Some of the batteries I keep charged with a single tender are 6-7 years old and have been left in an unheated garage all of those winters. The one in my sons 2005 is the one that came in it new.

 

I store the bike in a semi heated garage.. 50-60 deg F. I ride the bike at least once a week in the winter.. just a short spin.

 

I keep all five of my bikes hooked to their own battery tender junior all the time. Amazon.com has these on sale for about $25. The garage is unheated, never a problem and my batteries last five years or more.

 

Lots of old wives tales on batteries. Cold slows down the chemical reaction in a battery. Sure that mean less power when you try and start a cold motor, but for a stored battery it means the acid isn't eating the lead plates as fast. Which is a good thing.

100% charged battery will freeze at -77°F. (Like in Chicago)
75% @ -35°F
50% @ -10°F
25% @ 5°F
0% @ 20°F

Just keep a battery at 100% as much as you can and you'll get the most life out of it.

 

I live in Alberta where it can get down to -35C. I have an unheated garage and I have always left batteries in bikes all winter long. I just throw the charger on every 2-3 weeks, and ride whenever possible. I typically get 5-10 years service out of my batteries but I got as much as 19 years once!

 

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Ontario Canada, the coldest
days are -20C / -25C The
batteries stay in the bikes in an unheated garage and I just run them every few weeks over the wintertime. So far no issues.

 

I pull mine, fully charge them and store on a shelf in the basement all winter. Come spring I fully charge them and install back in the bikes.