MacBook Air shows battery as power source when plugged in I have a MacBook Air (mid 2011) that I only use from time to time. The battery no longer works but I don't mind since I really only use this machine when connected via the power adapter.

The problem is that for some reason it says "Power Source: Battery" when it's definitely not running off the battery (if I disconnect the adapter the computer immediately shuts off). I've been ignoring this for some time because it didn't really get in the way; however, I'm now trying to update the machine from Sierra to High Sierra and the installer won't allow me to continue unless I connect to a power source that isn't a battery.

Is there any way to either force the machine to only register as running off a power cable OR get the High Sierra installer to bypass the battery check?

I don't use this machine that much so I'm trying to avoid having to take it in just so I can update it.
Try an SMC reset. It's responsible for a lot of stuff, including battery and battery reporting. 

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