The Lithium Ion Batteries used by the MINI E work best if used at a temperature around 74F. It is very important that they not get too hot because high temperatures can permanently reduce their performance, and there is the risk of them catching fire (mostly if it was damaged or there was a defect in manufacturing them). Batteries at low temperatures can have their performance and ability to hold a charge severely reduced, but this is temporary and has no long term effect on the batteries.
BMW is aware of this and they have mentioned that the housing the batteries are in is designed to keep them cool. I dont know the details of the system but I can make some speculations and what I'd like to see.
The Tesla Roadster goes to great lengths to keep the batteries at an optimal temperature, it has a liquid cooling system that runs all the time, and I've read numerous criticisms of it as being a big energy waster, the MINI E uses air cooling with temperature load and speed sensitive fans also it is mounted inside the cabin so it must take advantage of the heating/cooling used for the driver. I have wondered if this is a passive or active system, if the batteries start to get too hot will the car automatically turn on the AC? When the car is left out in a parking lot on a hot day this could end up using a lot of power, but would also mean you'd have a comfortable cabin when you got inside. The EV-1 had a feature where you could tell it to pre cool the cabin but only while it was plugged in.
Being in New Jersey I dont expect problems with over heating to be that much of an issue, but I do worry about poor performance in low temperatures. I'm hopeful there will be someway I could tell the car to warm up the cabin on a cold day maybe an hour before I plan to leave, so the batteries will be able to hold a full charge, and I wont have to blast the heat as soon as I get going. Also I wonder how much of a range hit I'll get on those days when I need to use the heater or AC all during my driving.
Its all just speculation at this point but this will be something Ill talk about in more detail once I have some real experiences to back things up.