Could be a variety of things... not sure, but if it was putting out too much voltage it might also cause the light to illuminate, not sure though. Maybe if it was fluctuating, also.

You could take a volt/ohm meter to the battery while it's running to see what it's putting out. Also might not be putting out enough, so the light is letting you know it's a problem, but it's still enough not to leave you stranded. Yet.