Charging battery through 12V cigarette plug?

[mikegolf]

I assume the 12V cigarette plug is wired directly to the battery, there is no voltage stabilization. I know there is a fuse, but the charger maximum output (3A = ~40W) is way below the fuse rating.

I'm looking for a way to connect the charger easily ... how bad is it to connect it through the 12V outlet? (Ignoring the fact I'll have the male plug on the charger output)

During the winter time (this means mostly rainy ~0 to 5* C weather) I need to charge the battery quite often - but still don't want to take it out, so when there is a short weather window, I can go riding.