JPH09289738A - Battery monitoring circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To protect a battery by controlling its current at both times of charging and discharging, detecting the voltage between its both terminals and discriminating its polarity, and regulating the battery level when the battery is overcharged and overdischarged. SOLUTION: A battery 1 is connected to a load circuit 2 or a charging circuit 3 through transistors 71', 72' connected in inverse series as a current controlling means. On the input side of an error amplifying circuit 10, a reference voltage based on a battery terminal 6 is applied to the inversion input terminal of an error amplifier 10. On the other hand, a load terminal 4 and the noninversion input terminal of the error amplifier 10 are connected through a resistor 26. The output of the error amplifier 10 is connected with the output of an overcharge detecting comparing circuit 9 through diodes 41, 42. In addition, it is connected to the gates of the transistors 71', 72', and controls the transistor 71', 72' at the same time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery monitoring circuit incorporated in a portable device such as a video camera or a laptop computer, for example, a lithium ion battery which is extremely dangerous when overcharged. Of 2
The present invention relates to a battery monitoring circuit most suitable for a secondary battery.

[0002]

2. Description of the Related Art Among rechargeable secondary batteries, a lithium ion battery having a particularly large stored energy per volume has been developed and has recently been adopted as a power source for many portable devices. Although this lithium-ion battery is remarkably superior in terms of performance, it needs to monitor and control the terminal voltage of the battery during charging and discharging rather strictly than the conventional secondary battery, so A battery monitoring circuit having controllability that matches the charge / discharge characteristics is required. For example, if the charging / discharging operation is carelessly applied to the lithium-ion battery, not only the battery life will be significantly shortened, but also 100% of the battery performance cannot be obtained. Furthermore, since this cell uses a flammable electrolytic solution, there is a possibility that a fire accident or a burn injury may occur if it is overcharged. For this reason, the dedicated charger is equipped with overcharge protection, but in the case of an accident such as when it is charged by an inappropriate dedicated charger or when its protection function is insufficient, A general configuration is to install a monitoring and protection circuit on the side of the ion battery to provide redundant protection.

FIG. 5 is a block diagram showing the function of a battery monitoring circuit installed in a conventionally used secondary battery. The battery 1 is connected in parallel to each of the load circuit 2 and the charging circuit 3. In the configuration shown in the figure, a switch means 7 having a switching function is inserted between the load terminal 4 and the connection point 6 of the battery 1. The switch means 7 compares and judges the magnitudes of the reference voltage of the reference voltage generating circuit 8 and the battery voltage, and performs on / off control according to the judgment result. With such a configuration, when the terminal voltage of the battery 1 becomes an abnormal voltage deviating from the specified value, the switch means 7 operates, and the battery 1 is disconnected from the charging circuit 3 or the load circuit 2. Abnormal conditions will be prevented or avoided. In the case of a lithium-ion battery to which a conventional monitoring circuit is applied, the following voltage value is set as a limit value for overcharging or overdischarging. When the charging circuit 3 is connected and charging is started, the potential of the battery terminal 6 starts to rise according to the charging characteristics, and reaches the overcharge voltage set value of 4.2 V after the elapse of the required time,
Since the output level of the comparison circuit 9 changes, the switch means 7 is turned off and the charging is stopped. Even if the battery voltage slightly exceeds this set value, a serious accident may occur. On the contrary, during discharging, the operation is the same as during charging, and the overdischarge voltage set value is selected to be 2.3 V, for example, and the load circuit is disconnected when this set value is detected.

Next, the basic operation of the conventional secondary battery monitoring circuit will be described with a specific circuit configuration. In FIG.
The switch means 7 of FIG. 5 is a field effect transistor (FE
T) 71 and 72 are connected in anti-series, and the comparison circuit 9 is comparators 91 and 92. During charging, the charging circuit 3 is connected to the battery 1 and the FET 72 is on. On the other hand, at the time of discharging, the load circuit 2 is connected, the FET 71 is turned on, and the electric energy accumulated in the battery 1 is discharged to the load. Since it is complicated to explain all the operation modes of the battery monitoring circuit, the description will focus on overcharge and overdischarge. The case of protecting against overcharge will be described first. The voltage obtained by dividing the terminal voltage of the battery 1 by the resistors 21 and 22 is used as the inverting input of the comparator 92, while the reference output Vref of the reference voltage generating circuit composed of the resistor 23 and the constant voltage diode 81 is used as the non-inverting input. Connecting. The voltage division ratio of the terminal voltage and the reference output Vref are selected so that the voltage divided by the resistors 21 and 22 becomes equal to the reference output Vref when the battery voltage reaches the overcharge voltage setting value. Since the output of the comparator 92 is connected to the gate terminal of the FET 72, when the terminal voltage of the battery 1 reaches or exceeds the overcharge set value, one of the inputs of the comparator 92 becomes low (resistor 21
The voltage across the terminals of the
Output goes high and the FET 72 turns off. As a result, the battery 1 is separated from the charging circuit 3 to prevent overcharging.

