When you want to build the lead-acid battery charger project for 6V battery or 12V. Although there are many methods to choose from. I like using IC LM317 Because easy to use, with minimal equipment. Cheap and good.
The LM317 acts as a current regulator, It is connected to the battery thought a one way diode D1, limiting resistor R1, and a bias resistor R2. The D1 prevents the circuit from battery discharging, when the power out off from this circuit. As the battery charges, the voltage across A point potentiometer R5 and some point rises, to turn on the SCR1. Then current from the regulator LM317 can flow to ground, so now the IC1 is in functions the voltage-regulating mode. The R6 be used to control the output voltage.
When the SCR1 turn on, it also provides LED1 through R3 with a path to ground.
But when LED1 is on, this circuit is in the voltage-regulating mode, while LED1 is off to be in the current-regulating mode.