In this paper, the study of Li-ion battery charge state forecasting using Grey model is presented. A test-bed for the battery charge forecasting is designed and devised. By utilizing this test-bed, the charge condition for charging Li-ion battery can be easily set and the data of the charged Li-ion battery can be simultaneously obtained. Experimental results show that the GM (1,1) Grey model can precisely predict the Li-ion battery voltage curve when its sampling period is smaller than the safe sampling period. How the safe sampling period is determined is described and the mathematical formula for determining the safe sampling period is derived in this paper. A test example is also implemented to assess this proposed Li-ion battery charging forecast method. Experimental results show that Li-ion battery charge state forecasting using Grey model is very attractive as theory predicts.