Did you check the voltage while the engine was running across the battery terminals with a voltmeter? Should be 14+ volts if it is charging. If 14+ or more, probably the battery. If less than 14, there is a fuse and resistor in the charging path that you can check. I have a YT6700, so different engine, but the diagram >may< apply to the YT3600 as well (attached). If you still don't figure it out, follow the wiring to the charge coils under the flywheel (again, I assume the YT3600 is similar to the YT6700 here, but not sure). If your coils are toast, then not sure if you can get new ones. You may be able to find substitutes? Let us know what you find out.