Motor Mouth: How speed and temperature affect EV range

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Ford’s F-150 Lightning has one of the most interesting digital range infographics found in an EV. Range being the anxiety that electric vehicles bring to motoring, all EVs are, of course, chock-a-block with readouts, charts, and bar graphs plotting the state of the battery, kilometres remaining, and the amount of kilowatt-hours being consumed per 100 kilometres, this last simply the battery-powered equivalent, for those still confused by lithium-ions, of the L/100-km metric that shows how much gas-powered automobiles consume.

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But FoMoCo’s electrified pickup goes one stage further. Not only does it track how much electricity is being consumed — and, yes, it’s quite a lot — but, more importantly, it shows how it’s being consumed. As in what portion of the battery’s output is devoted to doing what. More specifically, the F-150’s “This Trip” infographic tracks Climate Use, Driving, Accessories, and Exterior Temperature. In other words, how much energy the air-conditioning system and seat heaters are using, how much energy is being consumed actually propelling the vehicle, the load imposed by accessories, and the effect of weather conditions on the battery’s performance.

I know, for instance, that for my most recent 158-kilometre scurry around the GTA, 20 per cent of the 60 kilowatt-hours the Lightning consumed was directed to keeping me warm (I do like me some seat-warmer), five per cent was for accessories, and a further ten per cent went to keeping the battery warm. That means — yes, your calculator is reading that right — only 65 per cent of the battery’s output was used to actually drive the vehicle.

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That’s right, less than two-thirds of the battery’s output was being directed to the wheels, the rest being consumed keeping me and the F-150’s electrolyte toasty. Now, a few caveat emptors. First, the F-150’s cabin is freaking huge, so it needs more heat and fan circulation to keep it warm, both mechanisms consuming precious lithium-ions. Ditto the battery, a huge 131-kilowatt-hour (usable) affair that will require more “conditioning” than, say, the smaller 58-kWh unit that propels say a bargain-basement Hyundai Ioniq5.

However, that 65 per cent calculation occurred at just five degrees C, cold certainly by Bahamian standards, but positively tropical compared with typical Februarys here in the Great White Frozen North. By comparison, Ford’s own Mustang Mach-E — the only other EV to share the Lightning’s “This Trip” infographic breakdown — uses only about 50 per cent of its battery output to drive the wheels at -15 degrees and below, the difference used up mostly in battery conditioning.

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All these numbers are backed up by a recent study by Geotab.com, “Digging deeper into how temperature and speed impact EV range.” According to its research — modelled after 350,000 trips in 500 different battery-powered sedans, as well as 2.8 million trips in 2,000 different electrified light cargo vans — “while both cold and hot temperatures impact range, colder climates have a larger impact.” Furthermore, the analysis says, 70 F (21.5 C) is the “vehicle trip efficiency sweet-spot.” Cold temperatures being more of a problem in Canada, it’s therefore pertinent that, according to Geotab, “at 5 F (-15 C), EVs drop to 54 per cent of their rated range, meaning a car that is rated for 250 miles (402 km) will only get on average 135 miles (217 km).”

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As for speed, because highway travel eliminates the benefits of brake regeneration, highway range is more dependent on aerodynamics than electric-motor efficiency or software sophistication. And according to its analysis, Geotab says that, at 68 F (19 C), optimum range is achieved by averaging just 19 miles per hour (30 kilometres an hour). That’s for a relatively slippery sedan. Vans — and presumably, equally bluff-bodied pickups and SUVs as well — eke out their best range at piffling 16 mph (26 km/h).

One interesting sideline is that, as speed increases, temperature affects range less. This has nothing to do with any technological advantage, but rather just the amount of time you’re spending in the car. Travelling 240 kilometres at the battery-powered sedan’s optimal speed of 30 km/h, for instance, takes eight hours; while the same distance at 120 km/h takes but two. Therefore, one-quarter as much energy is used heating the cabin for the same distance travelled.

