In this paper, we investigate the energy balanced data collection problem in WSNs, aiming to balance the energy consumption among all the sensor nodes in the data propagation process. Energy balanced data collection can potentially save energy consumption and prolong the network lifetime, and hence it has many practical implications for WSN design and deployment. The traditional hop-by-hop transmission model allows a sensor node to propagate its packets in a hop-by-hop manner towards the sink, resulting in poor energy balance for the entire network. To address the problem, we apply a slice based energy model, and divide the energy balanced data collection problem into inter- and intra-slice energy balance problems. We then propose a novel Inter-slice Mixed Transmission strategy and an Intra-slice Forwarding technique to address each of the problems. Finally, we design an Energy-balanced Transmission Protocol (ETP) to combine both techniques to achieve total energy balance in data collection. Through extensive simulation studies, we demonstrate that, while ETP achieves energy balanced data collection, the network lifespan is increased by 10 times and the network delay is decreased by more than 70% compared to the hop-by-hop transmission in a general square area WSN.