The actual battery voltage is lower than the applied charging voltage during charging due to the internal resistance r _i of the battery. When the FET 72 is turned off, the current flowing into the battery 1 is blocked and the battery 1
The terminal voltage of is reduced to the battery voltage before blocking. Therefore, the output of the comparator 92 returns from H level to L level, the FET 72 is turned on, and charging is started again. As a result, the terminal voltage of the battery 1 again exceeds the overcharge voltage set value, and the output of the comparator 92 becomes H level. Thus, at the end of charging, FET7
2 The intermittent operation will be repeated. In order to avoid this, the comparator 92 is provided with a hysteresis characteristic so that, once overcharged, the FET 72 is kept off until the terminal voltage of the battery becomes sufficiently lower than the overcharge set voltage value. It is normal. Once overcharged by FET
It is called an overcharged state from the time when 72 is turned off until the terminal voltage of the battery 1 becomes sufficiently low and the FET 72 is turned on again. The relationship described above is shown in FIG. On the other hand, in order to prevent over-discharge, the terminal voltage of the battery 1 is set to the resistance 34
The voltage divided by 35 and 35 and Vref are discriminated by the comparator 91, and the FET 71 is ON / OFF controlled. Like the comparator 92, the comparator 91 is usually provided with a hysteresis characteristic. It should be noted that the switch means in the figure is composed of a P-type MOSFET, but an N-type MOSF is used.
A configuration in which a switch means is provided on the common terminal 5 side by using ET has been conventionally performed.

When MOS type FETs are used as the FET 71 and the FET 72, it is considered that the reverse conducting diodes 71a and 72a are equivalently inserted in parallel because of their structure. When the load circuit is connected and discharged when the FET 72 in the overcharged state is off, the FET 7
Since the reverse conducting diode 72a in parallel with 2 is forward biased, even if the FET 72 is off, the battery 1
Can supply current to the load circuit 2. Of course, when the terminal voltage of the battery 1 is sufficiently lower than the overcharge voltage setting value beyond the hysteresis of the comparator 92, F
Since the ET 72 is in the ON state, discharge can be performed through the FET 72. In addition, when the FE
Even if T71 is off, if the charging circuit 3 is connected, the reverse conducting diode 7 inserted in parallel with the FET 71.
Since the battery 1a is biased in the forward direction, it is possible to charge the battery 1 from the charging circuit 3 through the reverse conducting diode 71a. When the terminal voltage of the battery 1 is sufficiently high beyond the hysteresis of the comparator 91 when the set value of the terminal voltage of the battery 1 is sufficiently high, the FET 72 is in the ON state and thus the battery can be charged through the FET 72.

[0007]

In recent years, portable devices such as telephones, personal computers, audio devices, and video cameras have become widespread, but there is a strong demand for miniaturization and power saving of these devices. Therefore, it is natural to improve the performance of the battery itself, but reducing the loss of the battery monitoring circuit is also an important issue. Further, from the viewpoint of safety, when the battery is a lithium ion battery, there is a risk of heat generation or fire due to overcharging. When the battery terminal voltage becomes, for example, 2.3 V or less due to over-discharging, it generally has a characteristic that it cannot be restored by recharging. Therefore, it is necessary to protect not only overcharge protection but also overdischarge. The allowable error range of the voltage value set for performing such protection is very narrow, and the circuit configuration for realizing this is more complicated. In the configuration of FIG. 6 which is an example of the conventional technique, the FE is used for charging or discharging control as described above.
Each T is turned on or off individually, but in discharging in the overcharge state and charging in the overdischarge state, reverse conduction is connected in parallel to the FET in the off state in each case. Discharge or charging current will flow through the diode.

