An electric car in the mountains: the pass, and what comes after

Lifting a mass to a height costs a calculable amount of energy: mass times gravitational acceleration times height. That is physics with no room for negotiation. One tonne raised a thousand metres is something under three kilowatt-hours of potential energy — and a car with passengers and luggage weighs several tonnes.

On top of that, the drivetrain does not deliver the energy without loss. Motor, power electronics and reduction gear run at a very good but not complete efficiency, and the pack sheds part of the energy as heat when working hard. The real demand is therefore above the calculated one, and the gap widens the steeper and faster the climb.

Meanwhile ordinary driving resistance carries on. A pass is rarely climbed quickly, so drag is small; rolling resistance stays. The gradient dominates the sum so heavily that the consumption readout shows figures never seen on the flat. That is neither a fault nor a reason for concern — it is the correct display for a process that is about to be reversed.

What went in on the way up sits at the top as potential energy in the car. Going down it is released again. A combustion car destroys it in engine braking and in the friction brakes; an electric drivetrain routes a large share of it back into the pack through the motor working as a generator.

Not all of it comes back. It travels the same chain as on the way up, only backwards: wheel, gearing, machine, power electronics, pack. Every stage takes its cut, and the pack accepts the energy only up to a limited power. As an order of magnitude, a good share survives — enough that a pass crossing costs far less overall than the climb makes you fear.

In practice: the consumption reading at the summit means nothing. What counts is the figure down in the valley on the far side. So plan an Alpine leg on the difference in altitude between start and finish, not on the sum of all the climbs — that second figure would be the arithmetic for a car that throws its energy away going down.

Regeneration needs room in the pack. If the pack is full there is nowhere for the energy to go — and the car reduces regeneration, partly or entirely. From that moment only the friction brakes retard the car, and that on a long, steep descent in a heavy vehicle.

Hence the rule that matters more than any other in the mountains: do not charge to full before a long descent. Charging in the valley below a pass means deliberately leaving room for the energy that will come back on the other side. How much room follows from the height difference and the mass — the same calculation as before, read forwards this time.

Cold produces the same effect. A cold cell must not take high charging current, or metallic lithium plates out; the battery management therefore limits regeneration until the cells are in their operating window. A descent on an early winter morning, straight after setting off, is the second case where the friction brakes take over — and the one almost nobody expects.

Fade is a friction-brake phenomenon: pads and discs get hot, friction falls, pedal travel grows. Slow a descent electrically and the brakes stay cold, fully available for the moment when they are actually needed. That is the greatest safety advantage of an electric drivetrain in the mountains, and it is rarely mentioned.

It is not unlimited. The electric path has a thermal ceiling too: machine, inverter and cooling circuit cannot absorb an extremely long, extremely steep descent indefinitely, and the pack fills up as they work. If regeneration steps back for any of those reasons, the car hands the job to the brakes — without warning, but not without signs.

The answer is the same as in any other vehicle and it is: slow down. Begin a descent at a speed you could hold without regeneration at all and there is no situation to resolve. And watching the display shows the regeneration power falling away before you feel it in the pedal.

Even in the mountains charging density follows traffic, and traffic runs along the valleys. There is rarely anything on a summit, often only a single point in a side valley, and that one can be unreachable in winter. So plan around the valley towns before and after the crossing.

Passes also close seasonally, and the alternative then runs through a tunnel or over a rail shuttle — a different length, a different altitude and a different time budget. Which route is open is published by the road authority of the country or region concerned; that is day-to-day information and has no place in a guide.

And temperature falls with altitude. A car that sets off in mild weather in the valley stands in frost at the summit — with everything that entails: more heating load, colder cells, limited regeneration. Plan the leg on the valley's figures and you have planned it too long.

Mass appears linearly in the formula for lifting work. Make the car a fifth heavier and the same climb needs a fifth more energy — with no exceptions and no driving style that alters it. Luggage, a roof box and a full car matter more here than anywhere else.

Going down, the same mass acts the other way, but not symmetrically: it returns more energy and fills the pack sooner, so the regeneration ceiling arrives earlier. A heavy combination on a long descent is precisely the case the leave-room-in-the-battery rule exists for.

And in winter, chains or winter equipment join in. What a country requires, which marking a tyre has to carry and when the obligation applies is set by each country and published through its traffic authority. For consumption it simply means more rolling resistance and a shorter leg.

A mountain run is where an electric car looks both best and worst, depending on which half of the route you look at. Take the two halves together and the picture is calm — and you have a car that spares its brakes on the way down instead of cooking them.