But I'd say that how much it's used is also an important factor.
I haven't booted up the older M1 recently to check, but I remember it was throwing replace battery warnings well before I got the upgrade and that I think that triggering below 80%.
I use Al Dente to further optimize battery and calibrate it now and then
I didn't have a spare laptop to work with at that time for a week so i held of[1]. This is probably a reason for lot of people opting out of upgrades.
After the M4 came there was hype for the new screen which performs better in sunlight and M1->M4 seemed worth it plus vRAM which is certainly handy.
[1]The screen was also deteriorating - no physical issues just wasn't as bright as it was new) and they said I should replace that too. The economics a new Air worth of money on a older laptop with just 1-2 years primary use life left is shaky.
Also both times I did it, it took 2-3 days, not the quoted 1+ weeks
2. Batteries age faster in the beginning and then their ageing rate plateaus. It's the same for electric cars. E.g. a Tesla can lose 5% efficiency in the first 30k miles, but will lose the next 5% over 60k+.
The only time battery is not a problem is when video streaming for hours, presumably the decoding is offloaded to the dedicated media chips on the board.
Also, the majority of time it’s been in my van, which is pretty chilly most of the time.
From my understanding, battery health is based on use and environment.
It typically runs around 110-130 for CPU and the battery is 85-90 on light usage like say browser or slack.
However if i run a full compile of the repo or just about anything a bit compute intensive CPU jumps easily to 170-200 range regularly and battery to 120.
I am now frequently running the fans higher manually to cool it down[1]
I think Apple is optimizing for being quiet over battery and laptop life and have kept the fans low speed even if it would benefit the hardware to speed them up earlier under load.
[1] This is not because I can the temps all the time in the top bar. The palm rests do become noticeably hotter and bit uncomfortable than the noise would be.
The lower power and heat of M-devices might result in meaningfully longer battery life, and I'm curious to find out.