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This lead-acid battery charging circuit is designed to charge two 12V/7Ah lead-acid batteries in series. The number of cells per battery is also an important parameter, and we designed the circuit assuming that each 12V battery contains 6 cells. When two batteries are connected in series, their voltages add, but their current capacity remains the same. Therefore, connecting two 12V/7AH batteries in series can be regarded as a 24V/7AH battery.
The circuit shown here is a current limiting lead-acid battery charger based on the popular LM317 adjustable voltage regulator IC. The charging current depends on the value of resistor R2, which here is set to 700 mA. Resistor R3 and POT R4 determine the charging voltage. Transformer T1 steps down the mains voltage and bridge D1 is responsible for rectification. C1 is the filter capacitor. Diode D1 prevents backflow of current from the battery when the charger is switched off or when the mains voltage is not available.
To set the charging voltage, turn on the charger, connect a voltmeter to the output terminals, and adjust R4 so that the voltmeter reads 28V. Now that the charger is ready, we can connect the battery. This charger is specially designed for connecting two 12V/7Ah/6 cell lead acid batteries in series or one 24V/7Ah/12 cell lead acid battery.
Remarks:
Assemble the circuit on a high quality printed circuit board.
T1 can be a 230V primary, 35V/3A secondary step-down transformer.
If a 3A bridge is not available, use four 1N5003 diodes to build a bridge.
The LM317 must be equipped with a heatsink.
R2 = 0.85 ohms is not a standard value. This is accomplished by connecting 6.2 ohm and 1 ohm resistors in parallel.
A 2A fuse is available for F1.