JP2673966B2 - Battery charger - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery charger that inserts and charges a battery pack such as a cleaner or an electric tool, and more particularly to an improvement in the circuit configuration of the battery charger. .

[Prior Art] Conventionally, for example, the outline of the circuit configuration of a constant current charging type battery charger is, as shown in FIG. 5, a primary side for rectifying and smoothing a commercial AC power source connected to AC terminals 1a, 1b. DC power supply circuit 1, an insulated gate field effect transistor FET as a chopper that intermittently interrupts a constant voltage current supplied from this circuit at a predetermined frequency, and the intermittent voltage applied to the primary coil is stepped down to a secondary A step-down transformer T that outputs a step-down AC power supply to a side coil, a secondary-side DC power supply circuit 2 that rectifies and smoothes the step-down AC current, a current detection circuit 3 that constantly detects a charging current value, and this current detection Based on the secondary side control circuit 4 that changes the value of the charge control signal S ₂ according to the current value to be charged based on the current value detection signal S ₁ output from the circuit 3, and based on this charge control signal S _2. , Insulated gate field effect transistor The primary side control circuit for varying the duty ratio of the gate signals G ₁ on-off to be supplied to Njisuta FET 5
And

Battery B to be charged and thermostat for full charge detection
When the battery pack 6 in which THMs are connected in series is attached to the battery pack fixing portion (not shown) of the battery charger main body, the terminals 6a, 6b, 6c of the battery pack 6 are automatically connected to the positive terminals of the battery charger. 7a, Negative terminal 7b, Thermo terminal
Conductive connection to 7c.

Assuming that the battery B is normal now, weak charging of the battery B is started via the diode D ₁ , the current limiting resistor R ₁ and the thermal protection element (eg, OCR) on the charger side. Then, the current detection circuit 3 detects the charging current value I by picking up the voltage across the shunt resistor R ₂ , supplies the current value detection signal S ₁ to the secondary side control circuit 4, and the secondary side control circuit 4 The charging voltage V is detected. Then, the secondary side control circuit 4 excites a relay coil (not shown) therein to close the relay switch SW. As a result, full-scale charging of the battery B is started. At the same time, although the circuit configuration is partially omitted in FIG. 5, the exciting current of the relay coil from the secondary side control circuit 4 side is the terminal 7c.
(6c), thermostat THM, flow through terminal 7b (6b).

This battery charger is driven by constant current charging,
On the other hand, the control is such that the charging current value to be charged is always a constant value. That is, when the analog voltage value of the current value detection signal S ₁ output from the current detection circuit 3 does not reach the predetermined value, the secondary side control circuit 4 uses the PWM in order to supply more charging current to the secondary side. A wave charge control signal S ₂ is generated, and this is supplied as an analog signal to the primary side control circuit 5 via the CR integration circuit 8 of the PWM demodulation circuit and the photocoupler (opto-isolator) PC ₁ . Then, the primary side control circuit 5 generates a gate control signal G ₁ having a larger duty ratio than before based on the charge control signal S ₂ , and supplies this to the insulated gate field effect transistor FET. As a result, the amount of current flowing through the primary side coil of the step-down transformer T increases, so that the amount of current flowing through the secondary side coil also increases, and a larger amount of charging current is supplied to the battery B. Constant current charging is always performed.

Here, for example, when the battery pack 6 having the short-circuited battery B is mounted, an overcurrent (short circuit current) flows through the shunt resistor R ₂ . In addition, an overcurrent may flow during charging even with a defective battery or the like. When such an overcurrent flows, the current detection circuit 3 detects it and supplies the overcurrent detection signal S ₃ to the primary side control circuit 5 via the photocoupler PC ₂ . Then, the primary side control circuit 5 forcibly narrows the duty ratio of the gate control signal G ₁ based on the overcurrent detection signal S ₃ . Therefore, the charging current on the secondary side decreases,
Supply of overcurrent to the battery B is reduced.

Note that R _{3 to} R ₆ are resistors and C ₁ is a capacitor.

[Problems to be solved by the invention]

However, the battery charger having the above circuit configuration has the following problems.

