To charge a battery you have to have more amps than is stored in the battery and it's doubtful that this unit puts out enough to do it even with the lights off, if you have a VOM you can check and see, the battery will be rated in amps, look at it this way, if the battery is rated at 12amps and the gen put out 3 amps it can only charge the battery to 3 amps, you would have 12v but only 3 amps, the same as the Gen is putting out and there would be no reserve stored in the battery, you need 14.3 volts to effectively charge a 12v battery,,,good luck with that.:-(