Sources in a dc microgrid are required to have a proportional current sharing and an improved voltage regulation. Conventional droop-based control techniques achieve the proportional current sharing, but the voltage regulation of an individual bus may be large due to a high value of droop gains. In the literature, secondary controllers are discussed to improve either the voltage regulation of one bus or the average voltage regulation of the entire system. Most of these techniques rely on the communicated values of both the terminal voltage and the output current of individual sources. This paper puts forth a new approach to address the aforementioned issues. A figure of merit called quality index is introduced, which constitutes a weighted average of terms representing the voltage regulation and the power sharing at each source bus/node. An online algorithm to find the optimal droop coefficient based on the quality index is suggested. It does not require the knowledge of the system parameters. Furthermore, it reduces congestion in the communication network, as only output current information is exchanged among sources. Equations for the current sharing error and the voltage regulation in a microgrid system are derived based on droop gain values of the sources. Operation of the proposed algorithm is verified using a computer simulation. The method is validated on a scaled-down dc microgrid prototype.