## 1 Introduction  
Wireless sensor networks are popularly applied for variant environmental monitoring applications such as forest fire detection [1], intruder detection [2], and factory or pasture monitoring [3]. Sensors are tiny devices that consist of multiple sensing components, a simple computation unit, and the wireless communication component [4]. The energy source of the sensors usually comes from their installed batteries. Using the limited energy resource efficiently is a critical factor to prolong the operation time of the sensors.  
Many energy-aware data gathering routing protocols have been proposed to prolong the operation time of the wireless sensor network. These studies can be classified into two classes. The first class is cluster-based routing protocols [5; 6; 7; 8; 9; 10]. Sensors in this class are organized into multiple clusters, and each cluster selects a cluster head (CH) as the coordinator to handle the communication sequence and to aggregate the data of the members. After each CH aggregated the data of all cluster members, the collected data is transferred to the sink which is a data center of the network.  
The second class is chain-based routing protocols that sensors are organized in the chain format [11; 12]. Sensors are chained as a Hamilton path. One of the sensors is selected as the leader, which is similar to the CH in the clustering methods to aggregate the sensors' data and to relay it to the sink. The data of sensors is propagated to leader hop by hop according to the sequence of the chained format. The number of sensors relaying data to sink reduces to one in the chain-based routing protocols. Similarly, the leader in the network is also periodically changed to share the relaying overhead.  
In addition to the above two class, some studies use movable vehicles to be the mobile sink to visit the sensors and gather their data [13; 14; 15; 16; 17]. Furthermore, some studies focus on visiting and charging the sensors [18; 19; 20] that is planning the routing path of the chargers. They can also be applied for data gathering when they charge the sensors. These kinds of methods generate much propagation delay time. They are not suitable for applying in large-scale networks and for the sparse deployed sensor networks. Although many mobile charging methods have already proposed to replenish the energy of sensors, they are not applied for the scenario that sensors are stationary collecting data continuously.  
The above methods are intuitively designed for the sensors that cannot replenish energy. Directly applying these methods to the rechargeable wireless sensor networks may lower the utilization of energy.  
Therefore, this paper proposes a charging-aware multi-mode routing protocol (CMRP) to collect data in wireless rechargeable sensor networks. The proposed CMRP method dynamically changes the data propagation mechanism according to the charging status of the deployed sensors. CMRP makes the sensors which are not in the charging area save their energy and lets the sensors which are in the charging area quickly propagate data to sink. It considers both the network lifetime and propagation delay.  
The rest of this paper is organized as follows. The related studies are reviewed in Section 2. The proposed charging-aware multi-Mode data routing protocol is provided in Section 3. Section 4 presents the simulation results to compare CMRP with the existing methods. Finally, the conclusions are presented in Section 5.