Since the forward voltage drop of the reverse conducting diode is generally about 0.8 V, internal loss of the reverse conducting diode occurs at both charging and discharging. When the battery 1 is a lithium ion battery, the generated voltage of one cell is about 3.6V, so 0.8V is about 25% of the generated voltage, and the loss due to the reverse conducting diode is a large value that cannot be ignored. is there. In addition, there is a problem in that an FET having an unnecessarily large current capacity must be used because the loss of the reverse conducting diode becomes heat. If it is possible to detect which of the charging circuit or the load circuit is connected to the terminal 4 and control the two FETs 71 and 72 connected in anti-series, the reverse conducting diode connected in parallel to each FET is connected. The internal loss generated as a result can be significantly reduced. However, it is difficult to realize such a configuration for the reasons described below.

The most reliable method for discriminating and determining the load circuit 2 and the charging circuit 3 connected to the load terminal 4 was to detect the voltage difference between the terminals of the switch means 7. That is, in the circuit shown in FIG. 5, when the voltage of the battery terminal 6 is higher than the voltage of the load terminal 4, the load circuit 2 is connected to the load terminal 4, and the battery terminal 6 is also connected.
Is lower than the voltage of load terminal 4, load terminal 4
Is a method of determining that the charging circuit 3 is connected to the. In FIG. 6, when the voltage detecting and comparing means of the battery terminal 6 and the load terminal 4 is provided and the output of the comparator 92 is in the direction of turning off the FET 72 when the overcharge state is set, the voltage of the battery terminal 6 is If it is higher than the voltage of the load terminal 4, it is judged that the load circuit is connected to the load terminal 4, and the FET
72 is for ON control. As a result, the discharge current passes through the FET 72 having a low on-resistance, and it can be expected that the loss due to the reverse conducting diodes connected in parallel is significantly reduced. As a specific circuit, a logic circuit that connects the output of the comparator 91 and the output of the comparator 92, which receives the voltage of the battery terminal 6 and the load terminal 4, may be provided between the gate of the FET 72. Details are omitted when in the over discharge state, but conversely, when the voltage at terminal 6 is lower than the voltage at terminal 4, F
By turning on the ET 71, the loss due to the reverse conducting diode 71a connected in parallel can be reduced.

However, it is actually extremely difficult to accurately compare the voltage difference between the battery terminal 6 and the load terminal 4. When a normal comparison circuit is used for comparing and judging voltage values, the comparison circuit has an offset voltage including noise as is well known, and this offset voltage is 100 mV at maximum.
It is assumed that there is a degree. Therefore, as long as the comparison circuit is used, the offset voltage must be taken into consideration when designing. For example, in order to reliably prohibit charging in the overcharged state, the voltage of the battery terminal 6 is higher than the voltage of the load terminal 4 by the maximum value of this offset voltage (for example, 100 m
It is conceivable to shift the setting of the comparison circuit in consideration of the offset voltage in advance so that it is determined that the load circuit is connected to the load terminal 4 when it is higher than V). In this case, assuming that the ON resistances of the two FETs are about 100 mΩ, when it is necessary to supply a relatively small current to the load circuit, for example, 1 A or less, the voltage drop due to the FET is 1
Since it becomes less than 00 mV, the comparison circuit determines that the load circuit is not connected and cannot attain the intended purpose.
Moreover, in this method, the phenomenon described below was actually caused and stable operation could not be expected. That is, when the voltage between the terminals of the switch means is lower in the load terminal 4 than in the battery terminal 6, the FET 72 is turned on when the FET 72 is turned on.
Since the forward voltage drop due to T is lower than the set voltage,
The comparison circuit uses the charging circuit 3 at the load terminal 4 instead of the load circuit 2.
Is determined to be connected, and the FET 72 is pulled back from the on state to the off state. Then, in the next step, the voltage of the terminal 4 drops, and the comparator circuit again determines that the load circuit is connected and guides the FET 72 to the ON state. As described above, the conventional method has a serious defect that a positive feedback loop is formed and oscillates and the operation of the monitoring circuit shifts to an unstable state. Although the direction of the current is opposite during charging in the over-discharged state, the same operation is performed as is apparent from the above description, and thus the details are omitted.

