Is it bad for battery life to leave notebook plugged in all the time?

alexbusername wrote:

I use my notebook plugged in to a docking station with a monitor, speakers, etc. I have heard that it is better to use the battery from 89/90% charge down to 5% and then recharge it. This requires regularly unplugging the notebook and disconnecting it from the dock and all the accessories when I use it. Is it bad for battery life to leave the notebook plugged in all the time (in my case connected to the docking station)?

General rule of thumb, if you are near the mains leave it plugged in, if you need the portability run on the battery. Full Stop. The less cycle counts the longer the battery will last with an expected 1000 *cycle count

You do not state what macOS you are running...

Since macOS Catalina 10.15.5 introduces battery health management, a feature designed to improve the lifespan of your Mac notebook's battery, it learns as it goes.

About battery health management in Mac notebooks https://support.apple.com/en-us/HT211094

The Battery Health button appears in >SystemPreferences>Energy Saver/“Battery” in the newer macOS —this feature is limited to Mac laptops with Thunderbolt 3 ports. That equates to all MacBook Pro models released since late 2016 and the Retina-capable MacBook Air models released since 2018.

With the release of macOS 11.0.1 Big Sur see >System Preferences>Battery

*Determine battery cycle count for Mac notebooks

https://support.apple.com/en-us/HT201585

That advice you cited is obsolete. It's ok to keep it plugged in. Occasional use of the battery is good, but not running it down.

Batteries - Maximizing Performance - Apple

You can leave your computer plugged in when convenient to do so. Neither the computer nor the battery will be damaged.

When the Covid-19 pandemic changed my work situation to mainly work from home, I started using my MacBook almost always plugged in. For good measure, I'll unplug it and let it run down to about 20% once every 3-4 weeks. Based on that, my battery health is showing "Normal".

Having had to replace a few swelling batteries (at Apple's expense) - I learned: a number of batteries are/were faulty, old batteries fail that way sometimes, and if you let your battery get too hot (e.g. in a car) that can cause such problems.