My friend Brad Meyer, who lives in the next town over from me in Massachusetts, has owned a Tesla Model S car for more than a year. His observations raise some serious questions about the advantages of an electric car, particularly in winter — and about the pricing of electricity in Massachusetts.
Electric power generation is most efficient (and least expensive) if power demand remains nearly constant; off-peak pricing can work well both for the power company and customers, by shifting time-insensitive tasks such as doing laundry and charging electric-car batteries into hours when electricity demand is otherwise low. California, where the Tesla is made, has an advantageous off-peak pricing scheme for electricity. Massachusetts has only a weak version of such a scheme. There are also serious cold-weather performance issues with the Tesla due to the slowing of the chemical reaction in its lithium-ion battery. Brad kept track of his car’s electricity use, and writes:
The Tesla’s info-center miles-remaining is based on an average of 293 watt-hours per mile. My measurement was from a winter period, and was calculated from a measurement of average watt-hours/mile over several winter months. My winter average was about 425. These numbers are not immediately available to me so I’m trying to remember them; they’re approximately correct.
Most of the time, I kept the car in a garage that is heated to about 38 degrees Fahrenheit in the winter, but there were times when, as an experiment, I left it out when the morning reading was about +5 F. When the car is turned on in those conditions, it begins to heat both the cabin and the batteries. There is very limited forward power available and regenerative braking is totally disabled, so you get no energy return at all to the batteries from slowing down and no braking effect except what you supply with the pedal for the first couple of miles.
I applied for a time-dependent electricity rate and saw that one part of the bill went from about 7.8 cents all the time to 3.9 at night and 9.6 during the day. But wait! That’s just the transmission charge. The generation charge, which should clearly change during the night when the machinery is just ticking along, is 15.9 cents all the time. This is the big fast one that we get from our power company (Eversource). So I’m charging my car at 19.8 cents per kilowatt-hour and I’m using 425 watt-hours/mile, which costs me 425 x 19.8/1000 = 8.4 cents per mile. You can run a Toyota Corolla for about that much in gasoline, or maybe a bit less.
The summer consumption of about 315 watt-hours per mile gives an electricity expense of around 6.2 cents a mile, which is better than most gasoline-engine cars but not dramatically so.
Don’t believe anyone who tells you he drives a Tesla for environmental reasons. That’s not what the car is about.
What is the Tesla about, then? As Brad demonstrated to me, it has astonishing acceleration, and so it counts as the first mass-production electric car with an appeal to car buffs. (Brad is one.) The acceleration is very strong at low speeds, unlike with an internal-combustion engine, which produces the most power near the high end of its rpm range. Brad tells me, though, that the Tesla’s power drops off with sustained acceleration, as protection cuts in to prevent the motors from overheating. And then there are the winter problems.
What might be done to improve the winter performance? Consider that a gasoline engine burns fuel on the spot to generate power, but only about 1/4 of the energy in the fuel is converted into mechanical energy to move the car. The other 3/4 becomes waste heat. About half of that is carried away in the exhaust, but the other half which the coolant carries away is ample to heat the passenger compartment. Though a fossil-fuel-burning or nuclear power plant is generally more efficient than a gasoline engine in a car, the waste heat is lost at the power plant. The electric car’s heater steals battery capacity, reducing the car’s range on a charge and increasing the cost per mile.
Electricity generation using hydro-, wind or solar power avoids the pollution, health, safety and environmental issues with fossil-fuel and nuclear power plants, but does not reduce the power demand to heat an electric car, or make the car run better in winter.
I’d think that it would make sense for an electric car to have the battery well-insulated against cold, and with a small electric heater. So, in the best-case scenario with today’s battery technology, an electric car could start up smartly if it had been charging, but would need a warm-up period if it had not been. The battery heater could keep the battery warm while the car is charging overnight, or be activated remotely or on a timer if the car is parked where it can’t be charged. Warming the battery in advance would avoid experiences like Brad’s when leaving an underground parking garage after attending a concert one evening: the car would slowly advance one foot up the exit ramp, then stop to gather its forces, then one more foot…
One advantage of an electric car, especially if the battery is already warm, is that the heater for the passenger compartment can be turned on immediately — or even in advance without starting the motor — rather than with a delay as with the heater in an internal-combustion-powered car.
The battery also needs to be actively cooled during use: lithium-ion batteries can overheat. The Tesla’s battery is liquid-cooled, and there is a battery heater, but evidently it lacks smart controls.
It is likely that technology will improve, but for now, the Tesla unfortunately cannot match the start-up-and go winter performance of a vehicle with an internal-combustion engine. Economy and range also suffer in cold weather. The same is likely true of other all-electric vehicles.
There are other Web pages discussing cold-weather performance of electric cars — search on <electric car battery winter> to find them. One specifically about the Tesla is here.
