The State-of-Charge (SOC) is an important performance parameter and evaluation index in rechargeable battery energy storage systems. Here, a novel smart estimation method based on coulomb counting is proposed in order to enhance estimation accuracy. Firstly, an enhanced model for the initial SOC (SOC0) estimation based on dynamic multi-parameter method is investigated, because SOC0 plays a key role in calculating real SOC in-time. All the design limits, such as voltage, temperature, are used as its constraints. And more importantly, the relaxation effect also is considered. The SOC0 with the main parameters satisfies Gauss function. Secondly, the quantitative relations of the energy efficiencies are measured and analyzed under the moderate discharging situation. The results show a negative quadratic correlation between energy efficiency and rate, and exhibit a reduction in energy efficiency as rate increases. Lastly, a test with several consecutive hybrid pulse power characteristic test profiles is carried. The experimental results indicate that the correction of SOC0 can efficiently limit the error below 4%, and also exhibit that considering on the effect of energy efficiency can further reduce its estimation error.