Hello.
I'm planning a convertion myself and I have a similar problem since I live in Sweden. It doesn't get QUITE as cold as Minnesota where I live, but fairly close to it (below -20C for extended periods and temporary dips down to 25C isn't uncommon in January). My findings so far is that no matter what batteries you choose you'll probably want to insulate them and possibly also add some kind of heater that keeps the temperature above a certain degree.
It's true that led suffers more from low temperature than, for example, LiFePO4 but even thought LiFePO4 seems to care very little for low temperatures they still have to be charged at above freezing point, as far as I've understood. Also, if you don't have a serious budget LiFePO4, and other kinds of really nice batteries, are very pricey and thus way above my budget. At least for now. Hopefully I can save enough gas money to pick something better when the first led-pack is worn down. That's the plan at least.
So my solution will be to go led, insulate the batteries and add some kind of heater that keeps them at least above 10C when charging and kicks the temperature up to 20C before I go somewhere. It will cost some kWh to avoid freezing the batteries, but if the insulation gets good enough, it'll still save big bucks in my daily commuting. Besides, I want to pre-heat the car in the winter anyway, otherwise heating up the passenger compartment will seriously cut down my range and, of course, deep freeze the private parts...
Happy tinkering!
