This paper proposes a fuzzy state-of-charge (SOC) estimation for Reflex™ battery charging, which allows rapid and efficient charging for sealed lead-acid batteries used in electric vehicles. The proposed SOC estimation can avoid undercharging or overcharging. Overcharge in particular can shorten a battery's lifetime. The charging control and the SOC estimation are based on fuzzy theorem and they are implemented by a digital signal controller (dsPIC30F4011). The battery's SOC is estimated while the system was charging the battery and the estimated error is maintained within an acceptable range (±5%). Moreover, this system used a method of equivalent internal resistance to calculate the internal resistance of the batteries. During the Reflex™ charging, the controller can self-tune the discharging time according to the internal resistances, which reduces the thermal rises and the lifetimes for the batteries. Finally, experimental results are provided to verify the performance of the system and the feasibility of the proposed SOC estimation in this paper.