Diatribes of Jay: Big “Gas Tanks” for Electrons

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Remember the old “horseless carriages” at the dawn of the automobile age? Remember how they looked just like carriages for horses, but without the yokes and bridles? Something similar is happening to electric cars, as we try to model them on cars run on gasoline, which they are not.

I’ve written on this general subject before. My earlier essay focused mostly on ways to differentiate electric cars from oil-driven cars by adding specialized accessories, including some fanciful ones. Today’s essay is about an essential engineering difference between the two types of vehicles, which affects their relative efficiency and economic value. Failure to take it into account could stunt the growth of the electric-car industry and set back the fight against global warming.

The problem is easy to state, but it’s not so easy to see all the ramifications. “Gas tanks” for electrons, otherwise known as “batteries,” are orders of magnitude more massive and heavier than their counterparts for gasoline.

It’s hard to get precise figures because electric-car makers treat almost everything about their batteries as proprietary. And why not? The whole point of electric cars is that there’s not much to them besides their batteries. Electric motors and generators have been around for a century, and the high-power solid-state electronics that controls the electric power and provides regenerative braking is mostly public-domain. A former battery researcher for Tesla, now working on long-life battery-run drones, has described a Tesla as a battery made in the shape of a car.

Yet despite all the secrecy, you can make some good estimates of battery mass/weight by comparing plug-in electrics with similar-size hybrid and gasoline cars. All today’s electric cars use lithium-ion battery technology, so all have roughly the same mass and weight per unit storage (kWh) or range (miles). All have roughly the same battery capacity per mile, namely, about three miles per kilowatt hour (kWh).

For my own Chevy Volt, estimating the size of the main (high-voltage) battery is pretty easy. The Volt is built on the Chevy Cruze platform, which GM just discontinued along with its Lordstown plant. But you can still find useful curb weights on the Web. Here are the data:

2019 Chevy Cruze, entry level curb weight: 2,870 pounds
2019 Chevy Volt, curb weight, 3,453 pounds
The difference is 583 pounds. Like the Cruze, which is an all-gasoline car, the Volt itself contains a small internal combustion engine to run its generator when the battery runs out, plus a massive electric motor/generator. So the difference in weight between the two vehicles is a reasonable estimate of the weight of the Volt’s main (high-voltage) battery, namely, 583 pounds.

Now the Volt’s battery range, in summer, is 54 miles. Tesla and its competitors advertise ranges of 250 miles. So let’s extrapolate linearly, thus: 250/54 x 583 pounds = 2,699 pounds. Therefore, to extend my Volt’s 54-mile battery to a 250-mile range, we would have to add an estimated 2,699-583 = 2,016 pounds of battery, for a total estimated curb weight of 3,453 + 2,016 = 5,469 pounds. (Coincidentally, that’s not far from the curb weights of some of the longer-range Teslas.)

That’s a lot of weight for what looks like a small car! In fact, the mass/weight of the Tesla’s (and equivalents’) long-range battery is almost as much as the entire mass/weight of a Chevy Cruze.

So when you drive a Tesla, you are likely carrying around with you a battery having the same mass/weight as an entire ICE car! You don’t notice much because Tesla’s engineers (like those of every other long-range electric car-maker) put all that mass/weight below the car’s center of gravity, where it improves traction and cornering. But all that weight is an energy-burner nevertheless.

Contrast the average tanks of gas. A gallon of gasoline weighs about 6.3 pounds. Ten or twelve gallons together—the usual tank size for a small car—weigh 63 or 75.6 pounds. Add fifty pounds or so for the metal tank itself, and they tote up less than 150 pounds. Compared to our estimate of 2,699 pounds for a 250-mile “electron tank,” that’s a factor of eighteen difference!

The first question this jarring comparison raises is, “Does everybody need that heavy a tank?” But before we answer that question, we need some engineering numbers. How much energy does dragging that heavy a tank around waste?

According to Newton’s First Law (inertia), it doesn’t take any energy at all to keep a mass going at constant velocity. But according to Newton’s Second Law, it takes energy to accelerate a mass or move it uphill.

When you’re traveling at a constant speed, the only energy-wasting effects of that big a tank are things Newton didn’t consider in detail: air resistance and friction, including tire resistance. Air resistance doesn’t depend upon mass/weight, only the aerodynamic shape of the car. So we’re left with only four effects that depend on mass/weight: accelerating to speed, going up hills, friction and tire resistance.