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Coupled with the fact that drag is proportional to the square of velocity, speed has much greater influence than temperature the faster you drive. At 50 kilometres an hour, for instance, the difference between 19 C and 0 C would be about a 20-per-cent drop, while at 110 km/h, it’d be closer to 8 per cent. Geotab even has a handy little range calculator that models for speed and temperature simultaneously. Feed in -4 F (-15 C) and 78 mph (Range Finder’s 125 km/h testing speed) and Geotab says the average battery-powered sedan can expect to retain 47.3 per cent of its rated range.

The analysis says 70 F (21.5 C) is the ‘vehicle trip efficiency sweet-spot’ — cold temperatures being more of a problem in Canada, it’s pertinent that ‘at 5 F (-15 C), EVs drop to 54 per cent of their rated range’

That’s pretty much in keeping with Driving’s own Range Finder testing, which suggests that -20 C results in a minimum 50-per-cent range decrease at highway speed. A test of Tesla’s Model Y in last year’s deep freeze, for instance, saw just 252 kilometres at highway speeds, while a Mustang Mach-E Performance managed even less. Even Mercedes’s big-batteried EQS580 4Martic managed just 332 kilometres — versus a rated 547 km — and that was only at -6 C.

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And finally, anticipating all the Tesla owners ready with their customary “but my Model S…” retorts, the company’s research suggests there’s not much difference between brands, the best EVs retaining 60 per cent of their range at -15 C and the worst about 45 per cent. In other words, all battery-powered vehicles suffer significantly when the mercury dips dramatically. So, if you’re heading out on your first winter road trip in an EV, the data suggests that you take whatever (optimistic) range your EV boasts and divide it in half.

And allow more time for charging, ’cause that slows down, too.

Author’s note: Many EV owners will note that they don’t drive long distances on the highway (and therefore range anxiety is of little concern), the implication almost always that no one needs to worry about decreases in range caused by speed and temperature. Nonetheless, ours is a large country and we Canadians do so like to travel by car. Indeed, long is the tradition of snowbirds heading south for the winter, often, at least in my experience (mostly with the francophone variety), by automobile.

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So, it’s worth noting that an Axios reporter and her husband did the snowbird thing starting on February 7 in a Kia EV6. Their 2,400-kilometre trip to Florida from Michigan involved 12 charging stops for an average of just 200 km per charge. This compares with a rated range of close to 440 kilometres. These frequent stops were hardly voluntary, author Joann Muller noting that “We were constantly thinking about where to charge next. It occupied our minds more than where to eat or spend the night.” It’s also worth noting that, in the coldest, most northern portion of the trip, only seat-heaters were used — “no cabin heat.”

The other point often brought up in discussions such as this is that ICE-powered cars also suffer the blight of aerodynamic drag. That may be true, but gas-powered cars, unlike EVs, get better fuel economy on the highway than in the city. That’s the result of multi-speed gearboxes that keep the engine revs low at increased speeds. As well, a gas engine is usually most efficient at its torque peak, something called brake-specific fuel consumption — the amount of fuel produced per horsepower generated — maximized. Most modern engines see this maximal efficiency around 2,000 to 3,000 rpm. Married to a wide-ratio eight- or twin-speed transmission, it is why ICE-powered cars reach their maximum efficiency at greater speeds than EVs.

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Indeed, it’s worth noting that, according to Geotab, the average EV’s range is almost halved — a 45-per-cent reduction compared to its optimum at 30 km/h — when travelling at 125 kilometres an hour; while the average diesel, in our testing, is down less than 20 per cent from its maximum efficiency (usually somewhere around 80 kilometres an hour). 

One last thing: ICEs don’t waste energy heating the cabin and its occupants. That’s because the heated wasted from combustion — for which the ICE is justifiably castigated — does, in cold weather, serve a purpose. The heat generated by combustion and friction in its moving part is simply blown into the cab. Wasteful it may be, but it does mean that ICEs are not markedly less efficient in the winter.