Broad answer: the charging status of any battery should be measured with a hydrometer which measures the specific gravity of the battery fluid. The stator needs to be measured with a multimeter to measure its output. The rectifier needs to be tested to make sure it is rectifying and regulating the electricity from the stator before it goes to the battery. The battery needs to be in good condition. The battery terminals need to be clean as does the frame ground. ALL plugs from the stator to the rectifier to the battery need to be clean and corrosion free. A melted plastic plug is a hint to a dirty plug, additional resistance and excessive heat. Start at the beginning and finish at the end. IOW, be methodic and don’t lose patience. Electricity is easy stuff when approached correctly. Good luck. –Ride On