Due　to　their　high　flexibility,　high　maneuverability,　and　line-of-sight　(LOS)　predominant　channel,　unmanned　aerial　vehicles　(UAVs)　serving　as　flying　base　stations　have　received　a　lot　of　interest　in　emerging　Internet　of　Things　(IoT)　networks.　This　article　studies　the　energy-efficient　cooperative　strategy　of　rechargeable　multi-UAVs　for　providing　seamless　coverage　and　long-term　information　services　for　IoT　nodes.　Considering　the　limited　cruising　duration　of　the　UAV,　multiple　rechargeable　UAVs　are　capable　of　constructing　a　closed　chain　for　the　sake　of　alternately　supporting　IoT　nodes.　Moreover,　a　joint　IoT　node　assignment　and　UAV　configuration　optimization　problem　is　proposed　in　order　to　maximize　the　energy　efficiency　of　the　system.　Since　the　proposed　problem　is　a　mixed-integer　nonconvex　problem,　we　divide　it　into　three　subproblems,　namely,　node　assignment　scheduling,　UAV　trajectory　planning,　and　transmit　power　control.　By　exploiting　sequential　convex　optimization　techniques,　we　reformulate　the　nonconvex　subproblems　into　three　convex　optimization　problems　which　can　be　solved　within　the　polynomial　time.　A　block　coordinate　descent-based　iterative　algorithm　is　proposed　for　solving　these　energy-efficiency　oriented　subproblems.　Finally,　the　simulation　results　corroborate　the　effectiveness　of　our　proposed　method.