That is, in order to limit the overcurrent state of the charging current generated on the secondary side, the secondary side current detection circuit 3 and the secondary side 1
The gate control signal G ₁ to be sent from the primary side control circuit 5 through the path of the photocoupler PC ₂ as means for transmitting information to the secondary side.
Although a method of forcibly narrowing the duty ratio of No. 2 and reducing the amount of power supply on the secondary side is adopted, it is difficult to apply a transient limit to an overcurrent state having a steep rise. Since the information transmission system is long, the response delay of detection is conspicuous, and it is not possible to deal with current limitation immediately. There are various types of batteries to be charged, and if an overcurrent state occurs during the charging process, excessive heat generation or ignition may occur.

Therefore, the present invention is to solve the above problems,
A first object of the present invention is to provide an overcurrent limiting control system having a fast response speed in a constant current charging system,
An object of the present invention is to provide a battery charger that can quickly suppress a transient occurrence of an overcurrent charge state on the secondary side. Further, a second object of the present invention is to provide a battery charger capable of avoiding a non-power-supply state and preventing power-down of a required circuit even when a current limitation as much as possible works in an overcurrent charging period. is there.

[Means for solving the problem]

In order to solve the above-mentioned problems, a battery charger according to the present invention includes a switching means for connecting and disconnecting a constant voltage fed from a first DC power supply circuit at a predetermined frequency, and a step-down for decreasing the intermittent voltage on the primary side. Transformer, a second DC power supply circuit for obtaining a DC charging voltage for constant current charging of a battery based on the step-down voltage on the secondary side, and a value of a charge control signal changed according to a change in current value to be charged A battery charger having secondary-side charging current detection means for causing the switching control signal to be supplied to the switching means on the basis of the charging control signal and controlling the opening / closing of the switching control signal for each cycle. A primary-side overcurrent detection means for detecting an excessively large intermittent current on the primary side of the transformer, and opening after closing of the switching means for each cycle based on the switching control signal. Also preferentially, and having a closing truncation means for controlling said switching means based on a detection signal from the primary-side overcurrent detection means open.

In addition to the above configuration, the secondary side overcurrent detection means for detecting an overcurrent state from the change in the current value to be charged on the secondary side, and the detection signal of the secondary side overcurrent detection means are set to the primary side. And a circuit system for generating an overcurrent limiting signal based on the detection signal thereof and interrupting the overcurrent limiting signal as an input signal of the switching control means.

[Action]

In the present invention, when a transient excessive current is generated on the secondary side in the constant current charging process, the switching means is limited by the charge control signal by the charging current detecting means on the secondary side.
Instead of limiting the amount of power supply on the secondary side, by detecting the change in the primary side current of the step-down transformer that directly goes back due to the overcurrent state on the secondary side, by using the primary side overcurrent detection means. Since the primary side current is directly controlled, overcurrent limitation with a very fast response speed is realized.

The mode of overcurrent limiting of the present invention is not to stop switching entirely by the switching means, but the switching means is closed once as usual by the switching control signal, and then the primary side is closed based on the detection signal by the overcurrent detection means. Since the closed state is closed early by the control of the closing means, the closing means with a small duty ratio of the switching means exists even in the cycle of charge limitation to the battery in the overcharged state. Therefore, the DC power supply circuit for actively energizing the circuit element on the primary side or the secondary side of the step-down transformer is connected without power down. For this reason, it is not necessary to supply power to all the circuit elements from the first DC power supply circuit of the source, and it is possible to avoid running long power supply wiring from the first DC power supply circuit to the secondary side circuit.

Further, a plurality of DC power supplies having different voltage values are required to actively energize various circuit elements, but energy loss poses a problem in deriving by direct-current stepping down from the first DC power supply circuit. . However, in the present invention, by using the step-down transformer having the shunt winding coil on the primary side or the secondary side,
With a simple circuit configuration, energy loss is small, and DC power supplies with different voltage values can be easily attached.

〔Example〕

Next, an embodiment of the battery charger according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.
In FIG. 1, the same parts as those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted.

The DC power supply circuit 1 on the primary side includes a noise removing filter circuit 1c, a diode bridge circuit 1d, and a smoothing capacitor C ₂
It is composed of A resistor R ₇ , a capacitor C ₃ and a diode D are provided between both ends of the primary side main coil Ta of the step-down transformer T ′.
_A noise elimination circuit 10 composed of ₂ is connected. A power insulated gate field effect transistor FET as a chopper and a shunt resistor R ₈ are connected in series between the primary side main coil Ta and the ground. The transistor FET is intermittently opened and closed at a predetermined frequency by a gate control signal G ₁ having a variable duty ratio sent from a primary side control circuit 20 described later.

