[jimcol]

My unit gets left in storage for up to 4 months at a time in Kansas City, Mo. Winters here are not as cold as they use to be, but we'll get several days close to 0-degree F. All I have ever done is disconnect the negative cable off the fully charged battery. Never failed me yet.
If keeping your battery charged is your only concern, I believe simple solutions are the best and most cost effective. Leaves a lot less stuff to fail.