According to physics, as outlined in a footnote, lifting my Chevy Volt up the big hill to Santa Fe should cost only 6.1% of the battery capacity. But in fact going up that hill lops 40% off my battery’s capacity. That’s the reliable drain on the battery driving the Volt up the big hill to town, confirmed on multiple trips.

When driving down the very same big hill, over the same distance, the battery drops only an additional 20% in capacity. Again, that’s a reliable number confirmed over multiple trips. Averaged over several trips, the only difference between the two directions—40% loss going up and 20% loss doing down—is going up versus going down.

Basic physics tells us I get the gravitational potential energy of the car’s weight back on the downhill run. So the total losses on the runs both ways, up and down, are just the losses from frictional effects. That’s a total of 60% of battery capacity, on a 34 mile run, about what you would expect from a battery with 54-mile capacity (0.6 x 54 = 32.4).

With these simple calculations, based on actual experimental results, we can come to some useful conclusions. First, energy losses due to friction—air/wind resistance, mechanical friction, and tire rolling resistance—are a big deal. Together, at 60% of battery capacity, they dwarf the 6.1% that theory tells us hauling my car up the mountain requires.

Without specific aerodynamic calculations, we don’t know how frictional losses divide between those that depend on total vehicle weight (mechanical friction and rolling resistance) and those that don’t, namely, air resistance. In windy New Mexico, wind certainly matters. But even if we arbitrarily assign wind, which doesn’t depend on weight, to twice the impact of other forms of friction, we conclude that things that do depend on weight account for at least 20% of battery depletion on the up-down run.

So here’s the question/problem posed by this post. Let’s say you use your electric car for 50 or so miles of travel on the average day. Let’s say you need more only a couple of days a month. And let’s say you charge up your car nightly by plugging it into your garage.

Which is better? Carrying around an extra two-thousand-pounds-plus of lithium battery, on the off chance you’ll use it a couple of times a month? Or carrying a couple of hundred extra pounds of mostly-unused internal combustion engine, plus three gallons of gas to run it for over 100 miles, when those three gallons plus tank weigh a total of less than seventy pounds?

At first glance, the extra battery seems the greater waste in weight and energy. That’s the reasoning that ultimately impelled me to lease my Chevy Volt after an eleven-year dither.

The waste of energy from carrying a constant big-battery burden around may seem inconsequential if the energy to charge your car comes from carbon-neutral sources. For example, you might have your own solar array (as I do) or windmill. Or the power company that charges your car might send you nuclear power, hydroelectric power, or the output of commercial solar arrays or windmills. Then you would still waste power, but it wouldn’t have any carbon footprint.

Yet if your own array or windmill is on the grid, your wasting your own clean energy may cause someone else to use electricity from fossil fuels. If the electricity from the power company that you use to charge your car itself comes from fossil fuels, your habitual waste of energy in carrying around two thousand or so pounds of battery that you don’t normally use puts more carbon into the air.

It would make things easier if car makers would offer plug-in electrics with an array of ranges, not just the Volt’s 54 miles, the Leaf’s 73 miles and the Tesla’s and Bolt’s 200-250 or so. In my case, for example, it would help to have a car that could reliably go the 70 miles to or from Albuquerque (including back uphill) without burning fuel. But in cases of only occasional extended-range use, it seems more environmentally sound to rely on a serial hybrid like the Volt than dragging-around a two-thousand-pound-plus millstone of excess electric battery every day.

The moral of this story is that gas tanks for electrons are heavy. They are much heavier than extra gasoline and a small internal-combustion engine to extend range when needed. So in order to avoid the energy waste and carbon footprint that comes from dragging thousands of pounds of excess battery capacity around when you don’t need it, drivers should consider their lifestyle carefully before buying unneeded battery capacity.

To make that possible, electric-car makers should offer a wide range of plug-in battery capacities, perhaps using snap-together modules. They might even offer snap-in battery modules that car owners themselves could add and remove when needed. The analogy between a gas tank and a battery, it turns out, goes only about as far as the old “horseless carriage” steering wheel, which looked like a modified bridle.

Footnote: Whenever I drive my 3,453-pound Chevy Volt up the 851 feet hill to town, I use = 3,453 X 851 = 2.939 million ft-pounds of energy, which converts to 3.984 x 10⁶ joules. That’s 1.1 kilowatt hours, or about 6.1% of my Volt’s 18 kWh battery capacity. Theoretically, when I drive the Volt back down I get that energy back in the form of gravitational energy and regenerative braking, less some unknown inefficiency in the car's systems.

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Diatribes of Jay

18 April 2019

Big “Gas Tanks” for Electrons

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