[Solved] Is it a battery problem or SMC problem?

Barmar Asks: Is it a battery problem or SMC problem?
I have a mid-2015 MacBook Pro running High Sierra. Except when I’m traveling, I always use it in clamshell mode and connected to power. I rarely put it to sleep (I have Energy Saver set to just put the display to sleep).

On July 21 I noticed that the light on the power connector had gone to amber, but Energy Saver said that the battery was fully charged. However, coconutBattery said the battery was at 95%.

Today I noticed that the lightning bolt was no longer showing on the battery status in the menu bar, and when I click on it it says “Battery is not charging”. Energy Saver says that the battery is at 97%, but coconutBattery says 81%. Here’s its full report:

Current Charge 6347 mAh
Full Charge Capacity 7092 mAh
89.5%