  We investigate a quantum battery system in which the coupled two-level
charger and battery are immersed in nonequilbrium boson or fermion reservoirs.
Based on the Redfield master equation, we consider the change of the energy
spectrum induced by the external driving to the charger. When the charger and
the battery possess the same transition frequency and the charger is driven in
resonance, a bistability can emerge with the closure of the Liouvillian gap. As
a result, the efficiency of the battery depends on the initial state of the
charger-battery system, and certain types of entangled initial states can
enhance the efficiency. In the non-resonance driving regime, the efficiency of
the quantum battery can be optimized by the compensation mechanism for both the
boson and fermion reservoirs. Our investigation is helpful to the design and
optimization of quantum battery in the nonequlibrium open system.
