Update 2020-08-05: It's a new feature by Apply and intended behavior
If you see 'Not Charging' when your Mac notebook is connected to power
Depending on its settings, your Mac might temporarily pause charging to help calibrate battery health management, a feature designed to improve the lifespan of your battery.
If your Mac notebook has Thunderbolt 3 ports and uses macOS Catalina 10.15.5 or later, you can use battery health management to help extend the life of your battery. Battery health management is designed to improve your battery's lifespan by reducing the rate at which it chemically ages. The feature does this by monitoring your battery's temperature history and charging patterns.
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. This is normal, and it's how battery health management optimizes charging. Your Mac resumes charging to 100 percent depending on your usage.
This is a new behavior, in my opinion –
and might be a bug (see update)
I have not noticed this behavior till recently (see below). I cannot remember observing a noticeable/quick discharge while connected to the power supply, and when this started to happen I thought the laptop is broken or the SMC/nvram corrupt…
What is happening now is that the Laptop (in my case also a MacBookPro 15" 2018) stops charging for a period of time (around 1 h). Maybe till it reaches ~93% remaining charge and then starts charging again.
I have noticed this since installing the 002 or 003 security update on High-Sierra, I have then tried a lot to troubleshoot this new behavior, updated to Mojave (with all updates), reset the nvram and power management but the issue persists.
The screen-shots below, from the iStat menu sensor data recording, show how the battery is draining (while still connected to the power supply). The DC input from the power supply is reduced to 2 W (20 V at 0.1 A) and power is (mainly) drawn from the battery.
above: Battery charge drops to ~93% while connected to power supply
above: The battery powers the system, although it's connected to power supply
above: Power input from the connected power supply
This only happens when the laptop is awake and always at the same time of day. When it is not awake at that time it will start draining the battery the moment it is awake.
This continues to happen with different chargers and when booting to recovery mode, and - I think - when booting an older system from external drive.
I suppose that maybe something in the SMC code was changed, which triggers this behavior independent from the OS.
All in all this new behavior has perplexed me quite a bit.
I do not see how a daily forced discharge to 93% can prolong the overall life of the battery!
Right now Apple is forcibly adding one full discharge/charge cycle about every two weeks, or ~25 full cycles per year. Maybe (and I hope so) they have done some research and determined that this behavior actually prolongs the battery-lifetime –
on the other hand this might just be a bug…
Update 2020-08-05: It's not a bug but "a feature designed to improve the lifespan of your battery", according to Apple
The way Apple back-ported this behavior to High-Sierra and Mojave, where there is no "Battery health management" in System Preferences/Energy Saver still makes me wonder if this is working entirely as intended.