What are the best charging habits for MacBook M3 to preserve battery health?

Use Optimized Charging and leave your Mac plugged in whenever it is next to power. Don't try and manually control charging by unplugging it and leave it plugged in. The only thing you are doing by removing from power unnecessarily is increasing your Charge Cycles which will result in a decrease in Battery Health faster. Your Mac will not overcharge, so you do not need to worry about that.

I have a 2020 M1 MacBook Air purchased when they first came out. It spends most of its time plugged in and as of today battery health is 90%.

I bought an M4 MacBook Air 6 months ago and treat it the same way and it is still at 100% battery health.

In addition to what others have stated, battery longevity can vary greatly from one batch to another and even in the same batch. Also, environment can have an impact on battery life as well when in heat or cold.

As others have said, just make sure optimize charging is on and just use your Mac and enjoy. When the Mac reaches a point where you no longer have enough work time on battery, then it is time to replace.

betwixtme wrote:

Thanks- and what about overnight? Leave it plugged in or not?

Yep, leave it plugged in. The only time you should be using the battery is when you are mobile or away from a power outlet.

I leave my computer plugged in whenever an outlet is available, day or night.

Please don't obsess over "health." It is neither exact nor linear.

Inexact in that the baseline metric used to calculate health is the average initial capacity of all batteries of that model, not that of your specific battery. We all know what "average" means:

Non-linear in that health seems to drop more rapidly between 100% and 90% than below that. I have a old Macbook Pro I bought new Apple-direct in the summer of 2013 that still has its original battery, gives very useable runtimes, yet its battery health has remained between 81 and 86 for about two years. And health readingis are up and down. Not linear.

Your better indictor is the runtime you get on one charge while doing your tasks. Everyone has different usage patterns so what someone else gets for runtime is rather meaningless.

See suggestions in: About battery health management in Mac notebooks - Apple Support