  To take quantum advantage of collective effects in many-body system, we
design an elementary block for building multipartite quantum battery, which
enables charging an atomic ensemble with optimal numbers in a common thermal
bath. One achieves maximum free energy as the stored energy in the steady
state, which is prior to each atom parallel charging independently. It ascribes
to quantum collective effects in the ensemble of atoms induced by the
competition between the coherent driving and decoherent dissipation. The
corresponding thermodynamic efficiency of the energy storage is analyzed. The
existence of the optimal elementary units of multipartite quantum battery
provide a guideline for designing a realizable charging scheme.
