BASIC IDEA
As we know from several surveys, the average car user drives 50–100 kilometers daily and only rarely embarks on longer trips. Therefore, creating an electric vehicle (EV) suitable for all purposes is contradictory. If a high-capacity battery (capable of storing enough energy for long trips) is installed in the car, it is expensive, heavy, and unnecessarily burdens the chassis, tires, and motors in the majority of use cases. On the other hand, with a small battery, more frequent charging stops are required during long highway trips (e.g., vacations), which is less convenient. This problem has long been recognized in EV circles. For example, Tesla attempts to improve highway range with the Cybertruck by offering a large and heavy (approximately 50 kWh capacity) additional battery box (Range-Extender) that can be mounted on the truck bed.
One might think that this dilemma will automatically be resolved when—as many expect in the near future—batteries with higher energy density are developed that are also cheaper, lighter, and safer. Numerous news outlets regard solid-state batteries as the “Holy Grail.” However, those who delve deeper into the scientific literature on battery research generally conclude that the parameters of batteries that need improvement often move in opposite directions. For instance, if a battery’s energy density is improved, its charging performance or lifespan often deteriorates, while its manufacturability becomes significantly more complex (and expensive). If, for example, the cost of a battery is reduced, its energy density and charging performance typically decline. I have been following this field for years, and despite almost weekly articles with headlines suggesting a “major breakthrough,” the actual practical changes are quite small. The NCM chemistry, known for at least 10 years, still offers the highest energy density. The LFP chemistry, heralded as a major breakthrough three years ago, is actually only cheaper but has lower energy density. The latest much-discussed sodium (salt)-based batteries will be even cheaper but also have even lower energy density. Experimental silicon nanostructure batteries show significant increases in energy density but are extremely expensive to produce and have a very limited lifespan. Therefore, I believe that the problem outlined in the introduction will persist in the future, and it is worth considering a long-term solution.
The idea presented here offers a solution to the problem created by the dilemma between large and small batteries.
[1] NACS
[2] Gooseneck
[3] Side camera
[4] Rear lights
[5] E-Motor-Generator
[6] Wheel
IMPLEMENTATION OF THE RANGE-EXTENDER BATTERY AS A TRAILER
A Range-Extender battery could also be implemented as a standalone trailer. A medium-sized trailer could house a battery with significant capacity. Additional storage space could be created on top. For good aerodynamics, it would be advisable to equip the trailer’s loading surface with a streamlined, openable/closable lid, similar to roof boxes. This trailer could be rented, swapped, or returned at larger charging stations. However, those who frequently travel long distances could also purchase their own Range-Extender Trailer (RET). Reserving the RET, paying the rental fee, and covering the cost of consumed electricity could be handled through already established apps.
Since the goal is to extend highway range, the RET would be equipped with its own electric motors, so it would not burden the towing vehicle and could even supply power to the towing EV from its own battery. The trailer would not be connected conventionally but via a newly developed “gooseneck” coupling for a fixed connection to the towing vehicle. Tesla’s proprietary connector standard (NACS) seems suitable for energy transfer and necessary communication. Due to the weight of the batteries, the trailer would be heavy, so a high-quality suspension and appropriately programmed control electronics could ensure stability at high speeds (up to 130 km/h) without swaying, bouncing, or resonating. Actively driven and braked wheels could eliminate the occasional “jackknifing” phenomenon seen with trailers. The usability of the idea depends heavily on authorities verifying and approving the higher permitted speed limit for this trailer.
The trailer would be equipped with rear and side cameras, allowing the driver to see what is behind and beside the trailer during towing. The trailer’s two wheels would each have separate motors, enabling it to maneuver in any direction (or more precisely, according to the towing vehicle’s steering angle) when reversing (e.g., by driving one wheel forward and the other backward). This greatly simplifies reversing with such a trailer, as the direction and angle of the turn are not determined by the relative position of the towing vehicle and trailer.
