From the site that EdelweissPirate linked above, the i3 charges from empty for £2.26. The average price of diesel right now is £1.30/litre. If I’ve done the maths right, £2.26 would take my old BMW 120d 32 miles, compared to the 75 for the i3.
My copy of Heywood contains a figure that indicates the following for a vehicle powered by a gasoline spark-ignited engine:
-at city speeds, the coolant heat load is about 52% of the fuel’s energy, the exhaust heat load is about 22%, and the brake power is about 19% (the remaining ~7% is lost through incomplete combustion;
-at highway speeds, the coolant heat load is about 38% of the fuel’s energy, the exhaust heat load is about 30%, and the brake power is about 24% (~8% loss to incomplete combustion).
This book is about thirty years old now. With careful fuel management and attention to intake port/induction design, AIUI the loss due to incomplete combustion in modern engines is more like 2%. Higher compression ratios seem to dominate these days too, resulting in even better engine efficiency. the result is lower exhaust temperatures…but perhaps even greater coolant heat load, owing to the higher temperatures developed during the compression and expansion strokes as a result of higher compression ratio.
Bottom line, my earlier claim that coolant heat load ~= brake power is actually a major understatement. If you replumbed your car so that the radiator was in the cabin, you’d be uncomfortably hot within the first mile of highway driving.
Thanks for all the replies. Didn’t early air cooled Volkswagens have some kind of auxiliary heating system. I think they had an optional propane (?) system. I may be mis-remembering. I looked at a few videos but they showed heat being captured from the air cooled engine. I’m thinking there was an optional heater with a separate fuel source.
I remember sleeping in a crowded old Volkswagen at a campsite. The gasoline fired heater growling. Imagining my fiery death. But maybe there should be a propane fired heating system offered in electric cars in colder regions? At least till the magic super battery arrives. With the level of computer controlled electronic controls in these cars, it should be fairly easy to integrate such a heating system into them.
My Super Beetle burned gasoline from my fuel tank for auxiliary heat. I don’t know if this was a factory option or aftermarket, though. There were lots of aftermarket options. The standard heat used exhaust heat exchange, which was miserable.
As an example - this morning’s commute it was -14C I drove 12.2km in 26min. (so avg. speed 28km/h) in stop-and-go traffic. No seat heat, but cabin temperature set to 22.5C; the Model 3 indicates avg Wh/km of 330. I started with 392km range and ended with 363. So I have no worries with a short commute (and even if I go driving around after work) that I will run out of range.
For the metric-challenged, I went 7.6mi and used 18.1mi of range. It was about 7F and I did about 17mph. For weather in the -10F range, I find my number is 3 times the distance for range lost.
Once again, this is really stop-and-go; heating power consumed depends on time not distance. For highway miles I assume this ratio would be much less than 2x range lost. So unless your commute is over 100 miles each way, you should have nothing to worry about. If you do commute close to 100mi, then you want to explore charging options at your destination for those really cold days.
(I had one of those Beetle heaters, but could never make it work. I remember scraping the interior of the Beetle windshield and driving at subzero temps with the driver’s window rolled down… I don’t miss those days.)
Wow. I’m impressed, would never have thought it was that extreme. Nothing like numbers. Ignorance fought.
Another video (in German) of a guy who sleeps in his electric car (not even a Tesla) to see how long the heat will last.
Apparently there was a scandal in the German media where they said you’d freeze to death if you get into a traffic jam in the winter and your battery runs out in the cold.
He found that his car could keep the heat going for more than three days on a full battery. Which he notes, is about the same length of time you could keep the heat going idling in a diesel with a full tank.
The heating/cooling is a small heat pump. It sucks heat from the outside or pushes heat to the outside. The problem is that, the colder it gets, the harder it becomes to suck heat from the outside. And also the battery becomes less efficient when it’s cold.