The secondary side DC power supply circuit 2 is composed of rectifying diodes D ₃ and D ₄ , a current limiting coil L and a smoothing capacitor C ₃ . The secondary side shunt coil Tb of the transformer T ′ has a logic power supply voltage for the secondary side control circuit 4 and the current detection circuit 3.
Secondary side DC stable power supply circuit 30 for supplying Vcc (5V)
Is connected. This DC stable power supply circuit 30 includes a rectifier diode D ₅ , a three-terminal regulator IC and capacitors C ₄ , C ₅
It is composed of Further, the primary side DC stable power supply circuit 4 for supplying the primary side shunt coil Tc of the transformer T ′ with the logic power supply voltage Vcc (5V) of the primary side control circuit 20 and the like.
0 is connected. The primary side DC stable power supply circuit 40 is composed of a rectifying diode D ₆ and a smoothing capacitor C ₆ .

The photocoupler PC ₁ that should transmit the charging control signal (feedback signal) S ₂ from the secondary side to the primary side is a light emitting diode LED ₁ as a light emitting element and a phototransistor Tr _{1 as a} light receiving element.
It is composed of The anode of the light emitting diode LED ₁ is connected to the Vcc power supply on the secondary side via the resistor R ₉ , and the cathode thereof is connected to the charge control signal output terminal 4a of the secondary side control circuit 4, and the resistor R ₁₀ emits light. Diode LED ₁ and resistor R ₉
Are connected in parallel.

On the other hand, the collector of the phototransistor Tr ₁ is a resistor R ₅ , PWM
It is connected to the second terminal of the control IC 23 via the demodulation circuit 21 and the error amplifier 22. The PWM demodulation circuit 21 is an integration circuit including voltage dividing resistors R ₁₁ and R ₁₂ connected in series and a charging / discharging capacitor C ₇ connected in parallel with them. This charge / discharge capacitor
The positive terminal of C ₇ is connected to the Vcc power supply on the primary side (applied to the 6th terminal of the control IC 23) via the 3-terminal regulator IC, and the voltage
It is pulled up to Vcc. The error amplifier 22 is composed of a PNP transistor Q ₁ , an emitter resistance R ₁₃ and a collector resistance R ₁₄ , and the collector voltage thereof is applied to the second terminal of the control IC 23.

The control IC 23 used in this embodiment is model number FA5304P (S)
(M) (manufactured by Fuji Electric Co., Ltd.), part of which is omitted, and the detailed circuit configuration is shown in FIG. The explanation of all the circuit elements of this control IC is omitted because it only complicates the understanding of the present invention, but only the circuit elements related to the present invention will be explained below.

The PWM comparator 23a as a means for sending the gate control signal G ₁ of the PWM wave to the transistor FET has four inputs (A, B,
C, D) and has an internal oscillator (OS
C) 23c oscillator output, CS terminal voltage applied to the B input, error amplifier output applied to the C input via the 2nd terminal,
And the DT voltage applied to the D input are compared, and the CS terminal voltage, the error amplifier output, and the DT voltage, which is the lowest voltage input, are preferentially compared. The output of the PWM comparator 23a is at a high level while the voltage value of the lowest input is lower than the oscillator output, and is at a low level during the high period. When the output of the PWM comparator 23a is low level, the output of the NOR gate 23b, that is, the voltage appearing at the 5th terminal becomes high level. NOR gate 23b has 3 inputs, these are output of PWM comparator 23a, low voltage malfunction prevention circuit (UV
The output of the LO) 23d and the Q output of the RS flip-flop 23e are applied. Undervoltage lockout circuit (UVLO) 2
3d outputs an output when a sufficient power supply voltage is reached in order to prevent malfunction of the power supply voltage of each circuit during an insufficient period of the power supply voltage of this IC at the initial stage. The set input S of the RS slip flop 23e receives the output of the comparator 23f, and its reset input R receives the output of the PWM comparator 23a. Then, an overcurrent generation signal IS, which will be described later, is applied to the detection input of the comparator 23f via the third terminal. The capacitor C _S connected to the 8th terminal is a timed capacitor that adjusts the length of the soft start when the power is turned on. Further, in the primary side control circuit 20 shown in FIG. 1, resistors, capacitors and diodes, which are not particularly described, play auxiliary roles such as bias, phase adjustment and destruction prevention.

