Spehro Pefhany

\$\begingroup\$ I see most smart devices (laptops/tablets/phones) now have the option to optimize your battery by charging it to 80% only. But wouldn't this reduce your capacity i.e. your brand new battery now only has 80% of its rated capacity, which means more frequent recharge and all that. Even if you do charge it to 100% for the occasional road trips or whatever reason that you need to have the battery to last longer, I still don't see the real benefit of capping your battery charge at 80%.

I need to replace the taillight/brake light on my car, and it's a bulb with two filaments (known as 12V P21/5W - see pic) My car has two taillights on each side. A standard 5W bulb for the top half, and the bottom one also doubles as a brake light when you use the brakes. So if you have the lights on, it lights up like the top half of the taillight (a normal 5W bulb) but when you press the brakes, it also lights up the other filament (maybe both at the same time?

Yes, they are and there are usually curves to explain what the effect is. The current at which they open is probably affected a lot more than the reaction time to a short, which is I^2t related. For example, here is the derating of the ATOF series of automotive fuses: The time curves do not show any temperature effect, but you can estimate the effect by scaling the current to the equivalent at the new temperature.