Each EV has an EPA rated range. But you know the saying, “Your mileage may vary.” That’s especially true of EVs. The factors that affect the range (in order of importance) are:
- how fast you drive
- whether it’s flat or hilly
- whether you run the heater or not
- how many times you start and stop
- how cold it is outside
- whether you use the seat warmer or not
- whether your headlights are on or off
My 2012 Mitsubishi iMiev has an EPA estimated range of 62 miles. On a nice summer day, driving just 25 mph, on flat ground, no stopping, no heater, headlights off, I can go almost 100 miles. On a cold winter day, driving 65 mph, up and down hills, frequent stopping, running the heater and the seat warmer and the headlights, I can only go about 35 miles.
You just have to plan ahead. Know where the charging station is, know how much of a charge you have, know what the terrain is like, pay attention to the weather. The car also has a range meter which estimates how much you have left, based on your past driving history.
Suppose you get in and start the car in the winter time, with a full battery, and the range meter says 62 miles remaining. Now turn on the seat warmer. The range meter drops to 58. Now turn on the heater, full hot. The range meter drops to 44. If you plan to drive 50 miles to the next charging station, you have to make some decisions. You can turn off the heater and drive normally or you can run the heater and drive slower.
But don’t worry too much because the car also has a margin of 5-10 miles past “empty”. I’ve gone into the margin several times, never been stranded. Worst case scenario, you might have to call a tow truck, or maybe push the car the last mile. It wouldn’t be the end of the world.
If you’re thinking about buying an EV, you need to ask yourself what kind of driving you expect to do and how long you need to go between charges. Suppose you drive 42 miles (21 miles out 21 miles back) each day, and it’s start-and-stop freeway driving, and you need the heater in the winter time. An EV with a range of about 75 miles could handle that, assuming you start each day with a full charge. But you’d probably be happier with a range of 150 miles. Then you could do your commute even if you started with just a half charge.
Most EVs have an EPA rating around 100-110 MPGe, which means it takes about 1 KWh to propel the car 3.5 miles. Here in Oregon, we pay about 9 cents per KWh. That’s 9 cents to travel 3.5 miles, or 90 cents to travel 35 miles. Meanwhile, it takes a gallon of gas to go 35 miles in an ICE car, and a gallon of gas costs about $2.79 . When my wife drives 35 miles in her car, it costs us $2.79 worth of gasoline. When I drive 35 miles in my car, it costs us 90 cents worth of electricity.
If you live in New England and pay 20 cents per KWh for electricity, then driving an EV there might cost you $2 to go 35 miles, compared to $2.25 for a gallon of gasoline.
I haven’t read the thread in depth yet, but can someone explain the math to me?
An electric car gets maybe 3 miles per kwh of battey. So a 65 kwh battery will drive about 200 miles.
An electric space heater is maybe 1000 watts, so running it will cost 1 kwh per hour.
But a space heater will heat about 100 square feet, technically 800 cubic feet when factoring in ceilings.
A sedan cabin is maybe 100 cubic feet. But even if you needed 1 kwh per hour (which seems excessive) to heat your car, that’s not too much battery.
If you’re driving 60mph, that means in an hour you’ll use 20kwh by driving and 1kwh by heating the car(again assuming you are using as much energy as a space heater which again seems excessive).
Heating a car shouldn’t use much energy. I’d assume it’s closer to 200-400 watts per hour to heat 100 cubic feet.
A car isn’t a house. A car gets parked in the cold weather, and we expect them to get warm within minutes. Also, cars aren’t as well insulated as houses. I’ve seen estimates saying the Tesla Model-3’s heater consumes 3 to 4 kW.
The impact is greater when driving at slower speed, as already discussed above. If you are driving in city/suburban roads and averaging 25 mph, that would be around 8 kW average. A 3 kW heater represents a 38% increase in power consumption.
