Driving an electric vehicle (EV) efficiently is forcing me to learn some new driving habits. And, in particular, I have to un-learn some cherished techniques used for driving a gas-powered car efficiently.
When I look for advice on driving an EV efficiently, all I get is a rehash of standard advice for driving a gas car. But the more I ponder it, and the more I pay attention to the instrumentation on my wife’s Prius Prime, the more I’m convinced that’s basically wrong.
An electric motor is fundamentally different from a gas engine. With an electric motor, you want to avoid turning your electricity into heat, rather than motion. That boils down to avoiding “ohmic heating”, also known as I-squared-R losses.
To minimize those heating losses, you want to accomplish any given task using or generating constant power. That task might be getting up to speed after stopping at a red light, or coming to a stop for a red light, in some given length of roadway.
Here’s the weird thing. Assuming I have that right — assuming that an efficient EV driving style focuses on providing or generating constant power over the course of an acceleration or deceleration — that implies a completely different driving style, compared to what is recommended for efficient driving of a gas-powered vehicle.
In particular, the standard advice for gas cars boils down to accelerating and decelerating with constant force. When you take off from a stop light, aim for a constant moderate rate of acceleration. When you are coming to a stop, aim for a steady rate of deceleration. Constant acceleration or deceleration boils down to constant force on the wheels, courtesy of Sir Isaac Newton’s F = MA (force is mass times acceleration).
But power is not force. As I show briefly, in the next section. In a car, power depends on speed. Constant force on the brake pedal (and so, on the brake rotors in a traditional car) generates far more power at high speed than at low speed. Similarly, a constant rate of acceleration consumes more power at high speed than at low speed.
And so, there seems to be a fundamental conflict between the way I was taught to drive a gas car efficiently, and what seems to be the right way to drive an EV efficiently.
In a nutshell, to drive an EV efficiently, you should be more of a lead-footed driver at low speeds. And taper off as the car speeds up. Conversely, hit the brakes lightly at high speed. And press the brakes harder as the car slows.
That’s the driving style that aims for production and consumption of power at a constant rate, over the length of each acceleration or deceleration. And that’s completely contrary to the way I was taught to drive a gas vehicle.
Think of it this way. Suppose you apply a certain level of force to the brake pedal of a traditional car. The resulting friction between brake pads and rotors will generate heat. That rate of heat production is, by definition, power, as physicists define it. You’re going to generate a lot more heat per second at 80 MPH than you are at 4 MPH. (In fact, 20 times as much.) Restated, for a given level of force, you are bleeding a lot more power off the car’s momentum at 80 MPH than at 4 MPH. And those big differences in power, over the course of an acceleration or deceleration, are exactly what you want to avoid in an EV with regenerative braking, in order to avoid I-squared-R losses.
Force and power: A brief bit of physics and algebra
1: Two definitions or laws of physics
Work = Force x Distance
Power = Work/Time
2: A bit of algebra
Substitute for the definition of work:
Power = (Force x Distance) / Time.
Re-arrange the terms:
Power = Force x (Distance/Time)
Distance/time = speed (definition)
Power = Force x speed.
For a constant level of force applied to or removed from the wheels, the rate of power consumption (or production) is proportional to the speed.
Upshot: To accelerate or decelerate at constant power, the slower you are going, the heavier your foot should be. The faster your are going, the lighter your foot should be. For the gas pedal and the brake pedal.
Ohmic heating: Why a long, hard acceleration trashes your battery reserve.
Anyone who drives a PHEV — with a relatively small battery — will eventually notice that one long, hard acceleration will consume a big chunk of your battery capacity. On a drive where you might lose one percent of battery charge every few minutes, you can knock several percent off in ten seconds if you floor it.
Another way to say that is that getting from A to B by flooring it, then coasting, consumes much more electricity than just gradually getting the car up to speed.
I’m not exactly sure why that is. But I am sure that it is universally attributed to I-squared-R or ohmic heating losses in the motors, batteries, and cables.
Any time you pass electric current through a wire or other substance, it heats it up. From the standpoint of moving your car, that heating is a loss of efficiency. The more current you pass, the more it heats up the wire. And that heating is non-linear. Watts of heat loss are proportional to I-squared-R, in the argot. They go up with the square of the current that you pass through that wire.
