I have the same device a month ago and I have same problem not as much of you but battery wasn't lasting 6-7 hours.
I think found a solution for that and now my battery last 8-9 hours web browsing and Netflix.
First I noticed that System Preferences > Displays here, as a default scaling setting "Looks like 1440x900" but this setting using
graphics not as 1440x900. It is using graphics as 2880x1800. So I changed scaled settings to "Looks like 1280x800" and it is using graphics as native screen resolution 2560x1600. Why this is not as a default setting but after this change battery last at least 1-2 hours long. Before that I contact with Apple Support and we monitor one full cycle with Activity Monitor to be sure nothing unusual. You should check that first some 3rd party app can decrease battery life background.
And one more thing I use is cancelling turbo boost of processor, because 2.0 ghz base clock speed is enough for web browsing and simple daily tasks. With this trick I can use my MacBook at least 9 hours until %5 percent at night with lowest brightness.
I hope these will help you too