Why does it take so long to go from 99 to 100% when charging?

[logicstudiouser]

Usually it ranges from 1-4 minutes per percent up till 99%, but I have always noticed on several macbook pros that as soon as it gets to 99%, it takes between 10 and as high as 30 minutes to go from 99 to 100%. What is that all about? And I know this is not isolated, I have seen this on several macbook pros.
It is not much of an issue, but I just find it bazarre, especially considering how much faster it is to go from 100% to 99% when the computer is not connected to power.

 

[Anitramane]

It's designed to charge and stay and fluctuate between 93-100 % to keep the electrons moving.

 

[simonsi]

Usually it ranges from 1-4 minutes per percent up till 99%, but I have always noticed on several macbook pros that as soon as it gets to 99%, it takes between 10 and as high as 30 minutes to go from 99 to 100%. What is that all about? And I know this is not isolated, I have seen this on several macbook pros.
It is not much of an issue, but I just find it bazarre, especially considering how much faster it is to go from 100% to 99% when the computer is not connected to power.

The newer battery monitoring/charging intelligence will detect when the battery is actually full as opposed to stopping at 99%+1, so essentially this is why they don't need re-calibration, if the battery will still accept charge it will recalibrate the "100%" point so stay at 99% until the battery won't accept any more. Then switch to "100%" and resume its trickle charge cycles from there.

Similar happens at the low end, it will attempt to continuously calibrate the low end of the battery, this is how it still displays "100%" on a battery which it also displays as having 90% of its original capacity....