MacBook Pro randomly discharges from 100% to ~90% when plugged

My 15 inch 2018 Mac Book Pro with Mojave 10.14 randomly starts draining battery when plugged in. When I try to move the cable, change the power cable to a different USB slot nothing helps. The battery indicator is in charging state, but when pressed, it says "Not charging". After some time, when it gets to 88-92% it suddenly starts charging again. I noticed that this happens independently of loads. Once or twice a day. This can happen when 2 IDE, browsers windows, media player are running simultaneously, and when it is in idle state showing only desktop. I'm using native 85Watt charging block and cable.

Additional info: In System Report -> Power, the Condition is displayed as Normal. With cycles count equal to 52. This used to happen even when the condition used to be better, with fewer cycles count. But less frequently.

Update 2020-08-05: It's a new feature by Apply and intended behavior

see HT211246

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.

This is normal behavior.

The key to your question is your estimation:

After some time, when it gets to 88-92%

That is right at the line (93%) where Apple estimates that charging will appear to stop. A couple of percentage points is well within the margin of error and it's safe to say it wasn't a scientific observation you were making.

From Apple Support: About Mac notebook batteries

My battery won't charge to 100%

Occasionally, the battery might not show a full charge (100%) in macOS, even after the power adapter has been connected for an extended period of time. The battery might appear to stop charging between 93 percent and 99 percent. This behavior is normal and will help to prolong the overall life of the battery.

The thing is, depending on use, it may go a little lower than 93%. Most things done with battery is done based on calculation of what's measured so 93% is a calculation. The point is, a hard 93% would not be that the demarcation of when to start charging. Like everything else, it is based on the measurement; and in this case, most likely the measurement of the remaining charge and max charge values read directly from the battery itself and not a calculation displayed by the OS.

Regarding Catalina

Catalina introduced a Battery Health Management feature and released it to the public June 1st 2020 in the 10.15.5 Update. The Support article I sourced the info from above is from May 26th, 2020 (Pre-Catalina, see image below).

^{Apple has a habit of not dating things or just showing you when things were updated so, took a screen grab for posterity}

It's also been in the public beta for some time judging by the ample blogs/articles written about it. Now, it wouldn't be uncommon for Apple to implement a portion of the the feature (like changing the max charging limit) into earlier versions of macOS. The full "Battery Management" feature allows you the ability to control it.

This feature was introduced in macOS Catalina, on by default. Are you sure you're actually on Mojave?

It's supposed to prolong your battery life, you can disable it in Energy Saver -> Battery Health.

More info here

PS: would comment, but don't have reputation