Battery run time
A battery is a water tank marked in amp-hours. Drain it with a thin pipe and it lasts all day; with a fire hose it is empty before lunch.
t = Q / I (rated capacity; derate 20–30% in practice)
Uses the rated capacity and a constant average current. Real packs deliver less at high currents, in the cold and as they age; cut-off voltage and converter losses also eat into the figure. Treat the result as an upper bound.