Battery charger device and method

A battery charger device is provided with a control switch which connects electrically a rechargeable battery load to a power supply which supplies charging current for charging the battery load. A pulse generator generates a series of pulses that control the control switch to connect and disconnect intermittently the power supply and the battery load. A maximum battery terminal voltage of the battery load is stored in a voltage memory means, and a fractional voltage is derived from the maximum battery terminal voltage. The fractional voltage is compared with a current battery terminal voltage from the battery load. A control signal is generated so as to prevent the control switch from receiving the pulses from the pulse generator, thereby disconnecting the power supply from the battery load so as to terminate charging of the battery load when the current battery terminal voltage is less than the fractional voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a battery charger device, more particularly to a 
battery charger device and method for charging different types of 
rechargeable batteries. 
2. Description of the Related Art 
Devices for charging and recharging various types and sizes of storage 
batteries, such as nickel-cadmium (Ni-Cd) battery cells and lead-acid 
cells, are known in the art. There are different types of battery charger 
devices which are presently available to the consumers. This is due in 
part to the absence of a standard for regulating the voltage 
characteristics of the storage batteries. The fast charger is one example 
of a conventional battery charger device and is capable of generating a 
relatively large charge current in order to charge the storage battery 
within a relatively short period of time. The large charge current, 
however, can cause rapid heating of the storage battery, thereby resulting 
in damage to the storage battery or a reduction in the useful life of the 
same if the storage battery was overcharged or if the fast charger was 
operated in cold weather conditions. 
The trickle charger is another example of a conventional battery charger 
device and generates a relatively small charge current, typically 10% of 
the maximum charge current which can be accepted by the storage battery. 
Such chargers do not require means for protecting the storage battery from 
overcharging and require relatively long charging periods. Thus, a storage 
battery which was charged by the trickle charger is sometimes 
insufficiently charged and cannot be used to drive an electrical load 
properly. 
U.S. Pat. No. 5,055,763 discloses an electronic battery charger device 
which is used to charge one or more storage batteries and which comprises 
a circuit with terminal means that is to be connected to the storage 
batteries. The circuit includes a source of electric energy, controllable 
switching means connected respectively between the energy source and each 
storage battery to be charged, and a microprocessor having a control 
connection to each of the controllable switching means for controlling 
communication between the energy source and the respective storage 
batteries to be charged. The microprocessor includes means for 
sequentially controlling the switching means to supply charging current to 
the storage batteries one at a time in repeating periods. The circuit 
further includes means responsive to the voltage across the terminals of 
each storage battery prior to and during each charging period thereof and 
operatively connected to the microprocessor. The microprocessor is 
programmed to calculate the difference between the battery terminal 
voltage of each storage battery prior to and when it is being charged. The 
circuit also includes means for storing for each storage battery the 
minimum value of the voltage difference during each charging period, and 
means to terminate a charging operation for a storage battery when the 
battery terminal voltage difference being calculated exceeds the minimum 
stored value of the battery terminal voltage difference by a predetermined 
amount. 
Note that in the above disclosed electronic battery charger device, the 
charging operation is continued until the battery terminal voltage 
difference exceeds the minimum stored value by a predetermined amount. 
This type of a charging operation is not suitable for lead-acid cells and 
can cause damage to the same. The battery charger device can only be used 
with nickelcadmium battery cells, thereby reducing the utility of the 
conventional battery charger device. 
In the above disclosed battery charger device, trickle charging is effected 
when the battery voltage is less than 0.7 volts. A storage battery with 
such an open circuit voltage is abnormal and should not undergo a fast 
charging operation. Therefore, the above disclosed battery charger is not 
capable of performing the trickle charging operation when a normal storage 
battery is installed. 
SUMMARY OF THE INVENTION 
Therefore, the main objective of the present invention is to provide a 
battery charger device which is capable of charging different types of 
rechargeable batteries and which can overcome the drawbacks which are 
commonly associated with the prior art. 
Another objective of the present invention is to provide a battery charger 
device which is capable of varying automatically the charging current to 
the rechargeable battery load in accordance with the charging state of the 
latter. 
