Piezoelectric energy harvester (PEH) has been commonly considered as energy sources for self-reliant systems or
wireless sensor nodes. They mainly consist a vibration source, an energy harvesting circuit and a storage device like
battery. Currently, PEH suffers from low efficiency energy harvesting circuit and lack of battery model. Most of the
energy harvesting circuits exploits switching techniques with inductor, which require peak detection or zero crossing
detection. This paper discusses the balance between the harvested energy and the energy costs of the switching. In
addition, a battery model is developed to predict the real-time charge on the battery.