I've bought a phone with a big battery so I could do exactly what the GP said, always charge between 30-80%. But that means I'm cutting half of the potential energy in the battery.

I'm lamenting the fact that we have to do this even though film capacitors can be drained from 100% to 0% without damaging the capacitor or reducing its capacity. This is far closer to how you want an energy storage device to operate. Aside from the fact that film capacitors store about a thousand times less energy by every metric.