Motorcycle Questions? Ask a Mechanic.
Likely a charging system issue. The first test is to remove the battery, charge it on a charger of at least 10 amps for 30 minutes and then load test it. An auto parts store or bike shop can do this for you if you don't have the equipment.
If the battery is verified good then re-install it. With the bike off the battery voltage should be very near 12.8 volts DC. With the bike on it should only drop to about 12.5 or so. Start the bike and rev it a little and you should see the voltage go up to around 13.6 or so. It should not go above 14.2. If it is much lower than 13.6 or higher than 14.2 then you have a charging system problem that will need to be diagnosed.
I hope this helps. Please let me know if I can be of further assistance, thanks, P.J.