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I have an (oldish) MacBook Pro that I use with an external monitor, which requires that the MBP power adapter be plugged in all the time. Does that damage the health of the battery? I ask because my battery, replaced about a year ago, will now hold almost no charge after being used almost exclusively with the external monitor during working hours and plugged in all that time.

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Welcome to Ask Different! This answer might help you out, but it doesn't tackle any issues specific to external monitors. – hairboat Jan 11 '12 at 20:43

While the external monitor itself is not the cause of the 'weakened' battery, leaving your MacBook Pro on AC power all of the time will definitely affect the battery's performance. It is essentially under a constant state of charging, even though this is regulated by the charging circuit, which will eventually cause the battery's chemistry to begin to break down.

Current batteries will typically last the longest when used regularly, but not completely discharged with each use.

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Recalling that using an external monitor requires that the MBP be on AC power, what should I do when I get my next MBP? Simply not use the external monitor part of the time and run down the battery? Or run down the battery at night away from the monitor? Or something else? I currently use the monitor all the time, 5 days a week, while at work. – jst Jan 12 '12 at 18:51
@jst u can install insomniaX to turn on your macbook while lid closed to use external display..!! – Ritesh Dec 16 '15 at 18:12

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