According to this site, the Model 3’s heater is closer to 4300 watts, than 1000 watts. That seems about right to me, because of what I’ve observed when pre-heating at full blast. Recently my car was connected to an outdoor charger during a snow storm at 30 amps and 198 volts getting me 22 miles of charge per hour. I turned the heat on to Hi (82F+), and the car continued to draw the full 30 amps, but the charge rate went down to 0 miles per hour. That’s something near 6000 watts being used to heat the car. My other car’s fan is a 40 amp fuse, so about 400 watts? The Model 3 has that fancy air blade system, so maybe some more wattage for the fans, and add in 500 watts or so for the seat heater.
Anyway, when it’s coming down hard and the snow is sticking to the windshield and turning to ice before the wipers can clear it, I don’t want a 1000 watt space heater doing the work.
Here are some hard numbers to put things in perspective. One day last week I drove home from work for 14.6 miles at an average speed of 21 MPH, and the outside temperature was 53F. That used 2.83kWh at 193Wh/mile. Excellent efficiency, probably because the car was doing the driving.
Today I did the same drive with snow coming down and an outdoor temperature of 25F. I had to run the heat to keep the windows clear. My average speed was 18 MPH, and I used 5.95kWh at 403Wh/mile. I drove myself the whole way.
Those are the two most comparable drives as far as average and max speed that I could find, with very different weather conditions. Most dry days, even very cold ones, my max speed is much higher on my commute.
So if it was -25 it wouldn’t matter because you maxed out the heat?
What about de-humidification? Does the car tell you the A/C is on when it’s slightly above zero? I would think 33F would be the most electrically taxing.
I had no idea the heaters were 4300 watts. I assumed because the cars interior is only about 100 cubic feet, that the heater would require a fraction of the energy of a space heater designed to heat 1000 cubic feet.
That makes more sense, but still it shouldn’t impact gas mileage ‘that’ much at highway speeds because if you drive 60mph for an hour you’ve used 20kwh of battery for the drive and 4kwh of battery for the heater. And that assumes the heater is on full blast the entire drive. At city speeds, it’d be more of an issue I agree.
That is pretty much what I observe. For example, I have a 14 mile trip with the outside temperature at 18F and the inside temperature at 65F, and I only used 281Wh/mile which is 85% efficiency. So, somewhat worse than the rating, but not too bad. That trip was only 18 minutes long, though, and heat is going to have a time component. It is going to take more energy to heat the car on a 45 minute rush hour drive, than the same off-time trip that only takes 20 minutes. Heat is also going to be a larger percentage of energy use when driving slow than fast.
Other things that will reduce range in the cold are higher air density; lower air pressure in the tires, if they were filled when it was warmer; potential higher rolling resistance from snow/slush; less efficient winter tires; energy loss from braking instead of regeneration; and possibly efficiency losses in the battery itself. I’ve only once had my battery cold enough to see a reduction in power, so at these normal 10F+ temperatures, I don’t think that is a big deal. That is different than the battery controller reserving some charge to keep the battery safe. For example, the minimum charge might be 10% when very cold, instead of 0%.
The worst thing for heating efficiency are lots of short trips, with long enough time in between for the car get cold again.
My husband almost never turns on the climate control in our hybrid electric, because it reduces the range so much. (It’s a c-max, it doesn’t have much electric range. And it has much less than it had when it was new.) It drives me nuts, the cabin is stuffy as heck, and if he’s been sweating the seat can get gross, too.
Whenever I drive the car the first thing I do is turn on the fan so I get some circulation.
A car’s cabin heater is heavily overdimensioned in order to heat up the interior very rapidly from a cold state. A car can heat up much faster than a room with a space heater.
Also a car has much poorer heat insulation relative to the walls of a room.
Some more numbers:
Bjørn Nyland reported a power consumption of 1,5kW overnight while sitting in his car in minus 31C / minus 23F. That was with some loosely attached foam and reflective heat shield insulation on his windows.
In another test during highway driving slightly below freezing in a smaller car, it reported a climate consumption of 800W. That was in a car that used a heat pump. Assuming a COP of 2 - 2,5 then the heat loss would be 1,5 - 2 kW.
Seems like a car needs the same heat power as a room to keep reasonably warm, and quite a bit more to heat up quickly.