How do I keep M3 MBP battery from charging fully?

Grant Bennet-Alder wrote:

If you turned Battery health management off and used the full battery capacity recently, you have created the required hurly-burly of battery activity required. This recent activity counts as your recent battery churn, and will likely suffice for Several Weeks.

I don't believe I've ever turned Battery health management off (I learned about it but never used it), except to toggle it off and on to see if the 80% behavior would come back. The energy usage graph in the Battery section for the past 10 days is empty (It's a shame it doesn't go back further), because this thing is on AC pretty much 24/7. It has a 15 cycle count since unboxing seven months ago. I wish the system's calculations were more transparent.

Now that you mention it, there is a possibility that over a month ago the charge cable got accidentally disconnected for most of a day with the laptop left idle but on, and the battery dipped significantly. I'm going to assume this may have been the event that triggered the current behavior. I'll be patient and wait a month or two more to see if the 80% behavior comes back.

FishingAddict wrote:

Have a look:

System Settings > Battery > Battery Health > click the (i) icon. That will reveal several options including Optimized Battery Charging and Manage battery longevity.

Source: Change Battery settings on a Mac laptop - Apple Support

Thanks. Let me say more precisely that I did find that control previously, it just doesn't seem to have any effect for me.

Grant Bennet-Alder wrote: 

The GOAL is to have some hurly-burly around using your battery, either by your actually using it, or by leaving it less than 100 percent charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower  (thought to be around twice monthly) if you do not do so by your ordinary use.

The feature tracks you actual usage. If you continue to not require 100 percent charge for a few weeks, it generally will revert to a lower "full" level. but it takes some experience, which takes some time.

This laptop spends almost its entire time plugged into the charger. As I mentioned, when I first got it, it happily charged to 80% and stayed there. It's only in the past few weeks that it's decided to charge to 100% and stay there, even though my usage profile has not changed a bit. In short, this mechanism is no longer working.