A study of the ring behavior appeared in the primary side control of flyback power converter for battery charging is presented. In order to have better isolation between the input and output ends of converter, instead of having direct output voltage sensing from the output connected in the secondary side, the primary side control scheme is usually applied to the regulation design of flyback power converters. However, such a design might not provide a good regulation of the output voltage due to the imperfection of the voltage sensing circuit design added in the auxiliary winding of transformer. In this paper, an equivalent circuit model for primary side control of battery charging is derived for the analysis of ring behavior appearing in the output voltage sensing circuit. Based on such a model, the oscillation or the so-called "ring" behavior is found to be attributed to the existence of non-ideal capacitance in the transformer winding. To reduce such an effect, three schemes are proposed for the design of the observer circuit for sensing the output voltage at the primary side of the transformer. By using the primary side control PWM controller chip SGPIOO designed by Fairchild Semiconductor Corporation, numerical simulations are obtained to demonstrate the effectiveness of the proposed designs.