I’ve wondered for a long time about the best way to charge my Mac laptops. As you probably know, there is a lot of conflicting opinion on this topic. 
So last night I watched the wattage in use as my M1 MacBook Air battery charged to 100% while observing with a “Kill A Watt” Energy Usage Monitor in the circuit.
- from 49% battery to 94% the wattage used was a pretty steady 31.4W
- at 94% battery, usage had dropped to 16.4W
- at 98%, it dropped further to 9.7W
- at 100% battery, the charger was using less than 4W
- while doing various tasks on the MBA with the battery at 100%, the wattage varied up to 10W as if current was being pulled directly from the house circuit as needed
- when I put the MBA to sleep, wattage dropped to less than 2W
- after being shut down, wattage dropped to 0.0W as measured on the Kill A Watt
- So it seems there are some smarts built into Apple’s battery and charger system. And you are probably okay leaving it plugged in even after the battery reaches 100%.
The fact remains that now the battery is holding at a full charge which is not supposed to be good for it.
I think I’ll return to my practice of making an effort when I can to begin charging at 40% battery level and discontinuing at 80%. My past experience with a swollen battery urges me to caution.