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Wednesday, February 21, 2024

All the Things That Drain Your EV Battery

Your wipers are working overtime to clear the screen of torrential rain and headlights are set to full beam to seek out corners of the unlit country road ahead. Meanwhile, the climate system and four heated seats do their best to keep you and your family warm on a freezing winter’s night. To make matters worse, the navigation system has just corrected itself to adjust the remaining range of your electric car to below the distance to your destination. The game of arithmetic brinkmanship every EV owner plays is up. And there’s a chance you won’t make it.

But what happens if you turn those heated seats off? Maybe the stereo, too? The cabin temperature could probably be lowered a touch. And if the rain clears, perhaps that’ll help eke out a few more miles. The kids could be deprived of the rear-seat infotainment system, and the iPads probably don’t both need to be charging. Or you could go full Apollo 13, turn the heating completely off, unplug everything, slow down, and hope for the best. 

The real question here, the one nearly every EV user understandably wants to know when nearing single-digit charge levels, is: Would any of this actually make a difference? And if so, by how much? Can you really earn a few extra miles by going without warm air or switching off the radio? The answer is complicated, and there are more than a few surprises.

ContentsBattery Heating and CoolingSecondary Vehicle Systems ScoringCabin Heating and CoolingLightingAudio and InfotainmentUSB chargers (and Wipers)Brakes and SuspensionAerodynamic Drag and SpeedWeightTiresEV IdlingBattery Heating and Cooling

After the motors of the drivetrain, heating and cooling the battery pack (and the cabin) of an electric car are the biggest drains on its power reserves, says Ashley Fly, a lecturer in vehicle electrification at Loughborough University in the UK. “The energy required depends on a lot of external factors, like temperature and sunlight. Heating and cooling power consumption could range from a few hundred watts when the ambient temperature is close to optimal, 1-2 kW when the environment is very hot or cold, or even up to 5+ kW if the vehicle is starting off very cold and the battery needs to be warmed up by a resistive heater.”

Why is battery temperature so important? When you have a cold battery, specific chemical reactions set performance limits. The cell chemistry currently used in the automotive industry does not handle high currents, charging or discharging, very well at low temperatures. If exposed, a phenomenon called lithium plating will happen. This is something similar to corrosion and will cause aging and degradation of battery performance. 

Fly stresses that the best course of action is to always preheat an electric car while it is plugged into the charger. That way, grid electricity is used to heat the battery and cabin, instead of sapping energy from the car itself. 

Once up to a set temperature, an EV can keep itself warm using far less energy than when starting from cold. “Heating a Tesla Model 3 Long Range battery pack from zero to 20 degrees Celsius without a heat pump needs around 2.4 kWh of energy, or 3.4 percent of its claimed usable energy,” Fry says.

Secondary Vehicle Systems Scoring

How an EV’s various secondary systems (cabin heating, lighting, driver assistance, audio, etc.) are used plays a key role in charge levels. To delve more deeply into how these systems affect range, beyond the obvious battery heating and cooling, we need to consult the Wh/km energy consumption rate of various electric cars, as compiled by the Electric Vehicle Database.

We then need to reach for the calculator—or, in our case, speak to Pete Bishop, chief technology officer of Silver Power Systems, an electric systems design company that specializes in EV battery analytics. Bishop created a spreadsheet for WIRED that details the power usage of more than 50 components and systems found in electric cars. This could then be used to calculate the approximate range reduction, expressed in kilometres per hour driven, each system is responsible for.

The data is taken from the technical workshop manuals of manufacturers and suppliers, plus information from owners’ forums and data collected in-house by Silver Power Systems.

Instead of offering a complex set of statistics for every electric car, we have settled on an average vehicle consumption of 180 watt-hours per kilometer (Wh/km). This is close to the Polestar 2 Long Range Single Motor and Mazda MX-30 (both 176 Wh/km), the Kia EV6 Standard Range 2WD (177 Wh/km), the BMW i4 M50 (179 Wh/km), the Porsche Taycan 4S (180 Wh/km), and the Tesla Model Y Performance (181 Wh/km).

