Here is a 12 volt lead acid automatic 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 0-18 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.
Schematic of the Automatic Battery
Charger Circuit
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.
Charger Settings
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.