Towing a trailer with an electric car

Towing capacity is not a property of the hitch but an approval from the manufacturer, recorded in the vehicle documents. With no entry there is no towing capacity, however solidly a fitted hitch appears to hold. That is the first and commonest misunderstanding with electric cars, because many early models were delivered with no approval at all.

The reasons are structural. The pack sits in the floor and brings the car close to its permitted axle load before any noseweight is added. Behind the rear axle sits a structure meant to protect the pack in a rear impact, and it cannot be drilled through at will. And the drivetrain has to withstand a sustained load that does not occur in the approval cycle — towing keeps the machine and the power electronics warm for a long time, not briefly.

So if you are buying a used electric car in order to tow, read the documents first and look at the car second. A retrofitted hitch on a car without an approval is not a bargain but a defect — and, if it comes to it, an insurance problem too.

Four terms are enough. Permitted towing capacity, split between braked and unbraked trailers, says how heavy the trailer may be. Noseweight says how much vertical load the ball may carry. Permitted combination weight caps the sum of the loaded car and the loaded trailer — and it is usually the limit reached first.

The fourth term is the D-value, and it ties the others together. It describes the force acting between car and trailer and is formed from both masses: D is gravitational acceleration times the product of the two masses, divided by their sum. Hitch, ball mount and vehicle each carry a permitted D-value; the smallest of the three governs.

The arithmetic is worth doing, because the combination is often limited by something other than you expect. A car with a high towing figure may in practice be pinned to very little payload in the car itself by the combination weight. In a heavy electric car with four people and luggage aboard that is the normal case, not the exception.

Two things happen at once. Mass rises, which raises rolling resistance and the work of acceleration — both roughly proportionally. And frontal area rises, provided the trailer is taller than the tow car's rear, which a box trailer certainly is and a caravan emphatically. The second effect dominates at cruising speed, because that is where drag consumes most of the power.

Then there is the turbulence in the gap. A trailer running close behind the car disturbs the flow less than one further back with a tall face; a flat open trailer less than a closed box. Which is why no single number exists: the same car with two different trailers consumes very differently.

As an order of magnitude — and strictly no more than that — reckon on consumption roughly doubling with a tall closed trailer at motorway speed. For planning that means half the leg length and twice the stops. And because speed acts here with the cube, easing off with a trailer is not a sacrifice but the single most effective measure available.

Rapid charging points are built as parking spaces: reverse in, plug in, wait. A combination does not fit, and reversing a trailer around a busy charging site is not a solution but an extension of the problem for everybody present.

There are three workable answers. The first is a site with pull-through bays, where the combination drives straight in and straight out — they are becoming more common but are not the rule, and operators mark them on their maps. The second is a site laid out for larger vehicles anyway. The third is charging at the destination, where the trailer can be dropped before the car is plugged in.

Uncoupling in the bay itself is the worst of the options. It blocks the bay twice over, many sites forbid it in their regulations, and a trailer parked without its car at a motorway service area is an object you would rather not leave alone for half an hour. So planning with a trailer means choosing the sites in advance, not en route.

A heavy combination on a long descent is the classic recipe for hot brakes. An electric drivetrain absorbs much of that work and holds the speed with no use of the pedal. For drivers who know combinations, that is the most noticeable difference from a combustion car — more so than pulling away on a slope, which was never really a problem anyway.

The constraint is the same as in the mountains: the energy has to go somewhere. A full pack or cold cells will not take it, and then the discs are back at work. With a combination that point arrives sooner, because the mass is greater. Before a long descent with a trailer, the leave-room-in-the-pack rule counts twice.

One quirk surprises many drivers. Most light trailers brake through an overrun coupling: under deceleration the trailer pushes against the drawbar and thereby applies its own brakes. A very smooth, gradual electric deceleration triggers that mechanism more weakly than braking with the pedal does. The trailer then contributes less braking than the driver assumes — one more reason to drive a combination with distance and foresight.

The thirteen-pin connector carries more than lighting: a permanent live and a charge line supply a caravan's leisure battery and fridge. Whether a car populates those lines, and on what conditions it releases them, depends on the manufacturer — in an electric car it is the vehicle control unit that decides whether the low-voltage system will feed an extra load.

More important is trailer detection. When the car knows a trailer is connected it changes several things at once: regeneration behaviour, trailer stability assistance, disabling of the rear parking sensors, sometimes a limit on power and top speed. A hitch retrofitted without being registered in the vehicle software provides lights and nothing else.

When buying used this is testable: connect a trailer or a test adapter and see whether the car announces it. If it does not, either the coding or the approval is missing — and which of the two decides whether a workshop can sort it in an hour or not at all.

A combination is legally a different thing from a car, and the differences sit exactly where you do not look for them. Which licence category covers which combination follows from the permitted masses; what speed limit applies to combinations is set by each country; and whether a speed plate on the trailer is required, or even permitted, differs too.

Then there is equipment. Extra mirrors, high-visibility vests, chocks, spare lighting — what must be carried is written in the rules of the country you drive through, not the country the car is registered in. The binding answer comes from that country's traffic authority or motoring organisation, and it changes, which is why no list appears here.

And the trailer itself needs its own documents and, in some countries, its own plate with its own inspection. If you are hiring one, establish before departure whether its papers cover the journey — a question for the hire company, not one that can be resolved at a border.

If you are buying an electric car in order to tow, invert the usual order: approval first, everything else second. A car with no entry is out, however well it otherwise fits.