Emitting diode L at the inverting amplifier 3a and photocoupler PC ₂ current detection circuit 3 for detecting a charging current value as a voltage value
It is composed of the driver circuit 3b of the ED ₂ . The current detection circuit 3 is composed of an operational amplifier OP that receives the voltage drop value (negative voltage) of the shunt resistor R ₂ at its inverting input via the input resistor R ₁₅ , its feedback resistor R ₁₆ and feedback capacitor C _8, and its inverting amplification. Output is current detection signal to terminal 4a of secondary side control circuit 4
It is supplied as S _{1 and} is also supplied to the shunt regulator Re connected in series with the LED ₂ via the resistor R ₁₇ . The voltage across the shunt regulator Re is maintained at about the charging voltage of the capacitor C ₉ , and when the operational amplifier OP exceeds a certain value, a current flows through the shunt regulator Re, which causes the light emitting diode LED ₂ to emit light. Photo coupler PC ₂
The collector of the phototransistor Tr ₂ is connected to the second terminal of the control IC via the resistor R ₆ , and the collector side is pulled up to the Vcc power supply.

 Next, the characteristic operation in the above embodiment will be described.

(Startup period) First, when the AC power is turned on to the primary side, the DC power supply circuit 1
A constant voltage V ₀ appears at the output of the control IC, the primary side Vcc power is supplied to pin 6 of the control IC via the voltage dividing resistor R _18, and the output (high level) of the low voltage malfunction prevention circuit 23d becomes NOR. It is applied to the gate 23b and the like. At the same time, 23g constant current source (10μA)
Due to this, charging of the time-limited capacitor C _S is started, and along with that, the CS terminal voltage also gradually rises with a predetermined time constant as shown in FIG. When the power is turned on, the oscillator 23c continuously outputs a triangular waveform as shown in FIG. At the initial stage of this start-up, the error amplifier output is at the upper limit value as shown in FIG. 3 based on the charging control signal S ₂ sent from the secondary side control circuit 4. Since the PWM comparator 23a compares the oscillator output with the lowest voltage among the CS pin voltage, the error amplifier output, and the DT voltage (e ₁ ), the CS pin voltage is preferentially compared in the initial stage of startup. , It takes a soft start.
The output of the PWM comparator and the gate control signal G _{1 which is its} inverted output
Pulse width (duty ratio) gradually increases, and along with this, the ON period of the chopper transistor FET also gradually increases, monotonically increasing the secondary side power supply amount. This gives 2
A DC voltage is generated from the DC power supply circuit 2 on the next side, and charging of the battery B is started according to the above-described aspect.

(Normal Operation Period) Although the charging current I causes a voltage drop of the shunt resistor R ₂ , the current detection circuit 3 detects the above voltage drop value. That is, as the value of the charging current I increases, the voltage drop increases, so that the voltage value of the current detection signal S ₁ increases. Moreover, when the value of the charging current I decreases, the voltage value of the current detection signal S ₁ decreases. The secondary side control circuit 4 sends this current detection signal S ₁ to PW
M modulates the PWM modulator that generates a charge control signal S ₂ has the (not shown), such as the terminal 4a of the secondary side control circuit 4 to photocoupler PC ₁ was appended to Figure 1 The PWM wave of the charge control signal S ₂ is being sent. Here, when the pulse width w ₁ matches the constant current value (for example, 6 A), the pulse width w ₂
Indicates that there is an undercurrent, and the pulse width w ₃ indicates that there is an excess current. In addition, t represents a period.

As note in Fig. 1, when the PWM wave is high (H), and light emitting diodes LED ₁ of the photocoupler PC ₁
Is in the off state (light emitting state), and is in the on state (non-light emitting state) when the level is low (L). Therefore, the photo coupler
The phototransistor Tr ₁ of PC ₁ is turned on / off according to the on / off of the light emitting diode LED ₁ , and this digital signal is
The base voltage of the transistor Q ₁ is analogized by the PWM demodulation circuit 21, and the PW of the charge control signal S ₂ is changed to PW as shown in FIG.
It has a waveform obtained by directly demodulating the M wave.

By the way, the PWM wave as the charge control signal S ₂ reaches the primary side as it is via the photocoupler PC _1, and the PWM demodulation circuit 21 provided on the primary side converts it into an analog signal. _It is possible to solve the problem of signal transmission due to the variation in the characteristics of ₁ and the poor temperature characteristics.
In other words, since the light emitting diode LED ₁ is not controlled to emit light in an analog amount of light emission but simply emits and turns off on and off, the variation in the characteristics of the photocoupler PC ₁ does not pose a problem. Even if the temperature characteristic is bad, a sufficient logical amplitude can be obtained, and this is not a problem. Furthermore, it has a long life and high reliability against aging. Therefore, it is possible to use an inexpensive photocoupler. In other words, the accuracy of the charging control is higher than that of the related art, and it is possible to flexibly cope with the condition of the battery to be mounted, so that it is possible to prevent accidental accidents such as battery damage and ignition.

