We study the energy transfer process in quantum battery systems consisting of multiple central spins and bath spins. Here with quantum battery we refer to the central spins, whereas the bath serves as the charger. For the single central-spin battery, we analytically derive the time evolutions of the energy transfer and the charging power with arbitrary number of bath spins. For the case of multiple central spins in the battery, we find the scaling-law relation between the maximum power $P_{max}$ and the number of central spins $N_B$. It approximately satisfies a scaling law relation $P_{max}propto N_{B}^{alpha}$, where scaling exponent $alpha$ varies with the bath spin number $N$ from the lower bound $alpha =1/2$ to the upper bound $alpha =3/2$. The lower and upper bounds correspond to the limits $Nto 1$ and $Ngg N_B$, respectively. In thermodynamic limit, by applying the Holstein-Primakoff (H-P) transformation, we rigorously prove that the upper bound is $P_{max}=0.72 B A sqrt{N} N_{B}^{3/2}$, which shows the same advantage in scaling of a recent charging protocol based on the Tavis-Cummins model. Here $B$ and $A $ are the external magnetic field and coupling constant between the battery and the charger.
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