Look at the circuit diagram of battery charger using SCR. The battery is charged with small amount of AC voltage or DC voltage. So if you want to charge your battery with AC source, we need first limit the large AC voltage, need to filter the AC voltage to remove the noise, regulate and get the constant voltage and then give the resulting voltage to the battery for charging. Once charging is completed the circuit should automatically turned off. For detailed information about this circuit, read the post - Battery charger using Silicon Controlled Rectifier.