The state-of-charge (SOC) estimation of lithium-ion batteries is very important for the safe and reliable operation of the battery pack. In the industry, the ampere-hour integration plus open-circuit voltage (OCV) strategy is normally used to estimate the SOC. However, the use of OCV is very demanding, requiring several hours. In this paper, a new estimation strategy is proposed, which can calibrate SOC during battery operation to eliminate the accumulated error of ampere-hour integration. Specifically, when the current of the battery is constant for a given period, or the change is small while charging or discharging, the SOC can be corrected according to its voltage. The detailed theoretical analysis is given in this paper and an experimental platform is built for testing.