  We consider a quantum battery modeled as a set of N independent two-level
quantum systems driven by a time dependent classical source. Different figures
of merit, such as stored energy, time of charging and energy quantum
fluctuations during the charging process, are characterized in a wide range of
parameters, by means of numerical approach and suitable analytical
approximation scheme. Particular emphasis is put on the role of different
initial conditions, describing the preparation state of the quantum battery, as
well as on the sensitivity to the functional form of the external
time-dependent drive. It is shown that an optimal charging protocol,
characterized by fast charging time and the absence of charging fluctuations,
can be achieved starting from the ground state of each two-level system, while
other pure preparation states are less efficient. Moreover, we argue that a
periodic train of peaked rectangular pulses can lead to fast charging. This
study aims at providing a useful theoretical background in view of future
experimental solid-state implementations.
