Bike needs to run above 4000rpms for about 15-minutes to charge battery back up after single start.
1. Measure voltage at battery after starting. Leave bike running. Then measure voltage every 2-minutes after that for about 10-minutes. What is trend in measurements?
2. Measure AC voltage of yellow pairs of wires coming out if stator. What is this AC voltage at idle? What is AC voltage at 4000rpms?
These numbers will tell you where problem is.
Also get
battery charger.