iFixit says that heat is the biggest killer of batteries and therefore you should avoid placing your laptop on an insulator like a pillow. Why? Assume it’s only exposed to the 40 something Celsius of a laptop at heavy load and not extreme heat that can melt off the casing or something, what about heating it up causes it to fail sooner?
Plugging the device in all the time can also reduce battery lifespan. Why is *that*? I’d have imagined that once on wall power, the device can afford to either use the battery sparingly or not at all, depending on if the charger is powerful enough.
What are the chemical processes that take place to make these two things the biggest reasons batteries fail?
Lithium ion batteries do not like being stored at “full” states of charge. Temperature makes this worse – all chemical reactions go faster with heat, and the bad chemical reactions that happen in a totally full lithium ion battery go faster at higher temperatures. When the battery is charging or discharging, a chemical reaction is running one way or the other, but side reactions also happen. As the voltage goes up, bad side reactions start happening more and more as you charge. Likewise, if the voltage gets too low, the battery is ruined because bad side reactions also happen under a certain voltage threshold.
Some really nice laptops have settings that let you limit the battery charge to 80 or 90 percent, which is much easier on the battery. Many electric cars can do this too, it is all controlled in software.
In the early days of lithium ion powered laptops, the standard advice if you were just going to have it plugged in all day was to discharge the battery down to 70% or so by running the computer on battery, then pulling the battery. Unfortunately many laptops don’t have easily removable batteries (or even user removable batteries), and aren’t even designed to be able to run on AC power alone – the included chargers can’t handle the peak power needs of the computer so the battery has to help.
Placing a laptop on a pillow or other insulating type surface stops the fans from being able to vent that heat. If it gets hot enough, the computer will shut down to avoid damage to the processor more than the battery.
That said the processor’s heat can cause thermal runaway, in which high temperatures release energy that makes the temperature even higher until it’s enough to cause damage to the battery.
40 C will probably not hurt your battery, but a laptop that cannot vent its heat will get much hotter than that.
*That* said, modern devices are smarter about battery management, as you said. The degradation isn’t so severe anymore.