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Is it bad to keep a MacBook plugged in all day to a charger?

MacBook always plugged into charger

Short answer: Yes.

? What happens when the charger is constantly connected?

Every new MacBook battery is lithium-ion based. Lithium-ion is a great battery chemistry, but one of its downsides is that it degrades at a faster rate when kept at its charge extremes.

The charge extremes can vary, but typically they are between 0% - 20%, and 90% - 100%. This means that when a MacBook battery is between those ranges of percentages, it will degrade faster than normal.

? Is there any way to keep the battery from being harmed?

It typically only takes a few hours for a MacBook battery to charge up to full, A good strategy is once fully charged, just unplug it for a while, and plug it back in when it gets to 40% - 50%. Another option is using BatteryBox - it bypasses the MacBooks internal battery and powers it directly, reducing the number of cycles and stress on the MacBooks battery.

This could extend the lifespan of the battery relative to simply leaving it plugged in all the time.

? Will my battery overcharge if left plugged in?

The harm doesn't come from overcharging the battery. This does not happen as the battery controller chip inside of the MacBook Pro and Air are intelligent, and stop the battery from charging past the overcharging safety threshold.

? Storing the battery for long periods of time

If you don’t plan on using your Mac notebook for more than six months, Apple recommends that you store the battery with a 50% charge. Leaving it fully discharged or fully charged could harm the battery when stored for so long.