Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
BATTERY CHARGER
5 Technical field
The present invention relates to a charger for a rechargeable battery.
Background art
In general, a battery charger is so designed that it charges a battery by
10 supplying it with an electric current while the voltage across the battery is lower
than a predetermined level, and that it thereafter stops charging when the voltage
across the battery reaches that predetermined level as the result of the charging.
Fig. 7 is a block diagram illustrating how a conventional battery charger is
connected to a power supply device (hereafter referred to as the "battery pack").
15 In Fig. 7, numeral 60 represents the charger, and numeral 61 represents the battery
pack that is charged thereby. When the voltage across the battery pack 61 is
lower than a predetermined level, the charger 60 charges the battery pack 61 by
supplying it with a current Ia, and meanwhile it keeps an LED (light-emitting
diode) 64 on to indicate that charging is in progress (this LED will hereafter be
20 referred to as the "charge-in-progress LED"). When the voltage across the battery
pack 61 reaches the above-mentioned predetermined level, the charger 60 stops the
supply of the current Ia and turns off the charge-in-progress LED 64, and in
addition it turns on an LED 65 to indicate that charging has been completed (this
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LED will hereafter be referred to as the "charge-complete LED").
For example, in a case where the battery pack 61 is a power supply device
that employs lithium-ion cells, the battery pack 61 incorporates a protection circuit
that serves to secure stable operation of those cells by protecting them against
5 overdischarge and other hazards. As a result, when overdischarge or the like is
detected, this protection circuit inhibits the discharging of the battery pack 61 from
within it. In this state, where both charging and discharging are inhibited, the
charging of the battery pack 61 cannot be restarted without first canceling this
inhibiting state. For this reason, the charger 60 is so designed that, even when it
10 is not connected to the battery pack 61, it outputs a voltage almost as high as the
voltage across the battery pack 61 in its fully charged state, and on the other hand
the battery pack 61 is so designed that it cancels the above-mentioned inhibiting
state by detecting a feeble current that flows into it when it is connected to the
charger 60 and receives therefrom the above-mentioned high voltage.
At this time, however, precisely because the charger 60 is designed to
output a high voltage on its current-supplying side, the charger 60, even when it is
not connected to the battery pack 61, keeps the charge-complete LED 65 on, falsely
indicating that charging has been completed. To overcome this inconvenience,
the conventional charger 60 is, as shown in Fig. 7, fitted with a mechanical switch
20 63 for checking whether the battery pack 61 is present or not, so that, when it is
not connected to the battery pack 61, it can keep both the charge-in-progress LED
64 and the charge-complete LED 65 off. Here, the mechanical switch 63 is a
switch that has a contact that is mechanically opened and closed depending on
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whether the battery pack 61 is present or not.
Having such a mechanical switch 63, the conventional charger 60 is prone
to malfunction due to imperfect mechanical contact in the mechanical switch 63,
and thus it does not offer satisfactory safety. In addition, the use of the
5 mechanical switch 63 increases the cost of the charger 60.
Disclosure of the invention
According to the present invention, a battery charger that outputs a high
voltage even when a power supply device having a rechargeable battery is not
10 connected to the battery charger so that the battery charger can supply the high
voltage to the power supply device whenever the power supply device is connected
to an output tern1inal of the battery charger is provided with periodic signal
producing means for supplying a periodically varying periodic signal to the output
terminal, and detecting means for checking whether the power supply device is
15 connected to the battery charger or not by checking whether the periodic signal is
present at the output terminal or not.
Brief description of drawings
Fig. 1 is a block diagram of a battery charger embodying the present
20 invention;
Fig. 2 is a detailed circuit diagram of the pulse current producing circuit
used in the battery charger of the invention;
Fig. 3 is a circuit diagram of the high-pass filter used in the battery charger
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of the invention;
Fig. 4 is a diagram showing the waveform of the voltage appearing at the
output terminal of the battery charger of the invention when no battery pack is
connected thereto;
Fig. 5 is a diagram showing the output characteristic of the constant-
voltage/constant-current circuit used in the battery charger of the invention;
Fig. 6 is a circuit diagram of the charge control FET and the protection
circuit incorporated in the power supply device; and
Fig. 7 is a block diagram showing how a conventional battery charger is
10 connected to a battery pack.
Best mode for carrying out the invention
Hereinafter, an embodiment of the present invention will be described with
reference to the drawings. Fig. 1 is a block diagram of the principal portion of a
15 battely charger embodying the invention. This battery charger is formed as an IC
(integrated circuit) 1. The IC 1 has a terminal (OUT) to which a battery pack 2 is
connected when it is charged. The battery pack 2 is connected in parallel with an
output capacitor 3, which is provided outside the IC 1.
