[CIRCUIT IDEAS]
Auto Turn Off
Battery Charger
DEEPAK GUPTA
Frontline Electronics, Pvt. Ltd., Salem
The aim of this project is to introduce a
technology for reducing the loss of power that is
happening always without human intensions. The
‘auto’ referred here comprises of ‘Automatic
controlled battery
charger.Over charging of
batteries not only reduces their life but also provide
serious threat to humans. The project is mainly
based on automation of charging batteries.
All mankind are responsible for preserving the
energy resources. That is, Electricity is a man
made energy resource. Nowadays lack of its
power is loosing by unwanted usage of human
society. ‘Auto turn off battery charger’ charger
can automatically, controlled the supply of power
to the electronic device during charging.
Here is a 12 volt Lead Acid battery charger that shut
off the charging process once the battery attains full
charge. This prevents overcharging of the battery
so that, the charger can be left unattended. If the
terminal voltage of the battery reduces below the
set level, say 13.5 volts, the circuit automatically
turns on to the charge mode.Charging current as
well as the power to the circuit is obtained from a 018 volt 2 Ampere step-down transformer. The low
voltage AC is rectified by the bridge rectifier
comprising D1 through D4 and made ripple free by
the smoothing capacitor C1. For charging
purpose, 18 volt DC is used while to power the
circuit, 9 volt regulated DC from IC1 is used. IC2
(CA3140) is used as a simple voltage comparator
to drive the relay. Its inverting input gets 4.7 volt
reference voltage from the Zener ZD, while the non
inverting input gets an adjustable voltage through
the POT VR1.So normally, the inverting input pin 2
gets higher voltage from the Zener (as adjusted by
VR1) and output of IC2 remains low. T1 then
remains off keeping the relay off. The charging
current passes to the battery through the NC
(Normally Connected) contacts of the relay.
When the terminal voltage of the battery increases
to 13.5 volts, pin 3 of IC2 gets higher voltage than
pin2 and the output of IC2 becomes high. This
activates the relay and the contacts break.
Charging current to the battery cut off and the relay
remains as such since the battery voltage(13.5V or
more) keeps the voltage at pin3 of IC2 is higher
than that of pin 2.
Note that before connecting the battery, set the
input voltage to IC2 using a fully charged battery or
variable power supply. Turn the switch S1 to the off
position and switch on the power. Then connect a
fully charged battery/ variable power supply to test
points TP observing polarity. Measure the input
voltage to pin 3 of IC2.
Slowly adjust VR1 till the input voltage to pin 3 of
IC2 raises to 5 volts. At this point, relay should
energize and Red LED turns on. Then connect the
battery for charging and switch on S1. If the battery
takes charge, current to pin 3 of IC2 will be low
since most of the current drain occurs into the
battery. This keeps the relay off. When the battery
voltage increases above 13.5 volts, no more
current passes into the battery, so that the voltage at
pin3 of IC2 rises and relay turns on.
V0
3
R5
1K
2
V1
GND
IC1
7809
1
D1
D2
50Hz
C1
470uF
R1
1K
B1
12V
C2
100uF
Ic2
1K
D3
RL
7
D4
D5
D3
RED LED
R2
Rv14 7K
3
6
2
GREENLED
ZD
R4
1K
4
1
5
230V
R3
470R
T1
BC548