The emerging wireless energy transfer technology based on Radio Frequency (RF) is a promising technology for wireless rechargeable sensor networks (WRSN) as it can charge sensor nodes simultaneously. In this paper, we use a mobile charger to stay at some locations and stay for certain time at each location to charge all the nodes in the network. We first define a power-charging function for the whole network and then get a set of candidate stop locations for the mobile charger by analyzing the property of this function. After the set of candidate locations are determined, we formulate two optimization problems: one is to minimize total charging time and another is to maximize the charging efficiency, subject to a charged energy threshold at each sensor node. Simulation results show that our method for choosing stop locations can greatly reduce the total charging time and improve charging efficiency.