Battery calibration and shutting down the mac

[MacInWin]

so if it's plugged in all the time there is no need to charge and discharge, which means the battery goes through fewer cycles? For example if I discharge and charge twice a week then the battery goes through one cycle a week. If it's lifespan is determined by cycles then keeping it from discharging and charging should make it last longer.

It is a little more complex than that, but certainly cycles are a critical factor in battery life. The chemistry of Lithium batteries is such that about two years after manufacture, the battery starts to decay, very slowly at first, and then it accelerates as the accumulated time passes. In addition, high temperature can eat into a battery life, as can deep cold. Another potential killer of Lithium batteries is impurity of the lithium. As few as a couple of hundred atoms of contaminate can cause the lithium to release hydrogen as a gas, causing the battery to swell. Swollen batteries are dangerous because they can rupture and catch fire. There is no way to tell in advance which battery will have those few hundred atoms and which won't. Most don't, but some do.

But, basically, if you keep it plugged in most of the time it won't use cycles, so the battery should last longer. But no battery lasts forever. Frankly, the battery may well, if it doesn't swell and you take any effort not to abuse the cycle use, will last the life of the Mac. And if it swells, you get a replacement as quickly as you can to avoid the fire potential.

 

[countfufu]

if I leave the charger plugged in after the battery is full the laptop runs from the power outlet rather than the battery. Does the battery discharge while the mac runs from the outlet?

 

[MacInWin]

if I leave the charger plugged in after the battery is full the laptop runs from the power outlet rather than the battery. Does the battery discharge while the mac runs from the outlet?

Don't know and it doesn't really matter. The fact is that when plugged in to a proper external power supply there is sufficient external power both to run the MBP/MBA and charge the battery. How the circuitry manages the power and any switching from mains to battery isn't perceptible to the user.