Future Internet of Things is paving the way for the proliferation of Wireless Sensor Networks (WSNs). To overcome the limited energy in batteries, WSN nodes are relying on everlasting environmental energy. Moreover, a Power Manager (PM) is also embedded in each WSN node to guarantees that the total consumed energy is equal to the harvested energy for a long period, leading to Energy Neutral Operation (ENO) with a theoretically infinite lifetime. In this paper, a new PM for WSN nodes powered by periodic sources (e.g. ambient energy is not available during the full harvesting cycle) is proposed. Not only respecting the ENO condition, our PM is able to balance the Quality of Service (QoS) during the whole cycle to provide regular data tracking, which is essential for WSN applications like monitoring. Simulations on OMNET++ show that our PM can improve the QoS during the absence of energy by a factor up to 84% compared to state-of-the-art PMs, while guaranteeing the same global QoS.