Q: My battery is draining fast after Sierra OS update.

Is this still happening after a System Management Controller (power management) re-set?

• Reset the System Management Controller (SMC) on your Mac - Apple Support

Does this happen when the unit should be asleep? ~ Or just while its expected to be running?

• Apps can affect Mac performance, battery runtime, temperature, and fan activity - Apple Support

Third party 'cleanmymac' and other flaky utilities can damage the system & make for bad issues.

Out-dated and irrelevant applications or utilities, also can slow the system. An Etrecheck report

may be helpful for others who see it, to offer suggestions. It can also be used to remove adware.

Hard to say what may be the background and history of the system. You could try the Etrecheck

app to see what the report it can generate says about the basics. Odd apps & adware can show.

About Etrecheck: http://etrecheck.com/#about

Also the Console 'utility logs' do show everything that goes on in Mac OS X. ~ An older article:

https://www.macobserver.com/tmo/article/mac_os_x_using_console_to_troubleshoot_p roblems/

• Hands on with macOS Sierra's Console: Now it's easier to get the Mac ...

www.macworld.com/.../hands-on-with-macos-sierras-console-now-its-easier... ~ newer article.

There are so many useful things built into the macOS. Most things generally tend to themselves.

Good luck & happy trails!

      Reset NVRAM/PRAM.  http://support.apple.com/kb/PH18761

      Reset SMC. Choose the method for “On Mac notebooks with non-removable battery”.

      https://support.apple.com/en-us/HT201295

      Try startup in Safe Mode.

       Start up in Safe Mode. https://support.apple.com/kb/PH21875?locale=en_US