[JerryHawk250]

The battery will charge will the bike is running just like an automobile. It could be a number of things. First thing to do is check with a multi meter that it is putting out voltage to the battery while running. Check the voltage on the battery while not running then check it while running. Voltage output should be higher while running.
Check connectors coming from the stator and voltage regulator have the pins pushed in all the way. I've had a couple on my Hawk that weren't set in all the way and would occasionally cut out.
Check all cables from the battery are tight and making good contact. Especially the ground.
There's always the possibility of a bad battery, voltage regulator or stator. But usually it turns out to be bad connections.