What is it that kills laptop batteries?
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries has some useful information. It suggests the following tips regarding lithium-ion batteries:
- Don't fully discharge the battery (partial discharges are best)
- Don't heat up the battery when it's fully charged
- Ideal storage is about half charged in a cool environment.
Tip 2 would support the idea of not using a laptop while fully charged and plugged in, as laptops generate significant heat, and 3 would suggest against leaving a laptop in a car that's in the sun.
In the end I would think it's just simple degradation that renders batteries useless. There's only about 500 charge cycles on a lithium-ion battery before it becomes next to useless. Sure this improves as the technology matures, but I don't believe anything radical has changed since the linked article was updated.
One of the best guides, covering pretty much any aspect of various battery types:
Battery University is an on-line resource that provides practical battery knowledge for engineers, educators, students and battery users alike. The papers address battery chemistries, best battery choices and ways to make your battery last longer.
As for your question, what kills laptop batteries:
Heat, while the inside of a laptop may seem to be a cosy place (temperature-wise), it is certainly not the best environment for a rechargeable Li-ion battery.
Age
Charging and discharging, the number of charge/discharge cycles of Li-ion batteries are limited, depending on the quality of the battery, usually between 300 and 500.
I leave my laptop on 24/7 for weeks at a time, and after a year, I get around 1 1/2 hours of charge, so it has lost about half an hour.
I think it mainly comes down to the circuitry and how advanced it is. I know that my laptop keeps saying 95% charged and every 6 or so hours seems to cycle or do something - like discharge 5% here and there.
I think the main thing that kills them is simply complete charge/discharge cycles. At the end of the day, you get about fifty without seeing any affect, then I would say you loose a few seconds for each charge past that.
As you said, the first two are related to the memory effect which does not affect lithium-ion batteries.
Lastly, if you are using a battery pack that uses standard cells, you can replace them yourself if the manufacturer wants a fortune for a new one. A friend's laptop was getting about 3-4 minutes of charge and Sony wanted to charge £290 which is ridiculous as you are better off getting a new netbook for that! I opened it up and replaced with identical cells which costed around £25. For example, here is the inside of an Apple battery:
