JPH06150977A - Charging circuit - Google Patents

Abstract

PURPOSE:To provide a charging circuit excellent in safety and not charged to the battery voltage exceeding the withstand voltage after a charge is completed by improving the charging circuit, and connecting a Zener diode in parallel with a lithium secondary battery. CONSTITUTION:An AC power source AC is 200V, the resistance value of a resistor R is 15 kOMEGA, the withstand voltage of a rectifying diode Ds is 220V, the breakdown voltage Vo of a Zener diode ZD is about 3.5V. A lithium secondary battery B is a vanadium-lithium secondary battery using vanadium pentaoxide for a positive electrode and a lithium aluminum alloy for a negative electrode, the voltage is 3V, and the capacity is 20 mAh. The secondary battery B is mainly charged at 3 to 3.3V, and the voltage quickly rises after a charge is completed. When the voltage reaches near 3.5V in this charging circuit, a current flows in the Zener diode ZD, the current rarely flows in the lithium secondary battery B, and the battery voltage is stabilized near 3.5V.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging circuit for a backup power source in a device using a rechargeable backup power source, such as a microcomputer-equipped device.

[0002]

2. Description of the Related Art In recent years, the number of electronic devices equipped with a microcomputer tends to increase rapidly, but various memory backup power supplies are used so that important memories are not erased when the power is turned off.

Examples of the memory backup power source include a lithium primary battery capable of discharging only, a dry battery, a chargeable / dischargeable nicad battery, a lithium secondary battery, a capacitor and the like.

Of these, the lithium secondary battery has an output voltage of 3 V or more, which is higher than that of the NiCd battery and the capacitor.
A memory IC that requires an operating voltage of 2.5 V or higher can be handled with a single battery, resulting in cost and space advantages. Further, it has a small self-discharge as a battery and is excellent in long-term reliability, and has recently attracted attention as a power source for memory backup.

FIG. 2 shows a conventional charging circuit for directly charging a nickel-cadmium battery from an AC power source.

As shown in FIG. 2, the conventional charging circuit is
Secondary battery B, overcurrent prevention resistor R, rectifying diode D
It is a circuit in which _S is connected in series. Then, the charging method is trickle charging, and a relatively large resistance is used to limit the charging current flowing into the battery after completion of charging.

Further, this charging circuit is peculiar to the NiCd battery, and the battery voltage does not rise above a certain value even though the charging current continues to flow after the completion of charging.

This is because, when the battery voltage reaches a certain value or more in the charging completed state, the charging current is used for the reaction of absorbing oxygen generated from the positive electrode inside the battery at the negative electrode and consumed. Therefore, the deterioration of the battery due to charging was small.

[0009]

However, when the lithium secondary battery is charged in the same manner as described above, the battery voltage continues to rise even after the charging is limited, even if the charging current is limited.

Then, the battery voltage rises until it exceeds the withstand voltage limit of the lithium secondary battery, and the battery is deteriorated due to decomposition of the electrolytic solution and corrosion of the battery case.

Further, the AC power supply usually has a high voltage of 100 V or 200 V. Therefore, when the overcurrent preventing resistor is broken, a large current may flow into the charging circuit and the battery may burst.

The present invention solves such a problem by ensuring that a lithium secondary battery is charged at an appropriate charging voltage and that the battery does not exceed its withstand voltage after completion of charging. It provides an excellent charging circuit.

[0013]

In order to solve the above-mentioned problems, the charging circuit of the present invention comprises an AC power supply and a series circuit of a resistance for overcurrent prevention, a diode for half-wave rectification and a lithium secondary battery. In the connected charging circuit, a Zener diode is connected in parallel to the lithium secondary battery, and the breakdown voltage of the Zener diode is set to substantially match the maximum allowable voltage of the lithium secondary battery.

[0014]

The current-voltage characteristic when the pn junction of the Zener diode is reverse biased is shown in FIG. As shown in FIG. 3, the Zener diode has a property that the current does not flow up to the breakdown voltage V ₀ but the current suddenly starts from around the voltage V ₀ .

Then, the Zener diode and the lithium secondary battery are connected in parallel in the charging circuit and combined so that the breakdown voltage V ₀ of the Zener diode and the maximum allowable voltage of the lithium secondary battery are substantially equal to each other.

When a lithium secondary battery is charged by the charging circuit configured as described above, no current flows through the Zener diode in the voltage region where the battery is charged (voltage lower than voltage V ₀ ) and the lithium secondary battery is charged. Current flows only in the battery, and the battery is charged.

When the charging of the lithium secondary battery is completed and the battery voltage rises to around V ₀ , a current suddenly starts to flow in the Zener diode and almost no current flows in the battery. In this way, the voltage of the lithium secondary battery can be controlled within the maximum allowable range after completion of charging.

Therefore, it is possible to prevent the current from continuing to flow in the battery even after the charging is completed, and the battery voltage is increased above the withstand voltage limit to cause decomposition of the electrolytic solution or the like, and the lithium secondary battery is safely charged. can do.

Further, in this circuit, after charging is completed, when a large resistance load is applied to the battery for discharging, a leakage current flows through the Zener diode, and the current flowing through the load is several tens of μA.
Becomes smaller.

When the leakage current flowing through the Zener diode with respect to this load current is relatively large, the leakage current is prevented by connecting a reverse current prevention diode between the lithium secondary battery and the Zener diode. be able to.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 shows a charging circuit of the present invention.