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For context, this scale is bookended by the Lightyear One (104 Wh/km) and Tesla Model 3 (151 Wh/km) and, at the other end of the spectrum, electric minivans like the Mercedes EQV 300 Long (295 Wh/km).

Cabin Heating and Cooling

“An electric vehicle’s main secondary energy use by far is to heat the cabin and battery,” says Matthias Tonn, chief program engineer for the Ford Mustang Mach-E. 

“When you compare an ICE to an EV, secondary systems become more dominant,” says Clemént Heinen, attribute lead in Polestar’s vehicle development team. "Whereas an electric car is driven by an efficient motor and battery pack, ICE cars use otherwise wasted heat generated by the engine to warm the cabin. The effects of those other elements, like the climate system, become very visible.”

Bishop’s calculations take into account a circulation fan; heating and cooling systems; heated front and rear screens; and heated mirrors, seats, and steering wheel. Heating and cooling systems are by far the largest drains of power in this category, requiring up to 3 kW and 4 kW, respectively, and robbing between 8.3 km and 11.1 km of range per hour of use.

Interestingly, heated seats are a far more efficient way to warm a car’s occupants, consuming 50 watt-hours each, taking just 560 meters of range per hour of use.

Lighting

Lighting, electric car owners will be pleased to hear, consumes very little power. Bishop’s calculations estimate that a vehicle’s entire exterior lighting system, when used in a typical manner, accounts for 48.80 watt-hours (Wh) of energy. For a vehicle with an energy consumption of 180 Wh/km, which includes EVs such as the Porsche Taycan 4S, Tesla Model Y Performance, Kia EV6 Long Range, and Volkswagen ID.4, this equates to 0.27 km/h—or just 270 meters of range per hour of driving.

Audio and Infotainment

Car infotainment displays have grown significantly in the past decade, to the point that some span the entire width of the cabin. And some cars, like the Porsche Taycan, can be bought with up to five digital displays. The latest generation of Tesla Model S and Model X cars also come equipped with powerful video game systems, boasting 10 teraflops of power, roughly equal to a PlayStation 5, which has an output of 350 watts.

All of this draws significantly more energy from the car’s battery pack than the simple music and navigation systems of just a few years ago. While a regular car stereo played loudly might reach 100 watts of power, its demands on the battery pack are tiny, with 100 watt-hours equating to approximately 0.5 km of vehicle range per hour of use.

At this point, it is worth addressing how premium sound systems with huge maximum power outputs don’t necessarily drain an EV battery more quickly than a regular stereo. According to Bishop, while it’s possible to buy cars with sound systems boasting more than 2,000 watts of peak output, such huge amounts of audio power—2 kW—in practical use have little effect on battery drain. Here it’s important to remember how peak output is often only reached for a matter of milliseconds, and it is the ability to do that, even for just a thousandth of a second, that contributes to the better sound of a more expensive audio system.

Additionally, it is useful to know that powerful sound systems make use of capacitors to regulate their electrical demands. These are trickle-charged by the vehicle and then used to give the system a quick jolt of electricity when extra power is needed—such as when reaching that headline 2,200-watt for a millisecond.

USB chargers (and Wipers)

USB ports are commonplace on most modern cars, often with a pair in the front and a further two or even three for rear passengers. We suggested earlier the possibility of removing an iPad from charge to preserve range, but there’s really no need. According to Silver Power Systems’ calculations, a regular car USB port is responsible for just 9 meters of range per hour of use. That’s about the same as using a windscreen wiper to clear a 15-minute rainstorm.

Brakes and Suspension

Secondary vehicle systems aren’t limited to those found in the cabin. The ABS, brake servo, power steering motor, and suspension compressor of many modern cars use electricity, but only a small amount. Broadly speaking, all of these combined account for about 100 watt-hours of power consumption, leading to approximately half a kilometer of range per hour.

