Is your MacBook not charging when it's plugged in? Don't worry, it's a feature, not a bug. In a recently published support document, Apple explained why some users see "Not charging" after connecting their MacBook to a charger.
The behavior stems from an update Apple released earlier this year that extends the lifespan of a laptop battery by slowing down the chemical aging process. This is accomplished by preventing the battery from reaching maximum capacity when it's charging.
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Available in macOS Catalina 10.15.5 or later, the battery health management tool monitors your battery's temperature history and charging patterns then determines the optimal maximum charging percentage.
"When battery health management is turned on, you might occasionally see ”Not Charging” in the battery status menu of your Mac, and your battery's maximum charge level might be lowered temporarily," Apple wrote. "This is normal, and it's how battery health management optimizes charging. Your Mac resumes charging to 100 percent depending on your usage."
Apple laptops are notoriously difficult to service so swapping out a battery when it inevitably runs out of juice can be an involved and costly procedure. When enabled, the battery health management feature should stall the battery degradation process so you can use your MacBook for longer without seeing a substantial decline in battery life.
Now, back to the issue at hand. That "Not charging" message you're seeing is probably misleading, so don't worry too much about it. Hopefully, Apple comes up with a less worrying notification, like "battery extender mode" so customers don't think there is something wrong with their MacBook or power adapter. Having said that, if your battery continues to decline when your laptop is plugged in, then you might have a faulty charger or you are not using a compatible charger or cable.