Steep climbs are the enemy of electric car drivers.

While modern electric vehicles are more than capable of powering up a hill, with ample torque, the effect on a car with limited range can be significant.

It's less of a problem with a range-extended electric vehicle like the 2014 BMW i3 REx, but a trip to the 14,130-foot summit of Mt. Evans in Colorado is still a tough task.

i3 owner Don Parsons has completed just such a trip, writing about it for Tom Moloughney's BMW i3 blog.

A few weeks earlier, Parsons had completed a trip over the Loveland Pass, cresting the continental divide at 11,990 feet. On that 128-mile trip, the car managed the distance on a couple of charges and just two miles of range-extending assistance before reaching home.

Mt. Evans is tougher. As Parsons writes, the trip from his house in Denver, at 5,280 ft above sea level, involves over 12,400 ft of ascents and nearly 3,700 ft of decents.

MORE: 2014 BMW i3 REx: Range-Extended Electric Car Drive Report, By Very First Owner

Around 52 miles into the trip, the range-extender kicked in--helping ensure the i3 had enough charge in its battery to power the electric motor up the mountain.

Don notes how the engine was audible, speeding up and slowing down depending on whether he accelerated out of switchbacks or braked into them.

At low speeds on the pass, there was enough juice--but towards the top, the thin air meant the range-extender started to struggle.

Combustion engines rely on a supply of oxygen to work, and the oxygen at 14,000 feet is pretty thin--as anyone who climbs mountains will be aware. At this kind of altitude, the range extender will be producing a great deal less than its quoted 38 horsepower, and can't provide enough energy for the car to move along at its usual pace.

In fact, it meant a maximum speed of only around 26 mph--though Parsons does stress that it wasn't a danger at this speed, as most other traffic was doing similar speeds for safety alone.

ALSO READ: First Review In: 2014 BMW i3 Range Extender Performance Limited

On the way down, brake energy regeneration helped replenish 25 percent of the car's battery--try doing that in a combustion vehicle--and also helped offset a brake issue that Parsons experienced on the way up. In all, the car used just a quarter of its gasoline capacity over the 128-mile trip.

It is not, realistically, the sort of journey BMW engineers would have envisioned when putting together their small electric car. Not least because the highest paved road in Europe is a good 3,000 feet lower than Mt. Evans.

Parsons says he's still keeping his BMW 335xi for longer-distance trips--it has greater range and all-wheel drive--and does wish the car has a REx hold mode like its European counterparts.

But what his trip shows is that the i3 REx can do almost everything a regular combustion vehicle can at its extremes, while being as clean and energy-efficient as only an electric vehicle can when used in normal, day-to-day driving.


Follow GreenCarReports on FacebookTwitter and Google+