[0011]

The present invention has led to the idea of a novel invention which has never existed in the prior art as a result of intensive efforts to solve the problems of the prior art described above.
That is, it is possible to achieve energy saving during charging / discharging, ensure stable operation, and configure an optimal monitoring circuit configuration for a secondary battery or the like. The present invention, from the voltage across the current control means to be inserted between the battery and the charging circuit or load circuit,
Detecting whether either the load circuit or the charging circuit is connected, the characteristics of the current control means between the battery and the charging circuit or the load circuit are turned on instead of the on / off operation digitally operated switch. The current control means having an analog switch function in which the impedance changes between off and off is used, and the impedance of the current control means is feedback-controlled by detecting the voltage difference across the current control means. This is a circuit configuration having a characteristic of blocking current.

The present invention can be represented by a block diagram as shown in FIG. The same numbers are given to those having the same functions as those in FIG. The most different point from the prior art is that the voltage difference between both ends of the current control means 70 is amplified by the error amplification circuit 10 as shown in FIG. The present invention that forms such a closed loop has the following features. The voltage of both ends of the current control means 70 is not directly input to the error amplification circuit 10, but the output of the error amplification circuit with respect to the preset reference voltage is adjusted so that the voltage across the current control means matches the reference voltage. It is possible to significantly improve the characteristics by performing feedback control that changes the. As a result, the operation of the current control means becomes close to the ideal diode characteristic, and even if the limitation of noise of the error amplification circuit is taken into consideration, it is possible to operate at a voltage considerably smaller than the forward voltage drop of the PN junction. Become. Therefore, it is obvious that the loss of the reverse conducting diode, which is a problem of the prior art, can be reduced.

In the battery monitoring circuit according to the present invention, in order to solve the above-mentioned problems of the prior art, a simple binary control as a current control means is not an on / off operation, but the impedance is continuously changed from a low impedance state to a high impedance state. It has a characteristic function that changes with a very narrow step width. Although a transistor is used as such a current control means, an FET having a particularly small on-resistance is particularly desirable, and in general, an analog switch having no parallel reverse conducting diode added structurally to the FET may be used. When a bipolar transistor is used, the loss in the forward direction is not much different from that of the diode, so that the loss is not reduced, but the structure of the present invention can realize the characteristic of reliably blocking the reverse current.

As an example of the current control means according to the present invention,
FIG. 2 shows the voltage-current characteristics across the current control means in the overcharged state. The voltage-current characteristic across the current control means according to the present invention in the over-discharged state is a point-symmetrical characteristic with respect to the origin O. In the region where the voltage across the current control means is small, the difference voltage between the battery terminal 6 and the load terminal 4 in FIG. 1 is controlled to be the same potential as the reference voltage Vref by the error amplifying circuit 10 so that it has a constant value. It becomes V1. This is a control characteristic represented by a straight line connecting points Q and R shown in the figure. In this state, if the voltage across both ends is increasing with respect to the increase in current, feedback control is performed by the signal of the error amplifier so as to reduce the impedance of the current control means, and as a result, a constant voltage is maintained. When the current further increases and exceeds I1, the impedance of the current control means becomes the minimum value and does not decrease any more and becomes a constant on-resistance, resulting in the characteristic of the straight line QP. On the other hand, when the current approaches 0, the impedance of the current control means increases, and when it reaches 0, the impedance is turned off. Even if a current flows in the opposite direction, the OFF state is continued because the impedance is further increased. This is the R to S state.

