  A fully operational loss-free quantum battery requires an inherent control
over the energy transfer process, with the ability of keeping the energy
retained with no leakage. Moreover, it also requires a stable discharge
mechanism, which entails that no energy revivals occur as the device starts its
energy distribution. Here, we provide a scalable solution for both
requirements. To this aim, we propose a general design for a quantum battery
based on an {\it{energy current}} (EC) observable quantifying the energy
transfer rate to a consumption hub. More specifically, we introduce an
instantaneous EC operator describing the energy transfer process driven by an
arbitrary interaction Hamiltonian. The EC observable is shown to be the root
for two main applications: (i) a trapping energy mechanism based on a common
eigenstate between the EC operator and the interaction Hamiltonian, in which
the battery can indefinitely retain its energy even if it is coupled to the
consumption hub; (ii) an asymptotically stable discharge mechanism, which is
achieved through an adiabatic evolution eventually yielding vanishing EC. These
two independent but complementary applications are illustrated in quantum spin
chains, where the trapping energy control is realized through Bell pairwise
entanglement and the stability arises as a general consequence of the adiabatic
spin dynamics.
