JPS6336219B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a charger for a rechargeable secondary battery (battery), and is particularly characterized by overcharging prevention means in a short-time quick charger using a constant current charging method. It is something that you have.

The demand for secondary batteries has increased with the increase in portable devices, especially portable video tape recorders (hereinafter referred to as "portable video tape recorders") that require relatively large currents.
Most of the power supplies for VTRs (referred to as VTRs) use secondary batteries. The secondary batteries used in portable VTRs are generally sealed lead-acid batteries (Pb batteries) based on past usage results.
This is disadvantageous for portable devices that aim to be smaller and lighter in terms of volume. On the other hand, sealed nickel-cadmium storage batteries (NiCd batteries) have very low internal resistance, so they have excellent discharge characteristics at large currents, stable characteristics over a relatively wide temperature range, and other advantages. Compared to batteries, NiCd batteries are easier to charge quickly and have a charge/discharge cycle life of 300 to 500 times or more, and many recent portable devices tend to use NiCd batteries.

Rapid charging methods for NiCd batteries can be roughly divided into constant voltage charging methods and constant current charging methods.

A typical constant voltage charging method is the joggle charging method, which charges the battery with a large current until the battery terminal voltage reaches a predetermined value, and then flows an intermittent pulsed charging current when the voltage exceeds the predetermined value. This is a method in which the charging current is reduced in response to a small voltage increase in the battery terminal voltage. This method has the advantages of efficient rapid charging and the ability to prevent overcharging, but on the other hand, it requires a highly sensitive voltage switch and a high frequency converter, making the circuit complex, and it requires a pulsed charging current. It has drawbacks such as the need to shield noise from airborne noise and the need for a line filter to prevent noise from being superimposed on the power supply line.

On the other hand, with the constant current charging method, with the advancement of semiconductor technology in recent years, high performance transistors, operational amplifiers, etc. have become available at low cost, so a constant current circuit can be configured with a simple circuit configuration. This charging method is being reconsidered because it handles continuous current, so it does not require noise countermeasures, which is a problem with the joggle charging method.

A typical constant current charging method is a method commonly called a V-taper method. In this method, the battery is charged with a nearly constant large current until the terminal voltage reaches a predetermined value, and when the terminal voltage exceeds the predetermined value, charging with the large current is stopped, and the CR discharge curve consisting of a capacitor and a resistor is used. This is an efficient rapid charging method that reduces the charging current.

Before explaining the details of the present invention, first, a V-taper type charging method will be explained.

FIG. 1 shows a charging pattern using the V taper method, and FIG. 2 shows a specific circuit example for obtaining the charging characteristics shown in FIG. 1.

In FIG. 1, the horizontal axis shows charging time, and the vertical axis shows battery terminal voltage and charging current. When a sufficiently discharged battery is charged using the V taper method, the charging current shows a change shown by i in FIG. 1, and the terminal voltage of the battery shows a change shown by υ.

During the period from time t ₀ to _{t 1} at the initial stage of charging, constant current charging is performed with a relatively large current corresponding to the battery capacity of 1C. At this time, the terminal voltage of the battery gradually increases as charging progresses, and when the charging capacity reaches around 0.7C, the terminal voltage increases rapidly. If you continue charging with a large current of around 1C in this state,
Gas generation causes a sudden increase in the pressure and temperature inside the battery. Sealed NiCd batteries are usually equipped with a safety valve, so they will not explode even if the internal pressure rises abnormally. However, once the safety valve is activated and releases gas, the battery's capacity deteriorates. In addition, continuous overcharging due to large currents can cause abnormal temperature rises and explosions due to ignition of hydrogen gas, so it is absolutely necessary to avoid using the battery in an unreasonable manner.

