Specifications

CIRCUIT IDEAS
S.C. DWIVEDI
N
ormally, 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 charg-
ing current. This type of control is not
adequate because of the irregular fluc-
tuations in the mains supply, rendering
the control ineffective.
A simple circuit intended for auto-
matic charging of lead-acid batteries is
presented here. It is flexible enough to
be used for large-capacity inverter bat-
teries. Only the rating of transformer
and power
transistor
needs to
be in-
creased.
The
circuit has
been basi-
cally de-
signed for
a car bat-
tery
(about 40
Ah rat-
ing),
which
could be
used for
lighting
two 40W
tubelights.
The circuit
includes
Schmitt
trigger re-
lay driver,
float
charger,
and bat-
tery volt-
age moni-
tor sec-
tions.
The
Schmitt
trigger is
incorpo-
rated 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, resis-
tors 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).
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 protec-
tion to the regulators.
The float charging section, compris-
ing 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 de-energised). In the
energised state of the relay, the emit-
ter and collector of transistor T4 remain
shorted, and hence the float charger is
ineffective and direct charging of bat-
tery takes place.
YASH DEEP
CIRCUIT IDEAS
AUTOMATIC BATTERY CHARGER
185