the first capacitor C1 while the second capacitor C2 accumulates charges can be reduced, thereby improving the measurement accuracy of the difference voltage between the first voltage and the second voltage.

[0096] In the second embodiment, because the switching control unit 211 operates the detection object selection switch 261 and the reference potential setting switch 262, various potential differences can be measured.

[0097] For example, when the current output unit 220 is not conducting current, the first voltage is obtained because only the SE21 in the reference potential setting switch 262 is turned on and only the SW11 in the detection object selection switch 261 is turned on. After that, the second voltage is obtained because only the SW12 in the detection object selection switch 261 is turned on. The difference voltage between the first voltage and the second voltage is measured, and the measurement result indicates both ends of the battery cell Ce1, that is, the voltage of the battery cell Ce1. The voltage of the other battery cells can be obtained in the same way.

[0098] In this case, by performing the preliminary sample and hold before obtaining the first voltage, the capacitance change rate due to the voltage change with respect to the DC bias characteristic of the ceramic capacitor is gradually changed to reduce the voltage drop across the capacitor due to the influence of the DC bias characteristic, thereby reducing the influence of the capacitor voltage drop due to the leakage current.

[0099] And because the capacitance of the first capacitor C1 is larger and the capacitance of the second capacitor C2 is smaller, the voltage drop of the first capacitor C1 while the second capacitor C2 accumulates charges can be reduced, thereby improving the measurement accuracy of the difference voltage between the first voltage and the second voltage.

[0100] The invention has been described with reference to a first and a second embodiment, but the differential voltage measuring device is not limited to the configurations of these two embodiments.

[0101] For example, in the above-described embodiments, regarding the first constant current 11 and the second constant current 12, a current flowing from the charging unit 115 or the current output unit 220 changes the secondary battery B, but a load current generated by a load connected to the secondary battery B may discharge the battery B.

[0102] It should be noted that the above-described embodiments are merely representative embodiments of the present invention. Therefore, those skilled in the art may make various modifications to the embodiments described herein without departing from the scope of the invention. As long as such modifications can realize the configuration of the differential voltage measuring device of the present invention, the modification is covered by the scope of the invention. List of reference symbols 100 Battery condition detection device (differential voltage measuring device) 111 differential amplifiers 112 changeover switches 113 first capacitor 114 second capacitor 115 Charging unit (power output section) 121 first ADW 122 second ADW 140 µCOM (control unit) 200 differential voltage measuring device 210 µCOM (control unit) 211 switching control unit 220 power output unit 230 changeover switches 240 differential amplifiers 250 ADW 261 Detection object selection switch 262 Reference potential setting switch 270 circuit breakers C1 first capacitor C2 second capacitor

Claims

[1] Differential voltage measuring device (100; 200) for detecting the voltage of a battery (B), comprising: a first capacitor (113; C1) and a second capacitor (114; C2), each formed by a ceramic capacitor, a differential amplifier (111; 240) for outputting a voltage corresponding to a difference voltage between a voltage held by the first capacitor (113; C1) and a voltage held by the second capacitor (114; C2), and a control unit (140; 210) configured to apply a first voltage to the first capacitor (113; C1) and a second voltage to the second capacitor (114; C2), the first capacitor (113; C1) holding the first voltage, and to detect an output of the differential amplifier (111; 240) corresponding to the difference voltage of the first voltage and the second voltage while the first capacitor (113; C1) holds the first voltage and the second capacitor (114; C2) holds the second voltage, wherein in a first time period, the control unit (140; 210) applies a third voltage to the first capacitor (113; C1) and, after in a second time period following the first time period, the control unit (140; 210) stops applying the third voltage to the first capacitor (113; C1) and applies the third voltage to the second capacitor (114; C2), and the first capacitor (113; C1) is discharged, in a third time period following the second time period, the control unit (140; 210) stops applying the third voltage to the second capacitor (114; C2) and applies the first voltage to the first capacitor (113; C1); and in a fourth time period following the third time period, the control unit (140; 210) stops applying the first voltage to the first capacitor (113; C1) and applies the second voltage to the second capacitor (114; C2). [2] Differential voltage measuring device (100; 200) according to claim 1, wherein the second capacitor (114; C2) has a smaller capacitance than the first capacitor (113; C1). [3] The differential voltage measuring device (100; 200) according to claim 1 or 2, wherein the control unit (140; 210) is further configured to control a switch to exclusively switch a connection destination of an input terminal to which a voltage is applied to either the first capacitor (113; C1) or the second capacitor (114; C2), to apply the first voltage or the third voltage to the first capacitor, and, while the first capacitor (113; C1) holds the first voltage or the third voltage, to apply the second voltage or the third voltage to the second capacitor (114; C2). [4] The differential voltage measuring device (100; 200) according to any one of claims 1 to 3, further comprising a current output unit (115; 220) configured to switch a first current supplied for generating the first voltage, a second current supplied for generating the second voltage, and a third current supplied for generating the third voltage to output. [5] Differential voltage measuring device (100; 200) according to one of claims 1 to 4, wherein the first capacitor (113; C1) is discharged by leakage current.

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