The factors that detect the completion of battery charging are voltage,
It is limited to three things: temperature and internal pressure. Detection of internal pressure requires a pressure detection element, and an inexpensive detection method has not yet been realized. A generally widely used method is a voltage detection method, and in addition to this method, a temperature detection method is also used. The purpose of temperature detection at this time is to change the voltage detection level in accordance with changes in ambient temperature. Since the temperature detection method is different from the gist of the present invention, detailed explanation will be omitted here.

In FIG. 1, from the point at which the terminal voltage of the battery reaches the control start voltage υc, that is, from time _t1 until time _t2 , the charging current is gradually decreased along the CR discharge curve. Control start voltage υc
is set to a voltage that has some margin from a safe voltage that does not cause the internal pressure of the battery or the temperature of the battery to rise abnormally. If the CR discharge curve is made steep, it will take a long time to reach full charge, and if it is made too slow, the terminal voltage of the battery will rise above the control start voltage due to the charging current after _t1 . Therefore, the CR discharge curve shows that the battery terminal voltage after t ₁ is below the control start voltage, and
The curve is set to be as slow as possible.

The charging current after time _t2 is set to a current value of 0.1C or less, which will not cause any abnormality to the battery even if it is continuously charged for a long time.

If the charging current is set as explained above, rapid charging can be performed in a relatively short time and safely. Next, a specific circuit example for realizing the charging pattern shown in FIG. 1 will be explained using FIG. 2.

In FIG. 2, 1 indicates a battery to be charged; for example, 10 batteries are connected in series. 2 is a constant current charging circuit, which charges the battery with a constant current during the time _t0 to _t1 shown in FIG. The constant current charging circuit 2 is a well-known circuit composed of, for example, an operational amplifier, and detects the current flowing through the battery 1 as a voltage drop across a resistor _R7 having a relatively low value (for example, 1Ω), and detects the voltage at point b. The charging current is controlled so that the value and the reference voltage value at point c become equal. Transistor Q ₁ is charged initially from t ₀ to _{t 1}
It is in the OFF state during the period. Therefore, the reference voltage at point c is a value obtained by subtracting the voltage drop across diode _D1 from the potential at point d determined by the division ratio between resistors _R2 and _R3 .

The first voltage detection circuit 3 is a circuit that detects the terminal voltage of the battery, and outputs the voltage characteristic υ shown in FIG. 1 as an output e. The second voltage detection circuit 4 is a Schmitt circuit with hysteresis characteristics, and the output f of this circuit 4 is generated during the period _t0 to _t1 .
Outputs low voltage. Therefore, transistor Q ₁
is in the OFF state. When the terminal voltage e of the battery reaches the control start voltage υc, the output f of the voltage detection circuit 2 becomes High voltage, and the transistor _Q1
It becomes ON state. At this time, assuming that the saturation voltage between the collector and emitter of transistor _Q1 is zero,
The potential at point d is determined by the parallel resistance value of resistors R ₃ and R ₄ and the division ratio of resistor R ₂ . The potential at point d at this time is set so that the output current of constant current charging circuit 2 is approximately 0.1C. That is, the charging current value after time _t2 is determined. Transistor Q ₁ is ON
During the period from time t ₁ to t ₂ , the potential at point d becomes lower than the potential at point c. This is because the potential at point d drops immediately when transistor Q ₁ turns on, but the potential at point c is determined by the amount of charge accumulated in capacitor C ₁ and does not drop immediately. do not. Assuming that the input impedance of the constant current charging circuit 2 is almost infinite, the charge accumulated in the capacitor _C1 is discharged only through the resistor _R1 . Since the charging current to the battery is determined by the potential at point c, the charging current between time t ₁ and _{t 2} gradually decreases in accordance with the discharge characteristics of C ₁ and R ₁ . When the potential at point c reaches a potential lower than the potential at point d by the voltage drop of the diode _D1 , the potential at point c becomes constant from then on, and the current value after time _t2 is set.

Note that the battery terminal voltage after time _t1 is
It stabilizes at a potential that is a certain amount lower than the control start voltage υc. At this time, this circuit 4 is provided with a hysteresis characteristic so that the output f of the second voltage detection circuit 4 does not become Low again.
The input threshold level for reaching the potential is set, for example, to the potential υh shown in FIG.

