<< Should the battery be drain all the way down about once per month. >>
Not unless you want to pay to replace that battery soon.
Deep discharge shortens battery life.
That computer is a battery-CAPABLE device, It is not optimized as a battery-operated device (it is not an iPhone.)
Your computer performs best when connected to AC power, such as the power adapter. It can use the full output of the Power Adapter AND when doing especially challenging work could also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during very stressful work.
When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)
In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Modern Macs maintain optimum battery charge levels under program control, and will NEVER over-charge. Connected to Power is NOT necessarily charging.
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Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.
NOTE that charging to 80 percent is a side-effect, NOT the GOAL.
The goal is to have some hurly-burly (percolating activity) around using your battery, either by your actually using it, or by leaving it less than fully 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
If your recent usage patterns demand top battery capacity, the battery will charge completely to be sure you are not stuck out in the wild with no power.
Executive summary: “If you keep your Mac plugged in continuously for several days, it will start pausing the charge at 80%. If you run a small amount of the time on battery, it will keep doing that. If you run some of the time on battery, it will charge to 100%.” © 2025 neuroanatomist