In this paper, a balanced space-time block coding method for cooperative relay networks is proposed which guarantees fall diversity for any number of mobile relays with minimal delay, provided that few bits of feedback from the destination to the source and the relays are available. As in the case of orthogonal space-time block codes, all transmitted symbols are separately decoded both in the relays and the destination. Therefore, decoding complexity is linear. Many of the proposed solutions in the literature are distributed space-time codes which are designed for limited number of relays. In the proposed scheme, as many number of relays as necessary can be included to increase diversity. Moreover, the proposed scheme has better signal-to-noise ratio improvement compared to relay selection schemes. Lastly, the new method satisfies power consumption fairness among relay terminals and requires no knowledge of topology information.