Need to fully charge those batteries. It may take a day or more on a tender but leave it until they top-off. One of my USB plugs has a volt meter on it and I can run it with all my lights on, GoPro, LED bar, GPS and the CVT booster fan and it stays at 14.4 volts above 2500 rpm. At idle it does come down to 13.7 but that's to be expected. Need to see where yours is. You may have a bad coil in the stator or something with the rectifier/regulator.