I use a simple rule of thumb on lithium cells that we lose 1% capacity per 2C/4F below 20C/70F, therefore a freezing temperature of 0C/30F would lose 10%, and a really cold temperature of -20C/-10F would lose 20%. Again, I'm talking about BATTERY temperature, not ambient air temperature.
This works great until you start to factor in the power consumption of the battery heater, which is going to be vastly different for every car and situation. The ONLY way to know how much power that is would be to read it off the CAN bus on the car, or measure it with your own equipment. There are just too many variables to accurately guess. The same goes for cabin heating.
So, let's say that you're the advanced EV driver, and you know that to get maximum range, you needed to preheat the cabin and battery (and you've done that), and by some stroke of luck, the battery just happens to get over 20C/70F (even though it might be -20C).
So, you are therefore starting out with ALL the available energy that the battery has to offer. If you have a heated garage, this might actually be the case. The other variables to range (not what the GOM says, but actual range... how far can the car go given a set of metrics) can be estimated.
Speed - thankfully, this is very easy to calculate.
Wind - easy to factor
Snow, Rain - you're going to have to guess
Cabin/Battery heater - another big guess
Ambient Temp - easy to factor increased air density in cold air
Winter tires - you may have snow tires that increase rolling resistance
That's really it. The bottom line is to know how much energy you start with and monitor the consumption to make sure that you're going to make it. With experience, you'll learn that the worst combination of events can cut your range in half.
Knowing what makes an electric vehicle go the FARTHEST might help to understand why it goes less in cold weather.
Longest Range Pointers
1) Drag - Wind resistance - high elevation and hot ambient temperatures thin the air, making it easier for any object to pass through the air. In the airplane business, we call this calculation "density altitude". Here's an easy to use online version: http://wahiduddin.net/calc/calc_da.htm. The most "aerodynamic" vehicle will cut through the air with the least resistance.
2) Drag - Rolling resistance - drag from everything that rotates to move the car; tires, wheel bearings, u-joints or cv-joints, gears and bearings, gear lubricant. Generally, the hotter the lubricant, the lower its resistance; the higher the tire air pressure, the lower the tire resistance.
3) Gravity and Mass equals Weight - as long as there is gravity, it will take energy to accelerate mass and energy to propel mass at speed. Obviously, it also takes significantly more power to lift the mass away from the gravitational pull while driving uphill. Quite simply, lighter is better.
4) Ideal speed - every vehicle has a speed where the intersection of the power required to overcome drag and weight is the least. Heavy cars with high drag tires (but extremely aerodynamic) like a Tesla tend to be most ideal in the 20-25mph range and small, lower mass cars like BMW i3, LEAF and Spark EV are probably in the 10-15mph range. Obviously, anything that uses power that isn't being used to overcome drag and weight is a waste; the heater, a/c, etc.
5) Hot batteries store more energy - the hotter the better, however, this same heat tends to significantly shorten their life (Nissan LEAF in Phoenix) and in some cases (Boeing 787), make the battery fail.
That's it! Hit the ideal speed, at the lowest weight with the least drag and the most stored energy!!!!
), and sitting atop those toasty heated seats and holding a heated steering wheel, I'd could probably drive all winter without using ventilation heat. (I precondition during the work week.) The heated wheel probably draws much less power than the heat pump, and the wheel is literally more direct, more efficient, heating.