The above is the charging pattern and specific circuit example of the V taper method. This method can handle continuous charging current with a relatively simple circuit configuration.
Although it has the advantage of being able to effectively shorten charging time by utilizing the CR discharge characteristics, the current circuit method is inadequate in terms of recharging the battery.

As already explained, if the CR discharge curve shown in Figure 1 is made steep, it will take a long time to reach full charge, and if it is made too slow, the terminal voltage of the battery after _t1 will exceed the control start voltage. Therefore, the terminal voltage of the battery after _t1 is set to be less than the control start voltage, and the curve is set to be as gradual as possible. but,
The relationship between the charging current and the terminal voltage of the battery after t ₁ changes in relation to the charging history of the battery up to that point. For example, when recharging a battery that has been fully discharged to about 1.0V per cell, the charging pattern shown in Figure 1 is used, but if the battery has only a small amount of discharge or is in a fully charged state. When the battery is forcibly recharged, the charging pattern shown by the solid line in FIG. 3 is obtained.

In FIG. 3, the terminal voltage of the battery forcibly started to be recharged at time t ₀ rises rapidly and reaches the control start voltage υc in a short time. At this point, the aforementioned second voltage detection circuit 4 is activated,
The charging current decreases along the CR discharge curve shown by the solid line _i1 . However, when recharging a battery that is nearly fully charged, the CR discharge curve i ₁ , which efficiently charges a fully discharged battery, is too slow.
The battery terminal voltage does not drop immediately, and the solid line
It changes as shown in ₁ . At this time, the battery terminal voltage increases beyond the safe voltage, and the internal pressure of the battery also increases. As a result, the safety valve will operate and the battery capacity will deteriorate. When recharging a battery that is close to fully charged, it is sufficient to use the steep CR discharge curve shown by the broken line _i2 , and the change in terminal voltage at this time will be as shown by the broken line, making it possible to obtain characteristics that do not exceed the control start voltage. can.

However, it is not easy to change the CR discharge curve depending on the charging history. For this reason, as mentioned above, conventional charging circuits have a configuration in which the voltage detection circuit 2 has a hysteresis characteristic, and a battery that is nearly fully charged is not recharged with a large current. This method was inconvenient for those who actually use batteries. because,
When actually operating equipment, there are times when the equipment is operated until the battery is fully discharged, but after operating the equipment for a suitable period of time, it is often necessary to recharge the battery in preparation for the next operation. This is because it occurs.

By adding a simple circuit, the present invention
It can be recharged regardless of the past discharge amount, it can also be used as an overcharge prevention circuit, and after detecting the control start voltage, it automatically reduces the charging current according to the remaining charge amount, and then follows the CR discharge curve. It has characteristics such as being able to perform charging with high efficiency.

Hereinafter, specific examples of the present invention will be described.

FIG. 4 shows a specific circuit example of the present invention. The circuit shown in FIG. 4 is the circuit shown in FIG. 2 with the addition of a circuit block 6 surrounded by a dotted line, that is, a third voltage detection circuit 5 and a transistor _Q2 . Components with the same symbols as in FIG. 2 have the same functions.

The third voltage detection circuit 5 is a voltage detection circuit without hysteresis characteristics, and if the output voltage of the voltage detection circuit 3 is at a level equal to or higher than a certain set voltage value,
Outputs High voltage, and if it is below, outputs Low voltage.
This set voltage value is set to a safe voltage that is slightly higher than the above-mentioned control start voltage and does not cause the internal pressure of the battery to rise abnormally. When the terminal voltage of the battery is below the safe voltage, the output g of the third voltage detection circuit 5 generates a low voltage and the transistor
Q ₂ is in the OFF state. When the terminal voltage exceeds the safe voltage, the output g becomes a high voltage and the transistor _Q2 is turned on.