The voltage-current characteristic when the conventional switch means is ON is a straight line having a slope determined by the ON resistance, and is a characteristic obtained by extending the straight line PQ. Further, in the off state, current does not flow even if the voltage changes, so that the characteristic is as if the straight line SR were extended. As described above, it is well understood that if the switch operation is performed in consideration of noise and offset voltage, it may not be possible to discharge in the current range smaller than I1, or the charging current may not be blocked. Even if a low noise, low drift amplifier or the like is adopted, it is considered extremely difficult to reduce the noise and drift voltage to about 0.1 mV or less. That is,
If the on-resistance is 100 mΩ, a charging current of 1 mA may flow into the battery for a long time even after overcharging, resulting in a serious accident such as ignition or explosion. V in the present invention
The value of 1 is determined by the value of Vref, but it needs to be determined in consideration of the noise of the above-described comparison circuit, noise of the same level as the offset voltage, and offset voltage. For example, Vref = 100m
It is apparent from FIG. 2 that if V is set, the noise and the offset voltage can be sufficiently exceeded and the current in the opposite direction can be surely blocked, and that the current I1 or less can be discharged. The lower the actual setting of V1, the smaller the loss due to the current control means,
Even if a low-noise, low-drift amplifier or the like is adopted, it is considered extremely difficult to set the realistic value to at most about 0.1 mV or less. On the other hand, the maximum value is M
The effect of the present invention is weak when the value of the forward voltage drop of the reverse conducting diode connected in parallel to the OSFET is about 0.8 V or more. That is, the set voltage of V1 is 0.1 mV to 0.
A remarkable effect can be achieved between 8V.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to FIG. The battery 1 is connected to the load circuit 2 or the charging circuit 3 via MOSFETs 71 'and 72' connected in anti-series as current control means. Charging circuit 3 and load 2
3 may be connected in parallel in FIG. 3, or may be connected individually. Error amplifier circuit 10
On the input side of, the reference voltage seen from the battery terminal 6 is connected to the inverting input terminal of the error amplifier 10, while the load terminal 4 and the ratio inverting input terminal of the error amplifier 10 are connected via the resistor 26. In the figure, a reference voltage composed of the resistor 23 and the constant voltage diode 81 is shown, but as a reference voltage having a good temperature characteristic, a band which is often used in an IC incorporating a reference voltage typified by a shunt regulator or the like. Adopting a gap type reference voltage is desirable. The output of the error amplification circuit 10 is connected to the output of the overcharge detection / comparison circuit 9 through the diodes 41 and 42. Further MOS
MOS connected to the gates of FETs 71 'and 72'
The FETs 71 'and 72' are controlled simultaneously. Although the control of the MOSFET by the error amplifier 10 is analog control, the MOSF by the overcharge comparison circuit 9 is controlled.
The ET control operates as a switch that only turns on and off. The characteristics of the discharge current to the load and the voltage across the current control means 70 when the output of the overcharge comparison circuit 9 is at the H level in the overcharged state are controlled as shown in FIG. Further, when the voltage of the battery 1 is equal to or lower than the overvoltage value and the output of the overcharge comparison circuit 9 is at the L level, the MOSFETs 71 'and 7 are independent of the operation of the error amplification circuit 10, as is apparent from FIG.
2'is in the on state.

In the above embodiment, the minute voltage setting value V1 is set to 5
Although it was set to 0 mV, this minute voltage setting value is 100 μV
It was found that stable operation becomes difficult when the value is less than the value. Further, when the minute voltage setting value is 0.8 V or more, it is the same as the forward loss of the reverse conducting diode, so that the effect of the present invention is lost. In FIG. 3, the P-channel MOS is provided on the plus side of the load circuit 2 and the charging circuit 3.
Although the case where the FETs 71 'and 72' are connected in anti-series is shown, as another embodiment of the present invention, the N-channel MOSFET 7 is provided on the negative side of the load circuit 2 and the charging circuit 3.
The case where 1'and 72 'are connected in anti-series is shown in the circuit diagram of FIG. The operation of the over-discharge protection of FIG. 4 will be described below. In FIG. 4, the voltage of the battery 1 is equal to that of the resistors 30 and 22.
And 21 are divided. At this time, the ON resistance of the FET 75 is sufficiently smaller than the resistances 30, 22, and 21. Resistance 3
The comparison circuit 92 compares and determines the intersection voltage of 0 and 22 and the reference voltage of the intersection of the resistor 23 and the constant voltage diode 81. When the voltage of the battery 1 is within the normal range, the divided voltage is higher, so the output of the comparison circuit 92 is at the L level. Further, since one input is at the L level by the operation of the AND circuit 85, the output of the AND circuit 85 is at the L level and the transistor 75 is turned on regardless of whether the transistor 74 is turned on or off. .