The IC 1 also has a terminal (RLED) to which a charge-in-progress LED 4 is
20 connected, and a terminal (GLED) to which a charge-complete LED 5 is connected.
As will be described later, when the battery pack 2 is not connected to the terminal
(OUT), the LEDs 4 and 5 are both kept off. On the other hand, when the battery
pack 2 is connected, only the charge-in-progress LED 4 is kept on when charging is
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in progress, and only the charge-complete LED 5 is kept on after the voltage of the
battery pack 2 reaches a predetermined level.
The IC 1 further has a terminal (Vcc) that is connected to a power source
voltage 6 on which the IC 1 operates, a terminal (GND~ that is connected to a
5 ground level voltage, and a terminal (CT) that is connected to a capacitor 7. The
capacitance of this capacitor 7 determines the oscillation frequency f of the
oscillator (OSC) 10 incorporated in the IC 1. An oscillation signal having the
frequency f is fed to a control circuit 11 included in a pulse current producing
circuit 30.
The control circuit 11 turns on and off current source circuits lZ and 13 by
feeding them with a signal that is synchronous with the oscillation frequency f.
When the current source circuit 12 is off, the current source circuit 13 is on,
allowing a current Il to flow; when the current source circuit 12 is on, the current
source circuit 13 is off, allowing a current I2 to flow.
As a result, when the battery pack 2 is not connected to the output terminal
(OUT), the output capacitor 3, which is connected to the output terminal (OUT), is
charged or discharged, and this causes the voltage at the output terminal (OUT) to
vary periodically, as shown in Fig. 4, between a full-charge voltage (Ve) and a
predetermined voltage (Vt) lower than that. Here, the voltage (Vt) is set, for
20 example, to 99 % of the full-charge voltage (Ve), although the proportion is not
restricted to any specific value. Depending on the characteristics of the actual
circuit and other factors, the waveform may not always be triangular as shown in
Fig. 4, but may be square or sawtooth-shaped, for example. The threshold level
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(VreA will be described later.
By contrast, when the battery pack 2 is connected to the output terminal
(OUT), since the battel~ pack 2 has a low impedance, the voltage at the output
terminal (OUT) is not affected by the current Il or 12, and thus it is kept equal to
5 the voltage of the battery pack 2. A pulse-drive starting comparator 14 checks
whether the voltage of the battery pack 2 is equal to or higher than the voltage (Vt).
If the voltage of the battery pack 2 is lower than the voltage (Vt), the control circuit
11 is turned off so that the pulse current producing circuit 30 will be deactivated.
On the other hand, when the voltage of the battery pack 2 is equal to or higher than
10 the voltage (Vt), the control circuit 11 is turned on so that the pulse current
producing circuit 30 will be activated.
This causes the battery pack 2 to be charged by the pulse current producing
circuit 30 and thus causes its voltage to increase gradually. However, since the
pulse current producing circuit 30 is, as shown in Fig. 2, so designed that it
15 operates only when the voltage at the output terminal (OUT) is in the range from
the voltage (Vt) to the full-charge voltage (Ve), it does not occur that the battery
pack 2 is charged beyond its full-charge voltage (Ve). Specifically, the comparator
14 compares the voltage at the output terminal (OUT) with the voltage (Vt), another
comparator 18 compares the voltage at the output terminal (OUT) with the full-
20 charge voltage (Ve), and the outputs of these comparators 14 and 18 are fed throughan AND gate 27 to the control circuit 11 for its turning-on/off operation. Thus, the
pulse current producing circuit 30 operates only when the voltage at the output
terminal (OUT) is in the range from the voltage (Vt) to the full-charge voltage (Ve).
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Note that, in Fig. 2, such elements as are found also in Fig. 1 are identified with the
same reference numerals and symbols.
In Fig. 1, when the battely pack 2 is not connected to the terminal (OUT), an
overdischarged battely recovering circuit 16 keeps the voltage at the terminal
5 (OUT) equal to the voltage (Vt). This allows the battery pack 2, even when it is in
the state in which charging is inhibited due to overdischarge, to receive a high
voltage from the overdischarged battery recovering circuit 16 and thereby cancel
the inhibiting state as soon as it is connected to the terminal (OUT). A pulse
detection comparator 17 compares the voltage at the output terminal (OUT) with a
10 threshold level (Vrefl, which is set, as shown in Fig. 4, to a voltage between the
full-charge voltage (Ve) and the voltage (Vt).
