Welcome Conde54 to the HDTimeline forum. While others will weigh in shortly I would take a look in the Self Help tips tab near the top of the page to find info on the battery tender or the Search tab typing in keywords like battery tender for related posts.
The short answer is, if you are using the HD or Deltran Battery Tender you can leave it plugged in all winter long, it is micro-processor controlled to monitor voltage and regulate current to shut itself off and not overcharge the battery.