Reducing voltage sensing in a battery pack is beneficial for cutting the cost of the battery management system. In this paper, a methodology is designed to estimate the states of charge (SOC) of two cells connected in series when only their total voltage is measured. First, the feasibility is analyzed based on the nonlinear observability of the two-cell string under reduced voltage sensing. Furthermore, observability analysis is performed to different battery chemistries to determine their respective observable SOC ranges based on the voltage-SOC relationship. A trajectory-based nonlinear observer, the Newton observer, is then designed for SOC estimation and has been validated by experiments. The limitation of extending the method to strings with more than two cells is also discussed. The methodology is initially designed under the assumption of equal and known capacity and resistance among cells. The robustness of estimation is finally investigated when the above assumptions do not hold.