When the battery pack 2 is not connected to the terminal (OUT), the pulse
detection comparator 17 outputs a signal that oscillates with a period f in
synchronism with a signal fed from the pulse current producing circuit 30. On
15 the other hand, when the battery pack 2 is connected, the pulse detection
comparator 17 outputs a fixed high or low level depending on the voltage of the
battery pack 2. The signal outputted from the comparator 17 is passed through a
high-pass filter 19 to extract only the alternating-current components thereof.
The signal flowing out of the high-pass filter 19 is compared with a
20 reference voltage (Vs) by a comparator 20. The high-pass filter 19 is composed of,
as shown in Fig. 3, a capacitor 19a and a resistor 19b, for example. Note that, in
Fig. 3, such elements as are found also in Fig. 1 are identified with the sarme
reference numerals and symbols.
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Reverting to Fig. 1, when the battery pack 2 is connected to the terminal
(OUT), the pulse detection comparator 17 outputs either a fixed high level or a
fixed low level, and thus causes the comparator Z0 to output a low level. On the
other hand, when the battery pack 2 is not connected to the terminal (OUT), the
comparator 20 outputs a signal that oscillates regularly with a period f. The
output of the comparator 20 is fed to a delay circuit 22. The delay circuit 22 iS
composed of a switching transistor 23, a capacitor 24, and a current source circuit
25. The voltage across the capacitor 24 iS compared with a reference voltage (Vu)
by a power-supply-device detecting comparator 26.
When the battery pack 2 is not connected to the terminal (OUT), the signal
flowing out of the high-pass filter 19 oscillates, and this causes the transistor 23 to
be alternately turned on and off, and thus causes the capacitor 24 to be alternately
charged and discharged. As a result, the voltage across the capacitor 24 becomes
sufficiently low, and the power-supply-device detecting comparator 26 outputs a
low level.
On the other hand, when the battery pack 2 is connected to the terminal
(01~), the transistor 23is kept off, and therefore the capacitor 24iS charged by the
current source circuit 25. When the voltage across the capacitor 24 exceeds the
reference voltage (Vu), the power-supply-device detecting comparator 26 outputs a
high level.
In this way, by the use of the signal outputted from the power-supply-device
detecting comparator 26, it is possible to check whether the battery pack 2 iS
connected or not. The comparator 26 may also be so configured that, using
-
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completely inverted logic, it outputs a high level when the battery pack 2 is not
connected and a low level when battery pack 2 is connected.
When the comparator 26 feeds a high level to an LED driving circuit 28, the
LED driving circuit 28 turns on either the charge-in-progress LED 4 or the charge-
5 complete LED 5, and thereby indicates that the battery pack 2 is connected to theterminal (OUT). By contrast, when the comparator 26 outputs a low level, the
LED driving circuit 28 keeps both LEDs 4 and 5 off, and thereby indicates that the
battery pack 2 is not connected to the terminal (OUT).
The signal outputted from the power-supply-device detecting comparator 26
0 is fed also to a constant-voltage/constant-current circuit 29. When the battery
pack 2 is not connected to the terminal (Ol~), the constant-voltage/constant-
current circuit 29 is turned off. By contrast, when the battery pack 2 is connected,
the constant-voltage/constant-current circuit 29 is turned on, and exhibits an
output characteristic as shown in Fig. 5. As shown in Fig. 5, the constant-
15 voltage/constant-current circuit 29 is so configured that it does not perform
charging when the voltage of the battery pack 2 is lower than a charge-inhibition
level (Vj). The charge-inhibition level (Vj) is determined in accordance with the
type of the battery used.
More specifically, owing to the above-mentioned output characteristic, in
z0 the voltage range 55 from around the charge-inhibition level (Vj) to around the
voltage (Vt), constant-current charging is performed; as the voltage of the battery
approaches the voltage (Vt) with the progress of the charging, the charging current
is so reduced that the voltage is kept almost constant. Meanwhile, when the
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charge-completion point 54 is reached during the charging, the constant-
voltage/constant-current circuit 29 (see Fig. 1) feeds a signal to the LED driving
circuit 28 to request it to change the indication from "charge-in-progress" to
"charge-complete". Thus, the LED driving circuit 28 turns off the charge-in-
5 progress LED 4 and instead turns on the charge-complete LED 5 to indicate that the
charging has been completed.
When the charging further progresses and the voltage of the battery reaches
the voltage (Vt), the constant-voltage/constant-current circuit 29 is deactivated.