As shown in FIG. 1 (a), in the charging circuit of the present invention, a series circuit of a resistor R, a rectifying diode D _S and a lithium secondary battery B is connected to an AC power supply AC and a Zener is connected. The diode ZD is connected in parallel to the lithium secondary battery B.

Here, the AC power supply AC is 200 V, and the resistance R
Has a resistance value of 15 kΩ, the rectifying diode D _S has a withstand voltage of 220 V, and the Zener diode ZD has a breakdown voltage V ₀ of about 3.5 V.

The lithium secondary battery B is a vanadium lithium secondary battery using vanadium pentoxide for the positive electrode and a lithium aluminum alloy for the negative electrode, and its nominal specifications are a voltage of 3 V and a capacity of 20 mAh.

FIG. 4 shows a state in which this lithium secondary battery is charged by the charging circuit of the present invention.

As shown in FIG. 4, the lithium secondary battery was charged mainly between 3V and 3.3V, but the voltage rose sharply after the completion of charging. However, when the voltage reached around 3.5V, a current began to flow through the Zener diode ZD, almost no current flowed through the lithium secondary battery B, and the battery voltage became stable around 3.5V.

After charging is completed, the AC power supply AC is turned off, a resistance of 15 kΩ is connected to the lithium secondary battery B, and the battery is discharged at a current of about 200 μA, the average discharge voltage of the battery is about 2. The capacity became 20 mAh at 8V.

As described above, proper charging could be performed without the battery voltage of the lithium secondary battery exceeding the withstand voltage limit after the completion of charging.

(Embodiment 2) After charging the lithium secondary battery B with the charging circuit shown in Embodiment 1, the AC power supply AC
When the resistor is turned off and a resistor with a resistance of 150 kΩ is connected and the battery is discharged at a current of about 20 μA, the discharge capacity is 16.4 m.
It decreased to Ah.

This is because the leakage current of the Zener diode flows by about 5 μA when the voltage is 3V. In order to prevent the leakage current of the Zener diode, a reverse current preventing diode D is connected between the Zener diode ZD and the lithium secondary battery B. The charging circuit at this time is shown in FIG. The diode D was a Schottky barrier type and had a voltage drop of about 0.1 V when the current was several μA or less.

By the charging circuit shown in FIG. 1 (b),
When the lithium secondary battery B was charged, the battery voltage was stable at about 3.4 V after the completion of charging.
In this circuit, after charging is completed, the AC power source A
With C turned off, the lithium secondary battery B has a resistance value of 15
When a resistor of 0 kΩ is connected and discharged at a current of about 20 μA, the average discharge voltage of the battery is about 2.8 V and the capacity is 20 mAh.
Became.

As a result, the current flows to the Zener diode.
It was possible to prevent leakage current. (Third Embodiment) In the charging circuit shown in the first and second embodiments.
As a resistance for overcurrent prevention, a resistor R ₁And R₂Connect directly
The circuit used is shown in FIG. The resistance R₁And R₂
The resistance value of each was 7.5 kΩ.

As described above, when two or more resistors R for preventing overcurrent are connected in series and used, even if one resistor is broken, it is possible to prevent a large current from flowing from the AC power supply into the circuit. Therefore, it is possible to prevent the risk of the battery bursting.

Comparative Example As shown in FIG. 2, a lithium secondary battery B is charged by a charging circuit in which an AC power source AC, a resistor R, a rectifying diode D _S and a lithium secondary battery B are connected in series. did. In this case, the battery voltage rises to 100 V or more after the completion of charging, the decomposition of the electrolytic solution occurs,
The battery swelled or burst. In this example, the vanadium lithium secondary battery was used as the lithium secondary battery, but the same effect can be obtained even with other lithium secondary batteries such as carbon lithium secondary battery and manganese lithium secondary battery. can get.

[0036]

As described above, in the charging circuit of the present invention,
Since the Zener diode is connected in parallel to the lithium secondary battery and the breakdown voltage V ₀ of the Zener diode is set to substantially match the maximum allowable voltage of the lithium secondary battery, charging of the lithium secondary battery is completed. After that, when the battery voltage rises to around V ₀ , a current suddenly starts to flow in the Zener diode and almost no current flows in the battery.

Therefore, even if the current continues to flow to the battery after the completion of charging, the battery voltage does not rise until it exceeds the withstand voltage limit, and it is possible to prevent the lithium secondary battery from expanding or bursting, and to prevent the lithium secondary battery from expanding. Can be properly charged.

[Brief description of drawings]

FIG. 1A is a diagram showing a first example of the charging circuit of the present invention, FIG. 1B is a diagram showing a second example of the charging circuit, and FIG. 1C is a diagram showing a third example of the charging circuit.

FIG. 2 is a diagram showing a conventional charging circuit.

FIG. 3 is a diagram showing a current-voltage characteristic of a Zener diode.

FIG. 4 is a diagram showing a charge curve when a lithium secondary battery is charged and a discharge curve when the battery is discharged using the charging circuit of the present invention.

[Explanation of symbols] AC AC power supply B Secondary battery D _S Rectifying diode ZD Zener diode D Reverse current prevention diode R, R ₁ , R ₂ resistance

Claims (3)

[Claims] 1. A charging circuit in which a series circuit of an overcurrent prevention resistor, a half-wave rectification diode, and a lithium secondary battery is connected to an AC power supply line, and a Zener diode is connected in parallel to the lithium secondary battery. . 2. The charging circuit according to claim 1, wherein two or more resistors are connected in series as the overcurrent preventing resistor. 3. The charging circuit according to claim 1, wherein a reverse current preventing diode is connected between the lithium secondary battery and the Zener diode.