A charging pattern when the circuit block 6 that performs the above operation is added will be explained.

FIG. 5 shows a charging pattern when a nearly fully charged battery is forced into a recharging state. A method for forcibly recharging may be a method of canceling the hysteresis characteristic of the second voltage detection circuit 4. This method may be performed automatically when the battery is placed in the charger, or a manual recharging switch may be provided (not shown). In Figure 5,
When recharging is performed from time _t0 , the battery is charged with a large charging current _i3 near the rated capacity of 1C. During this period, the terminal voltage of the battery increases rapidly and reaches the control start voltage υc at time _t1 . At this time, the second voltage detection circuit 4 shown in FIG. 4 is activated, and the voltage decreases in accordance with the CR discharge characteristics between _t1 and _t3 . Time _t3 is the time when the battery terminal voltage reaches the aforementioned safe voltage υs. If the third voltage detection circuit 5 according to the invention is not added, the time t ₃
Since a relatively large charging current continues to flow along the CR discharge curve, the terminal voltage of the battery increases as shown by the broken line in FIG. 5.

However, according to the present invention, when the terminal voltage of the battery reaches the safe voltage υs, the third voltage detection circuit 5 is activated and the transistor _Q2 is turned on. Therefore, the charge accumulated in the capacitor _C1 is discharged through the resistor _R10 , which is relatively small compared to the resistor _R1 , and the transistor _Q2 , and the charging current to the battery reaches the current value shown by _i5 in Figure 5. Decrease. The charging current decreases and the battery terminal voltage drops to the safe voltage υs.
When the voltage drops below this level, transistor Q ₂ turns OFF,
The charging current decreases according to the original discharge curve of C ₁ and R ₁ . The current value _i5 is the maximum current value in a state where the terminal voltage of the battery does not exceed the safe voltage υs, and varies depending on the remaining charge level of the battery before recharging. According to the present invention, by adding the circuit block 6 shown in FIG. 4, the value of this current value _i5 can be automatically determined. This is because the transistor _Q2 in the circuit block 6 turns on when the terminal voltage of the battery reaches the safe voltage _vs , discharging the charge accumulated in the capacitor _C1 . If we ignore the on-resistance of transistor Q ₂ , the discharge curve in this case is the resistance R ₁₀ and capacitor C ₁
The discharge curve is determined by Although this discharge curve is shown as a linear current change over time _t3 in FIG. 5, it actually has a predetermined short time. By discharging the charge accumulated in the capacitor C ₁ through the resistor R ₁₀ , the charging current decreases rapidly over time. As the charging current decreases, the terminal voltage of the battery also changes. If the charging current is larger than the maximum current value i ₅ at which the battery terminal voltage does not exceed the safe voltage v _s , the battery terminal voltage will be even higher than the safe voltage v _s , so the transistor Q ₂ will It remains on and the current value further decreases. When the current value decreases to the value shown by i ₅ , transistor Q ₂ becomes OFF, and from then on the charging current changes to C ₁ , R ₁ as shown by i ₆ .
It changes along the discharge curve determined by . The value of _i5 varies depending on the remaining charge in the battery before recharging.
This is because when the charging current value is constant, the terminal voltage of the battery changes depending on the remaining charge level of the battery. In other words, if there is a large amount of remaining charge, the terminal voltage will rise rapidly, and if there is little, the terminal voltage will change slowly. When the terminal voltage rises suddenly,
Even if the charging current value is decreased along the discharge curve determined by C ₁ and R ₁₀ , the terminal voltage becomes larger than the safe voltage _vs by a very small value. Therefore, it takes a predetermined time for the terminal voltage to become equal to or less than the value of _VS , and this time is longer than the time required for a similar operation by a battery that exhibits a slow terminal voltage change. Since this time corresponds to the time to keep the transistor _Q2 in the ON state, that is, the time to discharge the capacitor _C1 , it appears as the value of the current _i5 . Therefore, the circuit block 6 shown in the present invention
By adding , the current value i ₅ is automatically set according to the remaining charge amount.

