G'Day Leandro,
I don't see how it could damage your battery unless the battery is way too small for the job (i.e. you will fully discharge it every time you use the dew heater).
What Ah rating is the battery? As long as the current draw for the dew heater(s) times the number of hours you expect to use them for between charges is less than the Ah rating of the battery (the bigger the margin the better!
) it should be fine.
I will stand corrected by anyone with more electrical nouse than me.
Al.