98 Grizz 600 Voltage issue question

Hi all,
I charged my battery up to 12.8 volts and installed it. As soon as I turn the key on, the voltage across the battery drops to 4.8 volts and of course no start. When I boost it with my truck it fires right up and runs perfectly. I believe that the fact that it runs well eliminates the stator as a suspect. I’d sure appreciate any ideas that anyone might have. Thanks