Final Conclusion
Given the sources and explanations below. I am officially going to do the following to optimize my battery life:
- Keep my battery as cool as possible.
- Don't worry about whether it's plugged in or not. When it doubt, keep it plugged in so it uses AC power instead of battery, unless getting hot.
It turns out that the two methods I originally posited are largely moot. The only thing that really matters is temperature.
"Temperature was the most significant factor contributing to the cell degradation, with state-of-charge (SOC) and discharge pulse length of secondary importance." (Liaw et al.2)
Furthermore, it turns out that the decay can be accurately mathematically modeled:
(See Ramadass et al.1 for explanation of terms)
However, the dominant model is that of the Arrhenius formula, which generically predicts time-to-failure as a function of temperature.
The figure below shows the capacity at various cycle counts. Just look at the capacity on the x-axis. The top graph is at 25ºC, the bottom at 50ºC.
After 600 cycles, the cooler battery had ~2x the capacity
While I could still find no evidence on the behavior of Mac power circuitry, there was helpful information on the official Dell website. Two items specifically stood out.
Q. When docked or AC adapter is plugged into a wall outlet, am I using my battery charge?
A. No.
http://www.dell.com/content/topics/global.aspx/batteries_sitelet/en/batteries_faq?c=us&l=en&cs=19#faq9
Q. Should I totally discharge, then recharge my Dell laptop battery occasionally to make it last longer?
A. No, discharging and charging does not increase the life of a Lithium Ion technology battery.
http://www.dell.com/content/topics/global.aspx/batteries_sitelet/en/batteries_faq?c=us&l=en&cs=19#faq27
However It is important to note that Apple and Dell charging circuits may be different. Although, given that Dell does this, I assume apple does as well. On this assumption, unless someone can provide sources to claim otherwise, I will assume that the Apple charging circuitry is smart enough to know this.
I encourage anyone to continue exploring this question and challenge my assumptions. Please see the sources below if you're curious for a more detailed explanation.
Sources
1 Ramadass, P., Bala Haran, Ralph White, and Branko Popov. "Mathematical Modeling of the Capacity Fade of Li-ion Cells." Journal of Power Sources 123.2 (2003): 230-40. Web. http://www.che.sc.edu/faculty/popov/drbnp/WebSite/Publications_PDFs/Web33.pdf.
2 Liaw, B., R. Jungst, G. Nagasubramanian, H. Case, and D. Doughty. "Modeling Capacity Fade in Lithium-ion Cells." Journal of Power Sources 140.1 (2005): 157-61. Web. http://electrochem.org/dl/ma/204/pdfs/0253.PDF.
[3] Ning, G. "Capacity Fade Study of Lithium-ion Batteries Cycled at High Discharge Rates." Journal of Power Sources 117.1-2 (2003): 160-69. Web. http://www.che.sc.edu/faculty/popov/drbnp/website/Publications_PDFs/Web38.pdf.
[4] Ramadass, P., Bala Haran, Parthasarathy M. Gomadam, Ralph White, and Branko N. Popov. "Development of First Principles Capacity Fade Model for Li-Ion Cells." Journal of The Electrochemical Society 151.2 (2004): A196. Web. http://www.che.sc.edu/faculty/popov/Publications/Premanand1.pdf
[5] Zhang, D., B. S. Haran, A. Durairajan, R. W. White, Y. Podrazhansky, and B. N. Popov. "Studies on Capacity Fade of Lithium-ion Batteries." Journal of Power Sources 91 (2000): 122-29. Web. http://www.che.sc.edu/faculty/white/2000studiesoncapcaityfadeofzhangharandurairajanwhitepodrazhanshkypopov.pdf.
It sounds like you've got a bad battery, no doubt, and should have it replaced, gratis. While this shouldn't be a problem, it won't hurt to have the most ammunition possible when you arrive at the store or have to place a call to Apple Care.
Take a screenshot of the "Service Battery" menu icon (Shift + command + 3), and collect all the data you can (like the kind you've included in your question). I've had three defective batteries replaced by Apple, two of them close to the 300 cycle mark, which is what Apple guarantees 80% battery health to. The screenshots and data have been instrumental in getting a rapid, no questions asked replacement. Of course, if your battery is showing the health problems that it is after only six (!?!) cycles, then you've got a good shot at it anyway.
Good luck!
Best Answer
Update 2020-08-05: It's a new feature by Apply and intended behavior
see HT211246
This is a new behavior, in my opinion –
and might be a bug(see update)I have not noticed this behavior till recently (see below). I cannot remember observing a noticeable/quick discharge while connected to the power supply, and when this started to happen I thought the laptop is broken or the SMC/nvram corrupt…
What is happening now is that the Laptop (in my case also a MacBookPro 15" 2018) stops charging for a period of time (around 1 h). Maybe till it reaches ~93% remaining charge and then starts charging again.
I have noticed this since installing the 002 or 003 security update on High-Sierra, I have then tried a lot to troubleshoot this new behavior, updated to Mojave (with all updates), reset the nvram and power management but the issue persists.
The screen-shots below, from the iStat menu sensor data recording, show how the battery is draining (while still connected to the power supply). The DC input from the power supply is reduced to 2 W (20 V at 0.1 A) and power is (mainly) drawn from the battery.
above: Battery charge drops to ~93% while connected to power supply
above: The battery powers the system, although it's connected to power supply
above: Power input from the connected power supply
This only happens when the laptop is awake and always at the same time of day. When it is not awake at that time it will start draining the battery the moment it is awake.
This continues to happen with different chargers and when booting to recovery mode, and - I think - when booting an older system from external drive.
I suppose that maybe something in the SMC code was changed, which triggers this behavior independent from the OS.
All in all this new behavior has perplexed me quite a bit.
I do not see how a daily forced discharge to 93% can prolong the overall life of the battery!
Right now Apple is forcibly adding one full discharge/charge cycle about every two weeks, or ~25 full cycles per year. Maybe (and I hope so) they have done some research and determined that this behavior actually prolongs the battery-lifetime –
on the other hand this might just be a bug…Update 2020-08-05: It's not a bug but "a feature designed to improve the lifespan of your battery", according to Apple
The way Apple back-ported this behavior to High-Sierra and Mojave, where there is no "Battery health management" in System Preferences/Energy Saver still makes me wonder if this is working entirely as intended.