The process water circuit in Fig. 1 uses deionized (DI) water to humidify the anode and cathode streams. Under sustained driving conditions, the system is water neutral, i.e. the process water used in the humidifiers is recovered within the FCS. It may not be possible to completely recover the process water under all driving conditions, particularly during periods of rapid deceleration and at very low loads. However, the system can still be made water neutral by recovering excess water (produced in the stack) during favorable driving conditions (high loads and acceleration) to compensate for the net water consumed during unfavorable driving conditions.