Actually, an Apple battery gives you the percentage of its original capacity that it currently has. You can access that number through the free CoconutBattery app.
If you want to do your own benchmark, I recommend using BOINC and supporting a grid computing project, like SETI@home. These basically use 100% of your CPU from the moment you turn the program on until the moment you turn it off. This drains the battery pretty quick. Just be careful to check temperatures to make sure the computer doesn't overheat, especially with a MacBook Air. You probably should quit extra apps, but it shouldn't matter too much.
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.
Best Answer
The short answer is no, although this depends on what you mean by battery life.
You are correct that placing more of a burden on your computer while it's being charged (e.g. upping the brightness level, converting videos, etc) will result in a longer charging time (how much will differ on what your MBP is doing at the time).
However, this doesn't increase the life of the battery itself. Of course, in terms of a human measurement of time (in terms of hours/days/weeks etc) there may be a perceived difference in the life of the battery, but the battery itself isn't lasting any longer in terms of its actual usage.
Put another way, it's common that two users buying the exact same model MBP with the exact same specs will have a different experience in terms of how long it is before they need to replace the battery. But one user may use their MBP on the road etc every day, while the other only uses it a few hours every couple of days. Or one user spends a lot of time compressing video, while the other only does basic browsing and word processing. The fact that one of these users may need to replace their battery in a couple of years doesn't mean the battery life was less, just that its battery life was used up sooner than the other.
For further info you may want to read the entirety of what Apple says about their batteries here.