  The coherent energy transfer between two quantum devices (a quantum charger
and a quantum battery) mediated by a photonic cavity is investigated, in
presence of dissipative environments, with particular focus on the the
ultrastrong coupling regime. Here, very short transfer times and high charging
power can be achieved in comparison with the usually addressed weak coupling
case. Such phenomenology is further magnified by the presence of level
crossings appearing in the energy spectrum and which reveal very robust against
dissipative environmental effects. Moreover, by carefully control the physical
parameters of the model, e.g. the matter-radiation coupling and the frequencies
of the system, it is possible to tune these crossings making this device more
flexible and experimentally feasible. Finally to broaden our analysis, we
assume the possibility of choosing between a Fock and a coherent initial state
of the cavity, with the latter showing better energetic performances.
