Normally, chargers available in the market do not have any sort of
control except for a ro-tary switch that can select different tap-pings
on a rheostat, to vary the charging current. This type of control is not
adequate because of the irregular fluctuations in the mains supply,
rendering the control ineffective. A simple circuit intended for
automatic charging of lead-acid batteries is presented here. It is
flexible enough to be used for large capacity inverter batteries. Only
the rating of transformer and power transistor needs to be increased.
The circuit has been basically designed for a car battery (about 40 Ah
rating), which could be used for lighting two 40W tube lights. The
circuit includes Schmitt trigger relay driver,float charger,and battery
voltage monitor sections. The Schmitt trigger is incorporated to avoid
relay chattering. It is designed for a window of about 1V. During
charging, when the battery voltage increases be-yond 13.64V, the relay
cuts off and the float charging section continues to work. When battery
voltage goes below 11.66V, the relay is turned on and direct (fast)
charging of the battery takes place at around 3A. In the Schmitt
trigger circuit, resistors R1 and R2 are used as a simple voltage
divider (divide-by-2) to provide battery voltage sample to the inverting
input terminal of IC1. The non-invert-ing input terminal of IC1 is used
for reference input derived from the output of IC2 (7806), using the
potentiometer arrangement of resistors R3 (18 kilo-ohm) and R4 (1
kilo-ohm).
Automatic Battery Charger Circuit Diagram:
LED1 is connected across relay to indicate fast charging mode. Diodes D3
and D6 in the common leads of IC2 and IC3 respectively provide added
protecion to the regulators. The float charging section, comprising
regulator 7812, transistors T3 and T4, and few other discrete
components, becomes active when the battery volt-age goes above 13.64V
(such that the relay RL1 is deenergised). In the energised state of the
relay, the emitter and collector of transistor T4 remain shorted, and
hence the float charger is ineffective and direct charging of battery
takes place.
The reference terminal of regulator (IC3) is kept at 3.9V using LED2,
LED3, and diode D6 in the common lead of IC3 to obtain the required
regulated output (15.9V), in excess of its rated output, which is needed
for proper operation of the circuit. This output voltage is fed to the
base of transistor T3 (BC548), which along with transistor T4 (2N3055)
forms a Darlington pair. You get 14.5V output at the emitter of
transistor T4, but because of a drop in diode D7 you effectively get
13.8V at the positive terminal of the battery. When Schmitt trigger
switches ‘on’ relay RL1, charging is at high current rate (boost mode).
The fast charging path, starting from transformer X2, comprises diode
D5, N/O contacts of relay RL1, and diode D7.
The circuit built around IC4 and IC5 is the voltage monitoring section
that provides visual display of battery voltage level in bar graph like
fashion. Regulator 7805 is used for generating reference voltage. Preset
VR1 (20 kilo-ohm) can be used to adjust voltage levels as indicated in
the circuit. Here also a pot meter arrangement using resistors R7, R8,
and R9 is used as ‘divide by 3’ circuit to sample the battery voltage.
When voltage is below 10V, the buzzer sounds to indicate that the safe
dis-charge limit has been exceeded. Link
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