The chemistry of the taper
Lithium cells accept charge fast when empty (constant current) and slow near full (constant voltage) — forcing the last 20% at full wattage plates lithium onto the anode, permanently cutting capacity. The taper is not the charger being lazy; it is the chemistry being protected. Every honest charge-time estimate includes it.
Heat is the real killer
Battery longevity research is consistent: heat and time at 100% kill lithium — fast charging with good thermal management is gentler than slow charging in a hot car. Practical translation: charge in the shade, off the blanket, and avoid storing devices (and banks) at 100% in hot cars for weeks.
The 20–80 habit, honestly assessed
Keeping batteries 20–80% meaningfully extends cycle life — meaningful for devices you keep for years, less so for travel banks used hard for three seasons. The rational policy: 20–80 for the everyday device, full charges when the trip needs them, and zero guilt about either. The batteries serve the travel; not the reverse.