Then, the pulse-drive starting comparator 14 activates the pulse current producing
10 circuit 30 to start the supply of a periodic signal to the battery pack 2. The pulse
current producing circuit 30 outputs the periodic signal in such a way that the
battery pack 2 is charged gradually by the periodic signal. Accordingly, in the
interval 52 shown in Fig. 5, the voltage of the battery gradually rises. When the
voltage of the battery reaches the full-charge voltage (Ve), the pulse current
~5 producing circuit 30 (see Fig. 2) is deactivated, and thus the charging is completed.
In Fig. 5, the broken line 57 indicates the load characteristic of the feeble
current I that is fed from the overdischarged battery recovering circuit 16. This
line 5 7 shows that the overdischarged battery recovering circuit 16 supplies a
current to the battery back 2 even in the interval 55, but that this current is far
20 smaller than the charging current supplied from the constant-voltage/constant-
current circuit 29. This line 57 also shows that, when the battery pack 2 is not
connected to the terminal (OUT), the overdischarged battery recovering circuit 16
charges the output capacitor 3 in order to raise the voltage at the terminal (OUT)
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.
- 11 -
up to the voltage (Vt) and thereby activate the pulse current producing circuit 30.
As described above, the IC 1 can not only charge the battery pack Z, but also
check whether the battery pack 2 is connected or not without using a mechanical
switch and thereby turn on and off the LEDs 4 and 5 properly. Moreover, free
from malfunction due to imperfect contact such as experienced with a mechanical
switch, the battery charger of the embodiment provides higher safety.
Furthermore, the elimination of the mechanical switch leads to the reduction of
the cost of the battery charger.
Next, a description will be given as to the protection and other circuits
10 incorporated into the battery pack 2. Fig. 6 shows an example of the internal
circuit of the battery pack Z. The battery pack 2 essentially consists of a battery
proper 32 such as a lithium-ion cell, a protection circuit 31, and an n-channel FET
(field-effect transistor) 33 for discharge control. When the battery 32 is in the
process of discharging, the FET 33 is normally kept on, and the discharged
electricity is extracted via a positive (+) terminal 35 and a negative (-) terminal 36.
The voltage of the battery 32 is fed through a resistor R1 to a terminal (V) of
the protection circuit 31. When the voltage of the battery 32 reaches the
discharge-inhibition level (Vg) as the result of the discharging of the battery 32, a
comparator 37 shifts its output from a high level to a low level. This level shift is
fed through an OR gate 38 to a terminal (FE), and is outputted therefrom to turn off
the FET 33. This inhibits the discharging of the battery 32 and thereby prevents
the deterioration of the characteristics thereof.
However, in this state, in which the FET 33 iS off, it is not possible to start
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-12-
the charging of the battery 32 immediately. First of all, it is necessary to apply a
voltage close to the full-charge voltage of the battery 32 between the positive (+)
and negative (-) terminals 35 and 36. This causes a feeble current I to flow
through the body diode 34 of the FET 33, and thus causes a voltage drop there.
This voltage drop is fed through a resistor R2 to a monitor terminal (MO) of theprotection circuit 31, where a comparator 39 compares the drop with a voltage (V.1~
and outputs, in this case, a high level. This high level is fed through the O R gate
38 to the terminal (FE), and is outputted therefrom to turn on the FET 33. Now it
is possible to start the charging of the battery 32. Note that the body diode 34 iS
10 merely a parasitic diode, as explicitly illustrated, that exists in the FET 33. The
protection circuit 31 also has a terminal (GND) through which it receives a
reference level.
It is to be understood that the embodiment described specifically above is
merely one example of how the present invention can be applied. For example, it
15 iS also possible to use, in place of the high-pass filter 1, any circuit that can detect
alternating-current components in a signal; it is also possible to incorporate the
oscillation capacitor 7 into the IC 1.
Industrial applicabilily
As described heretofore, according to the present invention, in a battery
charger, a periodic signal is produced and fed to the output terminal so that, by
checking whether the periodic signal is present at the output terminal or not, it is
possible to check whether a battery pack is connected or not. This makes it
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possible to detect the presence of the battery pack without the use of a mechanical
switch dedicated to such detection. The elimination of a mechanical switch leads
not only to the elimination of malfunction due to imperfect contact or the like and
thus to increased safety, but also to the reduction of the cost. In particular, in a
5 battery charger for a battery pack, such as employs a lithium-ion cell, that is fitted
with a protection circuit for protection against overdischarge and other hazards
and that keeps its output voltage almost as high as the full-charge voltage even
when no battery pack is connected to its output terminaI, it is possible to detect
the presence of the battery pack even when the battery charger is keeping its
10 output at such a high voltage.