Although the PWM demodulation signal is applied to the base of the transistor Q ₁ , its error amplifier output is supplied to the 2nd terminal of the control IC as described above. When the error amplifier output falls from the upper limit and reaches the balanced state (constant current charging state), the value of this error amplifier output becomes the lowest, so that it is compared with the oscillator output as shown in FIG. The duty ratio of the PWM comparator output and the gate signal G ₁ during the operation period is variably adjusted as shown in FIG. Thereby, the constant current charging of the battery B is performed.

(When Charging Current Increases) For example, when the impedance of the battery B suddenly drops during the charging period for the battery B, the charging current increases, but the charging current increases due to the charging control signal S ₁ .
It works to reduce the pulse width of the PWM wave and lower the voltage value of the error amplifier output on the primary side. Therefore, the gate control signal
The pulse width of the PWM wave is narrowed, the amount of power supply on the secondary side is reduced, the charging current value for the battery B is reduced, and the constant current value is restored. Such feedback control from the secondary side to the primary side follows a comparatively gentle current change in the normal operation period.

(When Overcurrent Occurs) When the charging current increases relatively steeply, the voltage value of the current detection signal S ₁ rapidly rises. Thus, the phototransistor Tr ₂ is turned on to emit light emitting diode LED ₂ of photocoupler PC _2, the voltage applied to pin 2 of the control IC (error amplifier output) decreases to force. As a result, the secondary side power supply amount is suppressed relatively quickly.
However, the information on the rapid increase in the charging current on the secondary side is transmitted to the control IC on the primary side via the current detection circuit 3, photocoupler PC _2, etc., so the delay in the response speed of the circuit is large. There is a long time lag in the suppression control of the secondary power supply amount, which may cause damage or damage to the battery B.

In order to solve this problem, the present embodiment is provided with a circuit that limits the occurrence of overcurrent as quickly as possible. When an overcurrent occurs on the secondary side, at the same time, the current flowing through the coil Ta on the primary side (FET drain current) also rapidly increases, increasing the voltage drop across the shunt resistor R ₈ and increasing the voltage of the control IC. The voltage value of the overcurrent generation signal IS rises to the No. terminal (see Fig. 4). When the voltage value of the overcurrent generation signal IS exceeds the reference voltage e ₂ of the comparator 23f, the output of the comparator 23f becomes high level, and accordingly, the Q output of the RS flip-flop 23e becomes high level, and further the NOR gate 23b. Output becomes low level regardless of the output of the PWM comparator 23a. That is, since the gate control signal G ₁ becomes low level, the transistor FET is forcibly shut off and the drain current does not flow. Therefore, the PWM comparator 2
Even when the output of 3a is at a high level, the transistor FET is forcibly cut off until the next cycle, so the secondary side power supply amount is promptly reduced, and the overcurrent state is immediately eliminated. Therefore, even if the charging characteristics of the battery B are various, it is possible to complete the battery charging without causing damage or ignition and without any trouble.

Also in the next cycle, when the secondary side is in the overcharged state, the same overcharge limiting operation as described above is performed, but in the new cycle, as shown in FIG. 4, the gate control signal G ₁ is once Since it starts up, the transistor FET is turned on once, and if it is overcharged, it is forced to return to the off state after a slight time lag. For example, if an overcharge condition is detected, over the next few cycle periods,
If the shutdown of the transistor FET is continued, the stable DC power supply 30 on the secondary side is disabled, and the secondary side control circuit 4 does not work during that time, and the monitoring function disappears at all. However, in this embodiment, since the gate control signal G ₁ with a small or minimum duty ratio is sent to the transistor FET even in the cycle of power supply limitation in the overcharged state,
The function of the power supply circuit of the control system can be maintained at a minimum.

〔The invention's effect〕

As described above, the present invention provides a step-down transformer.
Primary-side overcurrent detection means for detecting an excessively large intermittent current on the secondary side, and detection by the primary-side overcurrent detection means with priority over opening after closing of the switching means for each cycle based on a switching control signal. Since it is characterized by having a closing closing means for controlling the opening of the switching means on the basis of a signal, the following unique effect is obtained.