In one aspect of the present invention, a battery charger device comprises: 
a power supply means for supplying charging current to charge a 
rechargeable battery load; 
a control switch means for connecting electrically the power supply means 
and the battery load; and 
a charging control unit including: a pulse generator means for generating a 
series of pulses which control the control switch means to connect and 
disconnect intermittently the power supply means and the battery load; a 
voltage memory means connected to the battery load for storing a maximum 
battery terminal voltage of the battery load therein; a voltage divider 
means receiving the maximum battery terminal voltage from the voltage 
memory means and deriving a fractional voltage from the maximum battery 
terminal voltage; and a comparator means for comparing the fractional 
voltage with a current battery terminal voltage from the battery load, 
said comparator means generating a control signal which prevents the 
control switch means from receiving the pulses from the pulse generator 
means, thereby disconnecting the power supply means from the battery load 
so as to terminate charging of the battery load when the current battery 
terminal voltage is less than the fractional voltage. 
In another aspect of the present invention, a battery charger device 
comprises: 
a power supply means for supplying charging current to charge a 
rechargeable battery load; 
a voltage-controlled current providing device which connects electrically 
the power supply means and the battery load; and 
a charging control unit including: a pulse generator means for generating a 
series of pulses which control the current providing device to connect and 
disconnect intermittently the power supply means and the battery load; a 
voltage memory means connected to the battery load for storing a maximum 
battery terminal voltage of the battery load therein; first and second 
voltage divider means which receive the maximum battery terminal voltage 
from the voltage memory means and which respectively derive higher and 
lower fractional voltages from the maximum battery terminal voltage; first 
and second comparator means for comparing a respective one of the higher 
and lower fractional voltage with a current battery terminal voltage from 
the battery load; a current control unit which generates an increasing 
analog voltage signal if the current battery terminal voltage is greater 
than the higher fractional voltage and a decreasing analog voltage signal 
if the current battery terminal voltage is less than the lower fractional 
voltage, said current providing device receiving the analog voltage signal 
from the current control unit and controlling the amount of charging 
current supplied to the battery load so as to correspond with the analog 
voltage signal; and a switch control unit which generates a control signal 
for preventing the current providing device from receiving the pulses from 
the pulse generator means when the analog voltage signal from the current 
control unit is less than a preset cut-off voltage, thereby disconnecting 
the power supply means from the battery load.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the first preferred embodiment of a battery charger 
device according to the present invention is shown to comprise a dc power 
supply (11), a current providing device (12) (such as a current limiter or 
a constant current device), a power supply control switch (13) and a 
charging control unit (2). The charging control unit (2) controls operably 
the control switch (13) so as to start or stop charging of a battery load 
(3). 
FIG. 2 is a waveform diagram illustrating the various signals which are 
obtained when the first preferred embodiment is operated. The charging 
control unit (2) is used to interrupt intermittently the supply of 
charging current to the battery load (3). The charging control unit (2) 
memorizes the maximum battery terminal voltage (V1) of the battery load 
(3). The charging control unit (2) then compares a fractional voltage (V2) 
of the battery terminal voltage (V1) with the current battery terminal 
voltage (V3) when the supply of charging current to the battery load (3) 
is interrupted. The spikes in the plot of the current battery terminal 
voltage (V3) indicate that the battery terminal voltage (V3) drops when 
the supply of charging current to the battery load (3) is interrupted. 
Charging of the battery load (3) is continued as long as the current 
battery terminal voltage (V3) is greater than the fractional voltage (V2). 
Otherwise, the charging control unit (2) controls operably the control 
switch (13) so as to break the electrical connection between the power 
supply (11) and the battery load (3). 
Referring to FIGS. 3 and 4, the charging control unit (2) includes a 
battery voltage processing unit (21), a no-load detector (22), a maximum 
battery terminal voltage (VI) memory unit (23), a voltage divider (24), a 
comparator (25), a pulse generator (26) and an AND gate (27). 
The processing unit (21) includes a capacitor (210) and a resistor (211). 
The processing unit (21) is connected across the battery load (3), thereby 
charging the capacitor (210) to the current battery terminal voltage (V3) 
via the resistor (211). The voltage across the capacitor (210) serves as 
one of the inputs to an operational amplifier (220) of the no-load 
detector (22). The no-load detector (22) is used to detect if a battery 
load (3) is connected to the battery charger device. The other input of 
the operational amplifier (220) is connected to a zener diode (221). The 
operational amplifier (220) compares the current battery terminal voltage 
(V3) with the reverse bias voltage of the zener diode (221). A diode (222) 
is used to connect the output of the operational amplifier (220) and the 
memory unit (23). If the current battery terminal voltage (V3) is greater 
than the reverse bias voltage of the zener diode (221), a high logic 
signal is present at the output of the operational amplifier (220), 
thereby preventing the conduction of the diode (222) to indicate the 
presence of a battery load (3). The operational amplifier (220) does not 
affect the operation of the memory unit (23) at this stage. 
