My cobalt also runs about 15v on a cold start, will drop down to 12.8-13.4 later on. I googled around and found this is normal for newer GM vehicles. My battery looks like the original part, unless it was replaced with an OEM battery (2007), and it isn't bubbling over with electrolyte so I'm not worried about it.
Try checking it on a cold start, then after the engine has reached full operating temp (~196 degrees). It depends on the voltage regulator sensing when the battery doesn't need to be charged anymore, so if your alternator always stays at 15v then it could mean your battery is going bad. Or could just be a bad voltage regulator.
Last edited by economichitman; 01-05-2016 at 11:47 PM.