There are really two schools of thought on this question:
1. Some will tell you to disconnect the battery (fully charged) and leave it inside the ski outside during winter. When springtime comes, just charge the battery for 24 hours and then reconnect the wires. They will tell you that a battery in cold storage will last longer (my neighbor does that as well as the local mechanics in hundreds of skis and boats).
2. Some will tell you to remove the battery, store in a cool place and slow charge it every month or so. Seems like cold kills a battery forever.
In then end (reading a lot about it), I found out that these batteries rarely last more than three years anyways... My ATV is the perfect example: even if I do a preventive tender charge on it in extremely cold nights (like at 5 below degrees F and lower), I have never succeeded in keeping a battery for more than two years (although the winch is hard on a battery).
So this year, and since my ski battery was already two seasons old and that it was over solicited this summer because of engine troubles, I left it in the ski disconnected. If it is still good for a third season next year, this will be proof that cold does not hurt. If it is dead, I will replace it and then next fall, I will have to decide if I bring it inside or not...
Benji.