The memory unit (23) stores the maximum battery terminal voltage (V1) in a 
capacitor (231). The memory unit (23) includes an operational amplifier 
(232) which is used to compare the maximum battery terminal voltage (V1) 
stored in the capacitor (231) with the current battery terminal voltage 
(V3). If the latter is greater than the former, the operational amplifier 
(232) generates a high logic signal to charge the capacitor (231) to the 
current battery terminal voltage (V3). Otherwise, the voltage across the 
capacitor (231) is maintained at its present value. The maximum battery 
terminal voltage (V1) is received by the voltage divider (24) which 
derives the fractional voltage (V2) therefrom. The ratio of the fractional 
voltage (V2) to the maximum battery terminal voltage (V1) depends upon the 
magnitude of the resistors (241, 242). The comparator (25) is responsible 
for comparing the fractional voltage (V2) with the current battery 
terminal voltage (V3). The comparator (25) generates a high logic signal 
if the current battery terminal voltage (V3) is greater than the 
fractional voltage (V2). The comparator (25) and the pulse generator (26) 
are connected to the inputs of the AND gate (27). When the outputs of the 
comparator (25) and the pulse generator (26) are at a high logic state, 
the output of the AND gate (27) is similarly at a high logic state, 
thereby causing a transistor (271) to conduct. The output (A) of the 
charging control unit (2) is at a low logic state, thereby permitting the 
supply of charging current to the battery load (3). 
When the output of the comparator (25) is at a high logic state but the 
output of the pulse generator (26) is at a low logic state, the output of 
the AND gate (27) is at a low logic state. The transistor (271) is in a 
non-conducting state, and the output (A) of the charging control unit (2) 
is at a high logic state, thereby preventing the supply of charging 
current to the battery load (3). It has thus been shown that the output of 
the pulse generator (26) is used to control the intermittent supply of 
charging current to the battery load (3). The output of the comparator 
(25), however, is used to terminate the charging operation when the 
current battery terminal voltage (V3) is less than the fractional voltage 
(V2). 
FIG. 5 illustrates how the charging control unit (2) controls the operation 
of the control switch (13). The control switch (13) is used to connect 
electrically the power supply (11) and the battery load (3). When the 
output (A) of the charging control unit (2) is at a low logic state, a 
relay (131) of the control switch (13) conducts, thereby permitting the 
flow of charging current to the battery load (3). When the output (A) of 
the charging control unit (2) is at a high logic state, the relay (131) is 
opened, thereby preventing the flow of charging current to the battery 
load (3). 
Referring once more to FIG. 2, a continuous increase in the terminal 
voltages (V1, V3) is detected as long as the battery load (3) has not been 
charged to a saturation point. When the battery load (3) has been charged 
to the saturation point, the terminal voltage (V3) becomes less than the 
fractional voltage (V2), thereby causing the output (A) of the charging 
control unit (2) to change to the high logic state and control the control 
switch (13) so as to break the electrical connection between the power 
supply (11) and the battery load (3). In this embodiment, the fractional 
voltage (V2) is preferably 0.9 times of the maximum battery terminal 
voltage (V1). The ratio of the fractional voltage (V2) to the maximum 
battery terminal voltage (VI), however, may be adjusted so as to 
correspond with the characteristics of the battery load (3) to be charged. 
It has thus been shown that the battery charger device of the present 
invention compares the current battery terminal voltage (V3) with a 
fractional voltage (V2) of the maximum battery terminal voltage (V1) in 
order to determine whether the battery load (3) has been charged to the 
saturation level. Of course, indicators (not shown) may be incorporated in 
the battery charger device of the present invention so as to indicate if 
charging of the battery load (3) is ongoing or has been terminated. Such 
circuit modifications can be easily accomplished by one who is skilled in 
the art and will not be detailed herein. 
FIG. 6 is a schematic circuit block diagram of the charging control unit 
(2') of the second preferred embodiment of a battery charging device 
according to the present invention. In this embodiment, the output of the 
comparator (25') is connected to the input of the pulse generator (26'). 
Referring to FIGS. 6 and 7, the charging control unit (2') further 
includes a battery voltage processing unit (21'), a no-load detector 
(22'), a maximum battery terminal voltage (V1) memory unit (23') and a 
voltage divider (24'). 
