Is it safe to charge my macbook after the green light shows? Would it not overcharge the battery, or reduce its lifespan?
I've heard that doing this is completely safe and it increases the cycles or something. Is that correct?
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Yes, leaving the charger in will not hurt the battery in the short term. However, you should occasionally allow the battery to be used and not leave it plugged in all the time. The battery works best when it is cycled; Apple has a brief writeup on their use of Lithium Ion batteries in the MacBook line at http://www.apple.com/batteries/why-lithium-ion/, where they state that a complete charge cycle (not the same as charging to 100%) is what will tend to reduce the capacity. A charge cycle can be adding 10% to the battery each day for 10 days, adding 100% of a charge in one day, or anything in between.
Is it safe to leave in? Yes. To understand this better we need to know what is happening when you have your charger plugged in.
What do the LED's on the MagSafe Charger Mean?
Amber: Battery is charging.
Green: Battery is fully fully charged and no power is being sent to the battery.
Your battery will naturally dissipate it's charge without use and the charger will detect this drop and automatically begin its recharge cycle.
Is it safe?
I am assuming you are referring to the longevity of the battery and not whether you will incur some sort of injury.
What affects the life of a battery are two things:
I answered a similar question recently about What's the best way to prolong the lifespan of a MacBook Pro battery?
As of Catalina (10.15.5), there is a new feature; it's called Battery Health Management. It will actually stop charging your battery and lower the maximum charge amount further making the micromanagement of your battery unnecessary. See the linked post for further details.
Leaving it plugged in will surely have pretty much zero effect on your battery.