How much EV range do you regain going downhill? We put a Cadillac Optiq to the test

Open this photo in gallery:

What goes up must come down, and electric vehicles are no exception. The question is, if they consume extra energy to climb a hill or even a mountain, how much do they regain on the way back down?

“One nice thing about mountain driving is that after you climb the mountain, you go down the other side and the car recaptures that energy,” Chris Luth, the Tesla-owning director of the car-rental service AutoSlash, told The New York Times while extolling the advantages of electric vehicle energy regeneration.

But does it really recapture all the extra power lost, minus the distance travelled?

When you lift your foot from the throttle of an EV, or press the brake pedal, the polarity of the electric motor is automatically reversed. The momentum of the wheels rolling on the ground spins the motor and turns it into a generator, so instead of consuming electricity, the EV produces it. The captured electricity – kinetic energy – is fed into the main battery for storage, which extends the driving range of the vehicle. On a downhill grade, gravity will help pull the EV along and the steeper the grade, the more the pull and the greater the creation of kinetic energy for future use.

I drove a fully electric Cadillac Optiq to the summit of the Bombi Pass outside Castlegar, B.C., to find out just how effective this can be. The Bombi Pass is about 740 metres above the town, and the drive to get there is about 15 kilometres, at grades of about 6 and 7 per cent, along British Columbia Highway 3. I drove with my friend Neil Horner, who owns a couple of EVs and understands the physics of electricity and energy regeneration much better than I do.

“The rule of thumb is for every 1,000 metres that you rise, you lose 50 kilometres of range [on top of the distance driven],” he told me. We’d driven to Castlegar from Calgary along the Crowsnest Pass route near the U.S. border, where the highway often seems to only rise or fall, “and I think we’re losing a lot more than that,” he said.

In correlation, perhaps we’d gain 50 kilometres of range coming back down the mountain, or close to it, at least.

We drove the Optiq from the edge of town to the summit of the pass, and our total estimated range for the 15-kilometre journey dropped by 70 kilometres: that’s 15 kilometres to drive the distance and an extra 55 to climb the 740-metre ascent. This was thirstier consumption than Horner’s rule of thumb because we only climbed about three-quarters of the 1,000 metres.

Then we turned around and drove at the same speed back down the mountain, on the same road, to the point where we’d begun the test. The car would probably not have rolled on its own to the bottom, but I needed only a light foot on the throttle. Our total estimated range seen at the summit did increase, but the regeneration only produced an additional 15 kilometres of “free” power.

(To explain the math: before we began the ascent, the Optiq’s computer told us we could drive 314 kilometres before running out of battery power. After travelling uphill for 15 kilometres, the computer said we had a remaining driving range of 244 kilometres. When we returned to the bottom after another 15 kilometres, our remaining driving range was 259 kilometres.)

Driving up cost us an extra 55 kilometres of range, while driving down created only an extra 15 kilometres – meaning the regenerative braking recovered about 27 per cent of the extra energy spent on the climb.

Open this photo in gallery:

I asked Cadillac why the energy regenerated going downhill did not come closer to the extra energy consumed going up.

“Cadillac describes regenerative braking as a way to ‘conserve and capture braking energy for added range,’ not as a way to fully offset the distance travelled or a climb,” wrote spokesperson Aliyah Menezes in an e-mail, quoting a response from General Motors’ engineers.

Many additional factors affect the regeneration performance of the car, said the engineers, including aerodynamic drag (which increases exponentially with speed), rolling resistance of the tires, drivetrain and battery losses, the state of charge of the battery and the battery temperature and the use of climate control and other accessories.

(For the record, the Optiq was fitted with Michelin X-Ice winter tires, which would have created some extra friction. There are many low-rolling-resistance all-season tires available for EVs that would have helped improve the regeneration process.)

“During braking or on the way downhill, regen can only recover part of the gravitational potential energy,” wrote Menezes. “It cannot recover the losses due to the above non-recoverable variables.”

This all seems reasonable. If the road’s gradient had been steeper, the component of gravity pulling the vehicle – the down-slope force – would have been greater and probably more kinetic power would have been created, but then we’d have used even more energy on the climb.

However, it’s still a better deal than cruising downhill with a gasoline-powered engine, where there is no storage available for the potential kinetic energy that’s created. And if you ride your brakes to control the speed, you wear out the brake pads. With gas prices as high as they are, I’ll take whatever I can get.

Open this photo in gallery:

The writer was a guest of the automaker. Content was not subject to approval.