The charging control unit (2') is substantially similar to that shown in 
FIG. 4. The main difference between the charging control units (2, 2') 
lies in the configuration of the memory unit (23'). Referring to FIG. 8, 
the memory unit (23') includes an oscillator circuit (231'), an up-counter 
(232') and an operational amplifier (233'). The up-counter (232') receives 
pulse signals from the oscillator circuit (231') and generates an analog 
output which corresponds to the pulses received thereby. The operational 
amplifier (233') compares the analog output from the up-counter (232') 
with the current battery terminal voltage (V3). If the analog output is 
less than the current battery terminal voltage (V3), the oscillator 
circuit (231') continues to provide pulse signals to the up-counter 
(232'). If the analog output is greater than the current battery terminal 
voltage (V3), the operational (amplifier (233') generates a low logic 
signal which is received by the oscillator circuit (231'), thereby 
preventing the latter from further providing pulse signals to the 
up-counter (232'). The output of the upcounter (233') is thus maintained 
and corresponds to the maximum battery terminal voltage (V1). The 
succeeding operations are similar to those executed in the preceding 
embodiment. The maximum battery terminal voltage (V1) is received by the 
voltage divider (24'). The voltage divider (24') derives the fractional 
voltage (V2) from the maximum battery terminal voltage (V1). The 
comparator (25') compares the fractional voltage (V2) with the current 
battery terminal voltage (V3). The comparator (25') then generates a low 
logic signal if the current battery terminal voltage (V3) is greater than 
the fractional voltage (V2), thus preventing a transistor (251') from 
conducting. The pulse generator (26') utilizes an RC charge-discharge 
circuit to generate a pulse train output which serves as the output (A) of 
the charging control unit (2'). The output (A) is connected to the control 
switch (13) (Refer to FIG. 5) and is used to effect the intermittent 
supply of charging current to the battery load (3). 
The comparator (25') generates a high logic signal if the current battery 
terminal voltage (V3) is less than the fractional voltage (V2). The 
transistor (251') conducts at this stage and prevents a capacitor of the 
RC circuit of the pulse generator (26') from discharging, thereby 
preventing the pulse generator (26') from providing a pulse train output 
at the output (A) of the charging control unit (2'). Charging of the 
battery load (3) is terminated at this stage. 
FIGS. 9 and 10 are illustrations of the charging control unit (2") of the 
third preferred embodiment of a battery charging device according to the 
present invention. The charging control unit (2") is substantially similar 
to the charging control unit (2') of the second preferred embodiment. The 
charging control unit (2"), however, is provided with a reset control unit 
(27). The reset control unit (27) is controlled by the pulse generator 
(26") and includes a resistor (271) which is connected across a capacitor 
(272). The output terminal of the reset control unit (27) is connected to 
the no-load detector (22"). When the output of the pulse generator (26") 
changes from a high logic state to a low logic state, the reset control 
unit (27) generates a reset signal to the no-load detector (22"), thereby 
causing the output of the latter to change to a low logic state and thus 
reset the memory unit (23"). The analog voltage which was previously 
stored in the memory unit (23") is thus erased. Therefore, the maximum 
battery terminal voltage (V1) for the succeeding charging period can be 
stored in the memory unit (23") when the output of the pulse generator 
(26") reverts to the high logic state. 
The preceding embodiments relate to a limited current or to a constant 
current charging method. The configuration of the preceding embodiments 
can be modified so as to permit automatic adjustments in the amount of 
charging current in order to correspond with the charging characteristics 
of the battery load. 
Referring to FIG. 11, the fourth preferred embodiment of a battery charger 
device according to the present invention is shown to comprise a charging 
control unit (30), a pulse generator (4), a status control unit (5), a 
current control unit (6), a current status indicator (60), a 
voltage-controlled current providing device (7), a switch control unit 
(8), a charging indicator (71) and a dc power supply (72). 
FIG. 12 is a waveform diagram illustrating the various signals which are 
obtained when the fourth preferred embodiment is operated. As with the 
preceding embodiments, the charging control unit (30) memorizes the 
maximum battery terminal voltage (V1) of the battery load (10). The 
charging control unit (30) then compares higher and lower fractional 
voltages (V21, V22) of the maximum battery terminal voltage (V1) with the 
current battery terminal voltage (V3). A large charging current is 
supplied to the battery load (10) if the current battery terminal voltage 
(V3) is greater than the higher fractional voltage (V21). A small charging 
current is supplied to the battery load (10) if the current battery 
terminal voltage (V3) is less than the lower fractional voltage (V22). 
