why does my macair shutdown when battery over 20

Hello @ptutts and thank you for the background on this computer

It has been sometimes since working in Yosemite to bear with me if some of the instructions are off the mark.

If you can get to the Utilities Folder >> System Information

In there on Left Sidebar are listed many categories

We are looking for either Power or Battery categories

If Battery is available chose it and on the Right Side see what it is reporting Specific to Full Battery Cycles

A computer from 2016 in 2023 = 7 year old battery and not doubt well used over that timeframe.

Batteries are considered Consumable Products.

Over time it will degrade to a point where it needs to be replaced.

As Learned Colleague @HWTech has mentioned and agree with it, shutting down at 20% battery power remaining generally indicates the battery has reached a point where it can not lower supply reliable power to the computer

Apple Batteries are rated for 1000 Full Battery Cycles and / or 80% Capacity before needing Evaluation or replacement.

Option-click the battery icon on the menu bar to check the battery condition. On recent versions of macOS, if the condition is "Normal", then it won't show any condition....only if a battery is worn out or bad will recent versions of macOS show the battery condition "Service Recommended".

You can also try running the Apple Diagnostics to see if any hardware issues are detected.

More than likely the battery is failing even if the battery condition is "Normal" and the diagnostic passes. Many battery hardware failures are not detected by macOS or the diagnostics. A laptop which powers off at 20% tends to indicate a hardware issue with the battery.

Yes it is descriptive, though very little actionable information has been provided.

Like the Age of the computer, how many Full Battery Cycles the battery has experienced, if the Battery the Original Apple Battery or an After Market Replacement battery.

If an After Market Battery replacement, was it Purchased from Apple or an Apple Authorized Service Centre ?

Did they Replace the Battery or was it done by someone else ?

Welcome and thank you too