  A generalized collision model is developed to investigate coherent charging a
single quantum battery by repeated interactions with many-atom large spins,
where collective atom operators are adopted and the battery is modeled by a
uniform energy ladder. For an initially empty battery, we derive analytical
results of the average number of excitations and hence the charging power in
the short-time limit. Our analytical results show that a faster charging and an
increased amount of the power in the coherent protocol uniquely arise from the
phase coherence of the atoms. Finally, we show that the charging power defined
by the so-called ergotropy almost follows our analytical result, due to a
nearly pure state of the battery in the short-time limit.