Charging of the battery load (10) is terminated if the current battery 
terminal voltage (V3) is between the higher and lower fractional voltages 
(V21, V22). 
Referring to FIGS. 13 to 16, the charging control unit (30) is shown to be 
substantially similar to the charging control unit of the preceding 
embodiments. The charging control unit (30), however, has two sets of 
voltage dividers (31) and two sets of comparators (32). Each of the 
voltage dividers (31) derives the corresponding fractional voltage (V21, 
V22) from the maximum battery terminal voltage (V1). Each of the 
comparators (32) compares the current battery terminal voltage (V3) with a 
corresponding one of the fractional voltages (V21, V22). The outputs of 
the comparators (32) are received by the status control unit (5). The 
status control unit (5) is a logic control circuit which receives the 
pulse output of the pulse generator (4) and which informs the current 
control unit (6) if a large or small charging current is to be provided to 
the battery load (10). 
The current control unit (6) includes an up/down counter (61), a 
digital-to-analog (D/A) converter (62) and a filter (63). The up/down 
counter (61) initiates an up or down counting operation in accordance with 
the output of the comparators (32) and the logic output of the status 
control unit (5). If the current battery terminal voltage (V3) is greater 
than the higher fractional voltage (V21), the up/down counter (61) 
executes an up counting operation. The up/down counter (61) executes a 
down counting operation if the current battery terminal voltage (V3) is 
less than the lower fractional voltage (V22). The count output of the 
up/down counter (61) is received by the D/A converter (62) and is 
converted into a corresponding analog voltage signal. The analog voltage 
signal is received by the voltage-controlled current providing device (7) 
via the filter (63). The current providing device (7) controls the amount 
of charging current from the power supply (72) to the battery load (10). 
When the up/down counter (61) is conducting an up counting operation, an 
increasing charging current is supplied to the battery load (10). When the 
up/down counter (61) is conducting a down counting operation, a decreasing 
charging current is supplied to the battery load (10). The analog voltage 
output of the current control unit (6) is also received by the current 
status indicator (60) so as to indicate clearly the amount of charging 
current being supplied to the battery load (10). 
The switch control unit (8) is used to determine if charging of the battery 
load (10) is to be terminated. The switch control unit (8) includes a 
cut-off voltage indicator (81), a timer (82) and a comparator (83). The 
timer (82) permits charging of the battery load (10) for a predetermined 
time period after the fourth preferred embodiment has been switched on. 
Therefore, the instantaneous surge in the terminal voltage of the battery 
load (10), which instantaneous surge usually occurs when charging of the 
battery load (10) is initiated, is prevented from resulting in the 
erroneous operation of the charging control unit (30). The cut-off voltage 
indicator (81) is set so as to generate a voltage output which corresponds 
to the battery terminal voltage when only 10% of the normal charging 
current is being supplied to the battery load (10). The comparator (83) is 
used to detect a condition wherein the analog voltage output of the 
current control unit (6) is less than the preset voltage of the cut-off 
voltage indicator (81). Upon detection of such a condition, the comparator 
(83) generates a control signal to the voltage-controlled current 
providing device (7) so as to terminate charging of the battery load (10). 
Of course, the configuration of the switch control unit (8) may be 
modified so as to terminate charging of the battery load (10) after a 
preset charging period has elapsed. Such circuit modifications are known 
to one skilled in the art and will not be detailed herein. 
The advantages and characterizing features of the battery charger device of 
the present invention are as follows: 
1. The battery charger device compares the current battery terminal voltage 
with a fractional voltage of the maximum battery terminal voltage in order 
to determine whether the battery load has been charged to a saturation 
point. Charging of the battery load is terminated automatically when the 
current battery terminal voltage drops below the fractional voltage. The 
battery charger device of the present invention is ideal for use with 
different kinds of storage batteries since overcharging or undercharging 
of the storage batteries can be effectively prevented. 
2. In one embodiment of the battery charger device of the present 
invention, the amount of charging current can be varied automatically so 
as to correspond with the charging characteristics of the battery load, 
thereby adapting the battery charger device for use with different kinds 
of storage batteries. 
While the present invention has been described in connection with what is 
considered the most practical and preferred embodiments, it is understood 
that this invention is not limited to the disclosed embodiments but is 
intended to cover various arrangements included within the spirit and 
scope of the broadest interpretation so as to encompass all such 
modifications and equivalent arrangements.