I am having the exact same problem. Up until the latest Marshmallow 6.0.1 update my battery life was fine...only needed to charge once a day. Now I'm lucky to get a few hours, even in Ultra Power Saving mode, and that's if I barely use my phone for anything. The battery drain is happening when the phone is idle...for example overnight when no apps are running, or when it's sitting on my desk at work during the day. The biggest battery user is Android OS and sometimes Google Services. I know all the ways to minimize battery use but they aren't helping. My phone is pretty much useless at this point. I've seen others with this problem...would really appreciate hearing that a fix is in the works because it's obviously a widespread issue.