When it is determined that the voltage of the battery 1 has dropped below the over-discharge voltage value and the battery is over-discharged, the divided voltage becomes lower than the reference voltage, so that the output of the comparison circuit 92 becomes H level. At this time, if the current direction is the load direction, the transistor 74 is off. As a result, since both inputs of the AND circuit 85 are at H level, the output becomes H level and the transistor 75 is turned off. When the transistor 75 is turned off, the AND circuit 85, the resistors 31, 32 and 33, and the circuits other than the transistor 74 are held at the potential of the battery terminal 6. At the same time, power supply to unnecessary circuit elements shall be stopped. At this time, the inputs of the AND circuit 85 are both at the H level due to the resistors 31 and 32, so that the transistor 75 holds the OFF idle state.

The return from the rest state depends on the charging condition. Since the MOSFETs 71 ′ and 72 ′ are off, the potential difference between the voltage of the terminal 5 and the voltage of the battery 1 is applied between the gate and the source of the transistor 74 at the same time when the charging voltage is applied, and the transistor 74 is turned on. . When the transistor 74 is turned on, one input of the AND circuit 85 becomes L level, the output of the AND circuit 85 becomes L level, and the transistor 75 is turned on, so that power is supplied to all the circuits to start normal operation. . In this state, the operation of the error amplifier 10 is to prevent discharge,
The operation principle is similar to that of the error amplifier 10 in FIG. 3, and detailed description thereof will be omitted.

[0020]

According to the present invention, it is possible to configure a battery monitoring circuit capable of determining whether the battery is in a discharged state or a charged state. By controlling a semiconductor element that can be turned on and off bidirectionally (for example, a semiconductor element that is configured with field effect transistors connected in anti-series and is operatively one), overcharge and overdischarge are prevented. A monitoring circuit that can protect the battery can be configured. Furthermore, as shown in the voltage-current characteristics of FIG. 2, the loss of the MOSFET and the like can be suppressed to a minimum, and a stable voltage can be supplied to the load even during a minute current load, and the overcharge can be reliably performed even during a minute current charge. Can be protected from The present invention also provides a battery monitoring circuit that can protect the secondary battery from over-discharge.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing the principle of the present invention.

FIG. 2 is a characteristic operation explanatory diagram of a current control unit in the present invention.

FIG. 3 is a circuit diagram showing an embodiment according to the present invention.

FIG. 4 is a circuit diagram showing another embodiment of the present invention.

FIG. 5 is a circuit block diagram of a conventional method.

FIG. 6 is a circuit diagram of a conventional method.

FIG. 7 is a characteristic operation explanatory diagram of a comparison circuit which is a conventional technique.

[Explanation of symbols]

1 battery, 2 load circuit, 3 charging circuit, 4 load terminal, 5 common terminal, 6 battery terminal, 7 switch means,
70 current control means, 71, 72 MOSFET, 7
4, 75 transistors, 8 reference voltage, 9, 91, 9
2 comparison circuit, 10 error amplification circuit, 21-36 resistance, 41-44 diode, 81 constant voltage diode, 85 AND circuit.

Claims (7)

[Claims] 1. A battery monitoring circuit for inserting a current control unit between a secondary battery and a charging circuit or a load circuit to monitor the charging or discharging state of the secondary battery and controlling the current control unit when an abnormality occurs. In the above battery, the current control means can control the current during both charging and discharging, detects the voltage across the current, and determines the polarity to perform control during overcharging or overdischarging. Monitoring circuit. 2. The battery monitoring circuit according to claim 1, wherein a voltage characteristic across the current control unit has a forward voltage drop characteristic lower than a PN junction of a diode. 3. The battery monitoring circuit according to claim 2, wherein the voltage characteristic across the current control means has a constant voltage region and a region proportional to the passing current. 4. The battery monitoring circuit according to claim 1, wherein the current control means has a characteristic that its impedance changes at least continuously. 5. The method according to claim 3 or 4,
The battery monitoring circuit is characterized in that the current control means is controlled by the output of the error amplification circuit, and the current control means and the output of the reference voltage generation circuit are connected to the input of the error amplification circuit. 6. The battery monitoring circuit according to claim 5, wherein the input of the error amplifier is set so as to control the voltage across the current control means within a range of 0.1 to 800 mV. 7. The battery monitoring circuit according to claim 4, wherein the current control means is constituted by an anti-series connection of field effect transistors.