NOVELTY - The relative humidity and condensed water estimator (10) has controllers for dynamically estimating relative humidity and condensed water in a fuel cell system using fluid dynamics and mass balance equation of oxygen, nitrogen, hydrogen, and water. The controllers comprise an internal estimator air blower controller (20), an internal estimator humidifier controller (30), an internal estimator fuel processing system controller (40), and an internal estimator stack controller (50). The estimator is utilized in control of fuel cell system involving control of anode condensed water drain.    USE - Relative humidity and condensed water estimator for fuel cell.    ADVANTAGE - Provides an estimator for dynamically estimating relative humidity and condensed water in fuel cell system in order to overcome the flooding phenomenon that signifies overflow of water, and the dry-out phenomenon that signifies deficiency of water, which are caused by changes of the amount and the state of transferred water according to changes of operation conditions such as operation temperature and pressure of fuel cell. Provides an estimator which can estimate relative humidity and condensed water in fuel cell system based on typical sensor signals without a separate sensor. Improves the reliability of controlling condensed water drain valve in terms of fail-safety, based on the failure determination of typical anode condensed water level sensor. Prevents flooding in a stack when the water level sensor of the condensed water drain valve fails, thus preventing hydrogen recycle blower unit from breaking down. Improves the fuel efficiency and safety by preventing an excessive hydrogen leakage.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a controlling method of condensed water drain.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram illustrating a relative humidity and condensed water estimator for a fuel cell.    Relative humidity and condensed water estimator (10)    Internal estimator air blower controller (20)    Internal estimator humidifier controller (30)    Internal estimator fuel processing system controller (40)    Internal estimator stack controller (50)