Again, I’m not completely sure here, but my takeaway is that your heating losses, at very high power, are hugely disproportionate to your losses at low power. At constant voltage, I believe those losses increase with the square of the power being produced by the electric motors. In other words, ten times the power produced to move the car creates 100 times the ohmic heating losses.
And that’s how ten seconds of pedal-to-the-metal can use up as much electricity as 10 minutes of moderate driving.
That said, I have to admit that I’m relying on “what everybody says” for this. For sure, hard acceleration seems to trash your battery capacity far in excess of the distance that you travel at that rate of acceleration. Whether the root cause for that is I-squared-R losses, or something else about the car, I couldn’t say.
Either way, my takeaway is that if losses are proportional to the square of power consumed or generated, then to accomplish any given task (any fixed acceleration or deceleration episode), your aim should be to do that at constant power. Because that’s what will minimize the overall energy loss from that acceleration or deceleration episode.
Drive like you are pressing on an egg — that was a real thing. EV drivers should chuck the egg.
Source: Duke University Libraries, via Internet Archive.
Those of us who grew up during the 1970s Energy Crisis will probably recall public service announcements that asked you to drive as if there were a raw egg between your foot and the gas pedal. I managed to find a Texaco ad of roughly that era, laying out that egg-on-gas-pedal meme. The picture above is from that video.
As kids, that was pretty much beaten into us. Responsible driving means no jackrabbit starts, no tire-smoking stops. Easy does it. We’re in the middle of a prolonged gasoline shortage, after all.
So now I come to the part that is absolute heresy for someone of my generation. If you’ve absorbed the prior two sections, you realize that this advice probably isn’t correct for an EV. Why? To consume power at a constant rate over the course of an acceleration, you should start off with a brisk rate of acceleration, then diminish that as the car speeds up.
In other words, if you drive an EV, drive with a lead foot. Not all the time. But at the start of every acceleration. And the end of every deceleration.
If you drive an EV, chuck the egg.
The Prius Prime Eco display
Source: Underlying picture is from Priuschat.
With that understanding in hand, I’m finally starting to make some sense of the “eco” display on my wife’s Prius Prime.
In theory, this little gauge is giving you guidance on how to drive the car most efficiently. In practice, I could never make head or tail out of it, except that it seemed to be telling me to drive with a lead foot.
Which, I now understand, it was.
On this display, if you put your foot on the gas, it will show you your actual throttle (gas pedal) position, and the gas pedal position that will, in theory, give you greatest efficiency.
By contrast, when you put your foot on the brake, it doesn’t show you the brake pedal position. Near as I can tell, it shows you the amount of power than you are generating. That is, a constant brake pedal position will lead to a shrinking bar, as the car slows down and less energy is generated.
Watch what happens if I try to accelerate gently:
It’s possible that all the meter is actually telling me is that a very lightly-loaded electric motor will operate inefficiently. I don’t think the Prius eco monitor is actually trying to get me to drive at constant power.
Edit 3/11/2024: After driving around with a ScanGauge III, my conclusion is that accelerating at constant power is exactly what the eco-meter is trying to get you to do. It wants you to start off with a heavy foot, and then, as you accelerate, it wants you to back off. Near as I can tell, that bar is set up to keep you at around 23 HP of power output, or 50 amps of discharge current, or a “2 C” rate of discharge, for this battery.
In particular, my diagram above is labeled wrong. The two red arrows should be labeled “desired power output” and “actual power output”. You will notice that if you don’t move the gas pedal, the “actual power output” line will creep up as you speed up. The only way to keep that line in the same place is to back off the gas as your speed increases. So that line isn’t the throttle position, it’s the power output (power = force x speed).
But no matter how I arrive at it — from theory, or from finally paying full attention to the Prius eco meter — the whole drive-like-there’s-an-egg-between-foot-and-gas-pedal is clearly obsolete. Gentle acceleration may get you your best mileage in a gas-powered vehicle. But it’s not the correct way to drive an EV.
