  We investigate a quantum battery made of N two-level systems, which is
charged by an optical mode via an energy-conserving interaction. We quantify
the fraction E(N) of energy stored in the B battery that can be extracted in
order to perform thermodynamic work. We first demonstrate that E(N) is highly
reduced by the presence of correlations between the charger and the battery or
B between the two-level systems composing the battery. We then show that the
correlation-induced suppression of extractable energy, however, can be
mitigated by preparing the charger in a coherent optical state. We conclude by
proving that the charger-battery system is asymptotically free of such locking
correlations in the N \to \infty limit.
