Hi StarLane
Another option might be to prevent the build up of dew in the first instance, by using a secondary mirror dew prevention heater, such as the
Kendrick system.
You will still need a battery with some grunt, but you'll be trickling the current all night to keep the secondary above the dew point, rather than belting it with the heat from the hair dyer once it has dewed up.
Cheers
Dennis