The SOH of batteries is critical information for electric vehicle (EV) and hybrid electric vehicle (HEV) systems. This paper proposes an SOH estimation system based on time-constant-ratio measurement. There are two reasons of choosing time-constant as aging criterion. First, time-constant represents the response speed of terminal voltage during charging and discharging. Second, time-constant represents the change of internal chemical reactions. The proposed estimation resolves the issues of traditional SOH estimations. There are two widely used SOH estimations. The first one is full-charge-capacity (FCC) estimation. Unfortunately, to access FCC information need to take a long-term charging and discharging test. Therefore, FCC estimation fails to be a fast SOH estimation. The second estimation is internal impedance estimation. However, this SOH estimation may cause large error due to the effect of environmental impedance. For improving the above issues on different SOH estimations, this paper proposes an SOH estimation system based on time-constant-ratio measurement. The time-constant-ratio is a novel criterion for aging effect. The use of this novel criterion for SOH estimation helps to achieve the purpose of an environmental-impedance-free and fast SOH estimation. The proposed estimation is 0.14% the measured time of FCC estimation based on 1C charging and discharging. The measured results have average error below 1% and maximum error below 2%.