Basically I'd like to have one brightness setting configured for when I'm plugged in through the adapter and charging the macbook from wall power, vs a lower brightness setting when I'm running off battery to preserve battery life. This configuration swap would be in response to the changed power state of the laptop. Does MacOS support such behavior?
MacBook – way to have the macbook set its brightness based on it running from battery or from line power
auto-brightnessbatterymacbook propower-management
Related Solutions
You do not need to disconnect your MacBook Pro's battery. Your battery will stop charging once it is full. Apple's modern batteries are much smarter than previous designs.
To get the most out of your MacBook Pro's battery, follow the Notebook Battery advice from Apple: unplug and use your battery until empty about once a month, then charge back up to full.
At the time of answering, Apple's advice read:
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing.
If you need help following Apple's advice, use Battery Guardian; it is free and will remind you when to deplete your battery.
Don't worry. It will use the wall outlet if that offers enough power which it should using the original AC adaptor.
What if the power is going from 100% down to 99.6%, then back up to 100% constantly?
That was until about a decade ago or so. Modern laptops even don't charge your battery if it is eg. at 98% for reducing unnecessary cycles. Don't bother about your battery in your computer.
If you want to care for your battery, just remember every cycle shortens battery life, no matter if 10 minutes on battery or completely emptied it. The battery has also some natural aging, so it will die after some years even if you never touch it.
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Best Answer
While macOS does not support the exact behavior you are looking for (setting custom brightness when switching between power states), you can configure your Mac portable to slightly dim the brightness automatically when on battery power.
To do that, go to System Preferences → Energy Saver and enable Slightly dim the display while on battery power.