  We introduce the notion of reverse quantum speed limit for arbitrary quantum
evolution which answers a fundamental question: ``how slow a quantum system can
evolve in time?" Using the geometrical approach to quantum mechanics, the
reverse speed limit follows from the fact that the gauge invariant length of
the reference section is always greater than the Fubini-Study distance on the
projective Hilbert space of the quantum system. We illustrate the reverse speed
limit for two-level quantum systems with an external driving Hamiltonian and
show that our results hold well. We find several examples where our bound is
tight. We also find one practical application of the reverse speed limit in
discharging process of quantum batteries which answers the question: ``how slow
quantum batteries can discharge?" Our result provides a lower bound on the
average discharging power of quantum batteries.
