  Quantum battery works as a micro- or nano-device to store and redistribute
energy at the quantum level. Here we propose a spin-charger protocol, in which
the battery cells are charged by a finite number of spins through a general
Heisenberg XY interaction. Under the isotropic interaction, the spin-charger
protocol is endowed with a higher capacity in terms of the maximum stored
energy than the conventional protocols, where the battery is charged by a
continuous-variable system, e.g., a cavity mode. By tuning the charger size, a
trade-off between the maximum stored energy and the average charging power is
found in comparison to the cavity-charger protocol in the Tavis-Cummings model.
Quantum advantage of our protocol is manifested by the scaling behavior of the
optimal average power with respect to the battery size, in comparing the
collective charging scheme to its parallel counterpart. We also discuss the
detrimental effect on the charging performance from the anisotropic interaction
between the battery and the charger, the non-ideal initial states for both of
them, and the crosstalk among the charger spins. A strong charger-charger
interaction can be used to decouple the battery and the charger. Our findings
about the advantages of the spin-charger protocol over the conventional
cavity-charger protocols, including the high capacity of energy storage and the
superior power-law in the collective charging, provide an insight to exploit an
efficient quantum battery based on the spin-spin-environment model.
