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Use spotlight to search for activity monitor and open it.
In the app, look for CPU% and determine the highest grade of usage and what it relates to such as a know or unknown app or service running in the background.
But you might have a faulty battery and a reservation might be required for a full cycle count.
Determining battery cycle count for Mac notebooks
Please post the EtreCheck output as a "Reply" to this thread
Use the EtreCheck "Share" button to "Copy Report" (See the image below)
And then Paste in a "Reply" to this thread.
If, AND ONLY IF, you get the error:
"The message contains invalid characters"
then try posting to PasteBin.com, and give us a PasteBin URL link.
EtreCheck is a tool that helps Apple Support Community volunteers debug problems without any access to the troubled computers. Debugging problems can be a difficult task even when the machine is in front of you. Attempting it via a discussion forum is extremely difficult. EtreCheck is a great help that regards.
Question: My Macbook Air will not keep the charge for very long