Quite the opposite. Fully charged and never/seldomly discharged batteries degrade a lot faster. Battery chemistry...
That's also why manufacturers advise to store batteries for prolonged times at a charge level of 60-80%, not at 100%.
We are not talking of an iPhone. The less cycle counts the longer the battery will last with an expected 1000 *cycle count.
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Macbookb is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)
The computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may even decline during 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 AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge.
Your MacBook Pro uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.
A charge cycle is ever-so-slightly destructive to batter longevity. When operating as designed (and not using Battery Health Management) battery charge level is allowed to decline to about 92 percent level before initiating a recharge cycle to top up to about 99 percent.
(Taken from Apple forums)