How to get my car to idle a bit higher?

Im having trouble with the electrical system I have a new altenator and battery and my battery gauge go down when I stop in the lights

I did the alt is giving me 15v when I turn the lights on it drops to 11v

Is brand new and the battery I dont know what else to do

your vehicle should be drawing in somewhere between 12 and 14 volts with your headlights stereo,a/c,fog lights,ect.Make sure you have good ground wire connections especially if you have added anything aftermarket.Make sure when you ground that wire,that there is no paint,oil,grease,mud,dirt,rust,anything that will interfear with a good ground.Just make sure you have a clean bare metal too metalground.GOOD LUCK!!