So, does the trend continue? Will the batteries take 50 years to get down to under one hour of usage time?
% hrs Year
100% 7 5
80% 5.6 10
80% 4.48 15
80% 3.584 20
80% 2.8672 25
80% 2.29376 30
80% 1.835008 35
80% 1.4680064 40
80% 1.17440512 45
80% 0.939524096 50
Nope. Battery capacity drops off quickly after 80% for each discharge/charge cycle.
I worked in the lithium-ion battery industry for a while. Here are some things I learned:
Under typical use, the battery capacity falls slowly with each cycle. A cycle is considered a full discharge and a full charge. This is true for every battery.
Some battery chemistries do not lose their capacity as quickly as others. That's what's supposed to make Apple's batteries better than the competition's. Other notebook batteries last about 300 cycles before they reach 80% like Apple's did before the Unibody MBP design.
Back to capacity loss: For each cycle, the battery loses a little capacity. The curve of capacity loss vs. cycles isn't linear. It's pretty flat then starts to take a dive pretty fast. So what happens is you might hit 80% in 1000 cycles, then 75% in only another 400 cycles, then 70% in another 100 cycles. It goes down really fast.
There's other issues that affect capacity. For most batteries, staying fully charged without being discharged can effect their life. Just sitting on the shelf affects them (fresh vs. old). They are also affected by temperature exposure and by how they are discharged (fast vs. slow).
Take all of that together and you get batteries that "die" before they hit 1000 cycles.
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Larger battery, but the big difference is it is glued in. That makes removal that much more tricky, as discussed in this thread.
Did iFixit try a little heat with a hair drier or heat gun to remove the battery? Heat softens most adhesives. Apple probably expected this.
If one is replacing the battery, one doesn't need to worry about damaging it with the heat. It's a common technique.