The Battery Drain
Time Machine is an incredible built-in backup solution for macOS, providing hourly snapshots of your entire system. If you use a portable external hard drive, or if your Mac backs up wirelessly to a network-attached storage (NAS) device, Time Machine will dutifully attempt to perform these backups every single hour, exactly on schedule. However, if you are working from a coffee shop on battery power, a sudden, massive Time Machine backup spinning up in the background will drastically consume your CPU cycles, thrash your internal drive, and severely drain your battery life, potentially cutting your working time in half.
Restricting Backups to AC Power
To preserve your battery life and stop macOS from automatically executing heavy Time Machine backups when disconnected from a power outlet, you must adjust the Time Machine scheduling conditions within System Settings.
Click the Apple logo in the top-left corner of the menu bar and select System Settings. Scroll down the left-hand navigation pane and click on General. In the right-hand panel, click on Time Machine. You will see your configured backup drive(s) listed here. Look for a button labelled Options… in the bottom right corner of this specific window (on newer versions of macOS, this might be located under a dedicated “Settings” button within the Time Machine panel). Click it. In the options dialogue, look for a specific checkbox labelled Back up while on battery power. Ensure this checkbox is completely unchecked. Click Done. macOS will now intelligently pause all automatic backups until you plug your MagSafe or USB-C charger back in.