Aerodynamic Drag and Speed

“At highway speed, by far the biggest [energy] loss is aerodynamic drag,” says Fry. “For a Tesla Model 3, which has a drag coefficient of 0.23 and 2.22 m² frontal area, 9.5 kW of power is required to overcome aerodynamic drag. If we also consider a few hundred watts for tire friction, an estimated 90 percent combined efficiency of the inverter and motor, and another few hundred watts for the essential onboard computers, we need 11 kW to cruise at 70 mph.”

What if the car were driven slightly slower? Fry says by turning the cruise control down just 2 mph, to 68 mph, “drag power would reduce by 800 watts to 8.7 kW”—in other words, an 8.4 percent savings in energy consumption for a 2.6 percent reduction in speed.

Weight

Adding passengers and luggage can affect the consumption of an electric car. But, unlike an ICE vehicle, the regenerative braking system of an EV helps undo some of the energy losses experienced when lugging around more weight. Those extra kilograms increase the mass and momentum of the vehicle, boosting the amount of energy recovered back into the battery when coasting and braking.

“The number of passengers and luggage will change the energy required to bring the vehicle up to speed," says Fry. "But it is not reflected in our simple 70 mph cruise example [outlined above], except for a small change in tire friction.”

Tires

Although they can’t be switched off to save range, like air conditioning, tires play a key role in the efficiency of an electric vehicle. Gunnlaugur Erlendsson, founder of startup tire producer ENSO, says: “If you put a poor set of tires on the car, it will dramatically affect the range.”

His thoughts are shared by Ian Coke, chief technical officer of Pirelli North America, who says that in the EV market, when you're not using the correct tire you’re more likely to notice a loss in range and an increase in noise and other characteristics, “which will be exaggerated due to the powertrain.”

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Erlendsson’s ENSO is developing a tire specifically for electric cars, which the company says could increase the range of a Renault Zoe by 11.5 percent. If true, such a gain would help offset several thousand miles of battery deterioration, theoretically giving a car that is three to five years old the range it had when new, Erlendsson claims.

On how a tire can improve range without affecting handling and performance, Erlendsson says a number of factors come into play. “It’s the combination of raw materials, chemistry, construction, and tread design that ultimately delivers these range benefits, as well as lightweighting,” he says. “If you use better raw materials, you can use a bit less of them and reduce the total weight. And we can bring these improvements without compromising other metrics.”

EV Idling

Foregoing a traditional key and starter button when driving an EV often feels like experiencing a slice of the future. But just because there’s no startup procedure in cars like the Tesla Model 3 and Polestar 2, don’t assume the car isn’t consuming any energy until you put it in Drive.

By looking at the power demands of everything from the accelerator position sensor and tilt angle sensor to the dashboard display, powertrain control model, and security system, Bishop was able to determine that the average EV consumes energy at 260 watt-hours while idling. This is a fairly small amount of power, however, and equates to approximately 1.44 km of driving range per hour.

On a similar theme, Tesla’s Camp Mode—which lets owners leave the climate control running for extended periods of time while parked and asleep in their car—consumes approximately 10 to 15 percent of a Model 3’s battery in eight hours.

Total

Add everything together and we arrive at a figure somewhere around 16 km of range consumed per hour driven. That is, an electric car’s entire suite of secondary systems is responsible for depleting the battery at a rate of approximately 16 km per hour. But, yes, your mileage will vary. 

Ambient temperature is the single biggest factor, felt most acutely when driving an EV on a cold day without having plugged it into a charger beforehand.

Ultimately, hooking up to a charger and preheating the battery and cabin before driving is the biggest single thing you can do to sustain range—that and planning your route and charging stops before setting off. 

The good news is that in all likelihood turning off the heated seats isn’t going to make any meaningful difference to range, and it’s a much more charge-friendly way of keeping warm in your EV than blasting the heater.


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