After time _t3 , the battery terminal voltage gradually decreases. Since the third voltage detection circuit 5 does not have hysteresis characteristics, if the battery terminal voltage falls below the safe voltage υs, its output g becomes a Low voltage and the transistor Q ₂ becomes OFF. In other words, this is equivalent to the state in which the circuit block 6 is not added.

Therefore, the charging current between _t3 and _t2 gradually decreases according to the discharge curve determined by _C1 and _R1 . This shows that when the terminal voltage of the battery exceeds the safe voltage υs, charging can be sustained more efficiently than when the charging current is reduced to zero or extremely reduced. After time _t2 , charging is performed at a constant current of 0.1C or less, as described above.

Figure 6 shows when batteries with different remaining charge levels are recharged using a charger equipped with the circuit according to the present invention.
The charging pattern near t ₃ is shown. When a sufficiently discharged battery is recharged, the terminal voltage of the battery decreases as shown by the broken line υ ₈ after the control start voltage υc, and the charging current at this time follows the CR discharge curve as shown by i ₈ . decreases along. When a battery that is nearly fully charged is recharged, the terminal voltage of the battery rises to the safe voltage υs, and the charging current characteristic is expressed as i ₉ or i ₁₀ depending on the remaining charge before recharging. You will get it.

As is clear from the above explanation, according to the present invention, when the battery terminal voltage reaches a safe voltage, the charge accumulated in the capacitor that determines the charging current is discharged, thereby preventing overcharging of the battery. When the battery terminal voltage reaches a safe voltage, the charging current can be automatically reduced according to the remaining charge before recharging.
Recharging is possible regardless of the amount of discharge, and after detecting a safe voltage and once reducing the charging current, the charging current decreases according to the CR discharge curve, so efficient charging is possible even when recharging. , and other advantages.

Although an explanation of temperature compensation for the battery has been omitted here, the temperature rise of the battery is detected using an element such as a thermistor whose resistance value changes depending on the temperature, and the threshold level of the voltage detection circuits 4 and 5, i.e. Changing the level of the control start voltage or the safety voltage may be performed as appropriate in the same manner as in the conventional voltage detection method, and does not go against the spirit of the present invention.

In addition, in the explanation so far, we have explained a specific example of a circuit using a time constant circuit that discharges the charge accumulated in a capacitor, but the same result can be obtained even if a time constant circuit that causes charge to accumulate in a capacitor is used. It's obvious what you'll get.

[Brief explanation of the drawing]

Figure 1 shows a charging pattern using the V taper method, Figure 2 is a specific circuit diagram for realizing the conventional V taper method, and Figure 3 shows the relationship between battery terminal voltage and charging current during recharging. FIG. 4 is a diagram showing an embodiment of the charging circuit according to the present invention, FIG. 5 is a charging pattern diagram when using the charging circuit according to the present invention, and FIG. It is each charging current characteristic diagram when charging. 1...Battery, 2...Constant current charging circuit,
3, 4, 5... Voltage detection circuit.

Claims (1)

[Claims] 1. A voltage setting circuit composed of a constant voltage circuit and a time constant circuit, a charging circuit that supplies a charging current according to the output voltage of the voltage setting circuit, a battery voltage detection circuit, and a time constant switching of the time constant circuit. A secondary battery charger is a secondary battery charger configured with a circuit, and in the initial stage of charging, the secondary battery is charged with a relatively large constant current set by the constant voltage circuit, and the voltage of the secondary battery is
When the set voltage of the secondary battery is reached, the voltage input to the charging circuit is switched from the constant voltage circuit to the time constant circuit, and the output voltage of the time constant circuit is decreased along the first discharge curve. The time constant of the time constant circuit is switched to a shorter time constant, and the output voltage of the time constant circuit is changed to a second discharge voltage only when the voltage of A secondary battery charger characterized by decreasing power along a curve.