  We investigate the connection between quantum resources and extractable work
in quantum batteries. We demonstrate that quantum coherence in the battery or
the battery-charger entanglement is a necessary resource for generating nonzero
extractable work during the charging process. At the end of the charging
process, we also establish a tight link of coherence and entanglement with the
final extractable work: coherence naturally promotes the coherent work while
coherence and entanglement inhibit the incoherent work. We also show that
obtaining maximally coherent work is faster than obtaining maximally incoherent
work. Examples ranging from the central-spin battery and the Tavis-Cummings
battery to the spin-chain battery are given to illustrate these results.
