Update MacOS on a MacBook with dead battery I have a macbook 12'' running sierra that I use occasionally.
The battery died some time ago, showing an X in the status bar.

I want to update to a new version of macOS but the installer shows: 


  Your computer is not connected to a power source.
  You must plug in and connect your power adapter before installing.


Obviously the device is connected to the power source (the battery is dead). 



I have already done the SMC reset. Didn't help.

So:
Can I convince the updater to install despite the fact that it thinks I am on battery power?
Alternatively:
Can I convince the system to recognize my power connection as an actual power connection?
Well baring the obvious and replacing the bad battery you will probably have to install the OS on a different computer and reinstall the hard drive.  The Mac power module is not responding that the laptop is connected to AC but responding that the battery is bad.  Apple will replace the battery for $199 so keep that in mind while doing any work around.

You can also put this MacBook in USB target mode and run the installer from another Mac to perform the upgrade without fixing the hardware. The battery service there is relatively labor and parts expensive due to the thin assembly, delicate cabling and glue that makes the cells part of the case structure. You’re getting a partial frame replacement when the battery is exchanged for $200 and possibly new top case / keyboard if the cabling or parts don’t fit when it’s out for repair.