Hybrid ad-hoc network (HANET) is a new and promising paradigm for decreasing the energy consumption of the mobile terminals (MTs) in a cellular network. In this network, the multi-hop functionality of ad-hoc networks is employed in an infrastructure-based network to relay the packets of MTs experiencing bad channel qualities through other MTs with better channel conditions to/from the gateway. However, the proper cooperation of MTs in a HANET scenario cannot be taken for granted as they are normally controlled by selfish users who seek to maximize their own battery lifetime. In this paper, we propose an incentive mechanism based on coalitional game theory to encourage MTs to forward packets for each other by offering energy credits to the cooperative nodes by means of a virtual central bank (VCB); where the contributions of all MTs are recorded and tracked to differentiate non-cooperative nodes from cooperative ones. We also validate the effectiveness of our proposed technique through simulation results.