Power control method for a motor vehicle electric window heater

摘要

An improved method of electric window heater activation automatically and independently activates front and rear window heaters at a variable level based on the respective potential of fogging, within the ability of the vehicle electrical system to supply the requested current without discharging the storage battery. A defog controller develops front and rear fog factors indicative of the relative potential of fogging, and activates the respective electric heaters as required to drive the respective fog factor to zero. The fog factors are based on an estimate of the cabin air dewpoint temperature, the temperature of the respective window surfaces, and a temperature interval over which the fog factor signals only partial activation of the respective heater. The temperature interval is biased in a direction to provide preventative activation of the heaters at a relatively low level when the electrical power requirement is limited, and the activation level is limited as required to prevent battery discharging.