Originally Posted by HunterGreen
I have heard that it's not the kind of battery people think it is that would be so affected by the cold, and in any case, that the Prius takes care of its battery's temperature automatically. I'm not sure if that's true. But here in Vermont, temperatures that cold happen as the low overnight a dozen times a year, and plenty of us have Prii that are just fine. So unless you mean that -30C is a temperature your thermometer sits at for days at a time, rather than being the overnight low, I am pretty sure you don't need to worry. And even if you do, I still doubt it.
I can tell you from personal experience that at -20F and a 2 day 'cold soak' that battery performance is undoubtedly affected. It works, but usable range is greatly diminished and you can't get much electric 'boost' when accelerating/passing. And it takes, literally, hours of driving to warm up to even the freezing point.
The battery temp is, to a point, regulated by the car, but this is via the intake vent in the cabin of the Prius...so it sucks in the cold air (in hot weather) or warm air (in the cold season) and blows it by the battery to adjust the temp...this is passive most of the time, but can be active with the fans built into the vent system. But the battery is a large thermal mass and the temp changes are not rapid..this is good in that it can't overheat quickly, but it's bad in that it takes forever to warm up once it gets extremely cold.
If I lived in a very cold area like Mongolia, Minnesota, or Alaska I'd be sure to have access for an engine block heater and, preferably, a garage that could be heated at least to ~32F to minimize the cold MPG and performance impact.