(1) In the constant current charging method, the occurrence of an overcurrent of the charging current on the secondary side is detected by a sudden change in the intermittent current that retroactively occurs on the primary side via the step-down transformer, and high-speed detection of overcurrent generation is performed. Has been realized.

(2) When an overcurrent occurs, the current is limited by the switching control on the primary side of the step-down transformer. However, in order to avoid the non-power supply state, the switching control signal is used to normally close the switching means once in each cycle. After closing the closed state, the closed state is closed early by the control of the closing closing means based on the detection signal from the primary side or the overcurrent detecting means to open the closed state. Secure a growing period. Since the DC power supply circuit for actively energizing the circuit elements on the primary side or the secondary side of the step-down transformer is maintained without power down, power is supplied to all the circuit elements from the first DC power supply circuit of the source. Therefore, it is possible to avoid long wiring of power source wiring from the first DC power source circuit to the secondary side circuit.

Further, a plurality of DC power supplies having different voltage values are required to actively energize various circuit elements, but it is not necessary to step down the DC voltage from the first DC power supply circuit to derive it, and the primary side or 2 By using a step-down transformer having a shunt coil on the secondary side, it is possible to easily attach a DC power supply with a simple circuit configuration with little energy loss and different voltage values.

[Explanation of main symbols]

1 ... Primary side DC power supply circuit FET ... Insulated gate field effect transistor T '... Step-down transformer 2 ... Secondary side DC power supply circuit 3 ... Current detection circuit S ₁ ... Current value detection signal S ₂ …… Charge control signal S ₃ , IS …… Overcurrent generation signal G ₁ …… Gate control signal 4 …… Secondary side control circuit 6 …… Battery pack B …… Battery 20 …… Primary side control circuit 30 ... DC stable power supply circuit 40 DC power supply circuit PC ₁ , PC ₂ Photo coupler 21 …… PWM demodulation circuit 22 …… Error amplifier 23 …… Control IC 23a …… PWM comparator 23b …… NOR gate 23c …… Built-in oscillator 23d …… Low voltage malfunction prevention circuit 23e …… RS flip-flop 23f …… Comparator 23g …… Constant current source R ₂ , R ₈ …… Shunt resistor.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Takao Iwatsuki 3-11-8 Sumiyoshi-cho, Anjo-shi, Aichi Prefecture Makita Denki Seisakusho Co., Ltd. (72) Masatoshi Sugiura 3--11 Sumiyoshi-cho, Anjo, Aichi Prefecture # 8 Makita Denki Seisakusho Co., Ltd. (72) Inventor Toshio Matsumoto 3-11-8 Sumiyoshi-cho, Anjo-shi, Aichi Prefecture Makita Denki Seisakusho (72) Inventor Hideki Watanabe 3 Sumiyoshi-cho, Anjo-shi, Aichi Makita Denki Mfg. Co., Ltd. 11-18 (72) Inventor Hideo Niwa 570 Nakasuna-cho, Tenpaku-ku, Nagoya-shi, Aichi Okawa Electric Co., Ltd. (56) References: 61-134645 (JP, U) )

Claims (2)

(57) [Claims] 1. A switching means for connecting and disconnecting a constant voltage fed from a first DC power supply circuit at a predetermined frequency, a step-down transformer for stepping down the intermittent voltage on the primary side, and the step-down voltage on the secondary side. A second DC power supply circuit for obtaining a DC charging voltage for constant current charging of the battery based on the above, a secondary side charging current detecting means for changing the value of the charging control signal according to a change in the current value to be charged, and A battery charger including: a switching control signal based on a charge control signal to the switching means to control opening and closing of the switching means for each cycle, wherein the intermittent current on the primary side of the step-down transformer is excessive. By the primary-side overcurrent detection means and the primary-side overcurrent detection means with priority over opening after closing of the switching means for each cycle based on the switching control signal. A battery charger comprising: a closing closing means for controlling the opening of the switching means based on a detection signal. 2. A secondary-side overcurrent detecting means for detecting an overcurrent state from a change in a current value to be charged on the secondary side, and a detection signal of the secondary-side overcurrent detecting means according to claim 1. Is provided to the primary side, and a circuit system that generates an overcurrent limiting signal based on the detection signal thereof and interrupts it as an input signal of the switching control means.