The main focus of this work is directed towards maximization of the lifetime of a network of cooperating mobile sensors monitoring a pre-specified target. It is assumed that the main sources of energy consumption in the network are movement, communication and sensing. It is desired to gather information about a moving target in a 2D field and find a proper route to transmit it to a fixed base or destination point. In order to find the most efficient route for transmitting information, the field is discretized as a grid of nodes. It is assumed that the sensors and target are located in the nodes at any point in time. A direct graph is subsequently constructed whose vertices are the grid nodes, and whose edges are weighted properly based on their residual energy. The proper set of nodes are then obtained (which form the desired route for maximizing the lifetime of the network) by solving the shortest path problem in the resultant graph. Finally, a proper model is adopted for the batteries to plan the sensor movement by solving a nonlinear programming problem to minimize the energy consumption of the overall network.