Debugging a battery charger circuit
The 560 Ohm resistor feeding gate current to the thyristor via the 1n4002 diode, is almost certainly too high a value leading to the device not being turned on properly. This in turn will increase the dissipation and cause over heating. You need to reduce the value at least by 10 to about 56 Ohms. One simple way to arrive at the value is to monitor the battery voltage and reduce the value gradually until the voltage stops increasing. You may destroy the thyristor if you reduce it too far so no less than say 33 Ohms. The best way is to monitor the gate current and set it at the manufacturers recommended value from the data sheet.