This lead acid battery charger circuit is
designed in response to a request from Mr.Devdas .C. His requirement was
a circuit to charge two 12V/7AH lead acid batteries in series.Anyway he
did not mentioned the no of cells per each 12V battery. The no of
cells/battery is also an important parameter and here I designed the
circuit assuming each 12V battery containing 6 cells. When two batteries
are connected in series, the voltage will add up and the current
capacity remains same. So two 12V/7AH batteries connected in series can
be considered as a 24V/7AH battery.
The circuit given here is a current limited lead acid battery charger
built around the famous variable voltage regulator IC LM 317. The
charging current depends on the value of resistor R2 and here it is set
to be 700mA. Resistor R3 and POT R4 determines
the charging voltage. Transformer T1 steps down the mains voltage and
bridge D1 does the job of rectification. C1 is the filter capacitor.
Diode D1 prevents the reverse flow of current from the battery when
charger is switched OFF or when mains power is not available.
Assemble the circuit on a good quality PCB.
T1 can be a 230V primary, 35V/3A secondary step down transformer.
If 3A Bridge is not available, make one using four 1N5003 diodes.
LM317 must be fitted with a heat sink.
R2 = 0.85 ohm is not a standard value. You can obtain it by combining a 6.2 ohm and 1 ohm resistors in parallel.
F1 can be a 2A fuse.
To setup the charging voltage, power ON the charger and hook up a
voltmeter across the output terminals and adjust R4 to make the
voltmeter read 28V. Now the charger is ready and you can connect the
This charger is specifically designed for two 12V/7AH/6 cell lead acid
batteries in series OR a 24V/7AH/12 cell lead acid battery.