This trailer would also be an excellent foundation for creating other active (minimally burdening the towing vehicle) camper trailers. A camper with a large internal battery would not only be advantageous during towing but could also provide power during “boondocking” (off-grid parking) without relying on diesel generators. If the camper’s larger surfaces were covered with solar panels, the batteries could be recharged.
BUSINESS MODEL
RET stations (Range-Extender Trailer stations) could be operated alongside current charging stations on busy routes. Here, trailers could be rented and returned after use. While the RETs wait for the next renter, they could be charged with solar panels or the existing power grid at the charging stations. Since the RETs have significant battery capacity, they could provide additional power to regular chargers if needed. Thus, they could balance the load on the power grid for the charging station operator, requiring less reserved capacity (and thus reducing costs).
For example, the market price of a 150 kWh LFP battery is about 10 000 EUR. A simple trailer costs 2000 EUR. Such an active RET with two motors and sensors would naturally be more expensive (e.g., 5000 EUR). The cost of tires for 200,000 km is about 1000 EUR. Thus, the total operating cost for the rental provider over 200,000 kilometers is approximately 20 000 EUR.
If the operator expects the rental fee to recover the full cost of the RET over this distance, a rental fee of about 12.5 EUR per 100 km can be calculated. The price of the 150 kWh of electricity carried could, for example, be sold to the user at 0.5 EUR/kWh, resulting in electricity costs of 75 EUR for a full charge. (The significant difference between the purchase and sale price of electricity greatly improves the payback period.)
EXAMPLE USE CASE
Assume that the infrastructure for RET rentals is at least as developed as the charging network for electric vehicles in Western Europe. In this future environment, a family plans a one-week vacation 1,300 km away with their Tesla Model Y. The vehicle’s 75 kWh battery limits the length of highway stages. Even in the best case (with a consumption of 25 kWh/100 km), stages of 250–270 km are possible, resulting in 4–5 charging stops.
The driver reserves a 150 kWh RET online, with pickup at a charging station along the route (<20 km away). On the departure day, they attach the RET at the (confirmed during booking) charging station, charge the vehicle to 100% if necessary, and begin the trip. The trailer is flat and streamlined but still slightly increases overall consumption, so we calculate with 30 kWh/100 km highway consumption. With the total capacity of 75 kWh + 150 kWh, a range of 700–750 km is achieved. (Hopefully, the family can handle long pee breaks well 😊.)
A longer stop is needed to charge the car and swap (or charge) the RET. With current technology, this takes about 40 minutes, during which the family can have lunch. (Time is short, so it would be wise to order lunch online before arriving at the charging station.) The EV, RET, and passengers are “refueled,” and the journey can continue.
Approaching the destination (and the RET drop-off point), the driver transfers the remaining energy from the RET to the vehicle while still en route. Thus, dropping off the RET is only a brief administrative process.
The family vacations at a location with destination chargers, so they can use their familiar vehicle for smaller trips during the vacation. The return trip begins similarly with renting and attaching a RET.
The total cost for the 1,200 km outbound trip is 312.5 EUR. This is roughly equivalent to the cost of ~16 L/100 km gasoline consumption. This is double the highway consumption of a modern gasoline vehicle. However, maintaining a gasoline vehicle incurs significantly higher costs in daily use compared to an electric vehicle charged at home.
An alternative for the family would be to rent a gasoline vehicle for this long trip. Suppose the daily rental fee is about 50 EUR, and the consumption is 8 L/100 km. Excluding local trips, the total cost for the vacation would be 737.5 EUR (3337.5 EUR fuel + 400 EUR rental fee), which is still more than the 625 EUR total cost for EV + RET.
GOOSENECK
[7] 800V actively cooled cable
[8] Rubberized outer casing
[9] Gooseneck vertebra elements
[10] Fixing pin
The gooseneck is a flexible rod built into the front of the trailer, rigidly (but detachably) connected to the towing vehicle. It is a mechanical solution resembling a spine, protecting the high-voltage cable running through its center. The gooseneck consists of vertebra-like metal elements and is covered by a flexible outer casing to protect it from contamination.
It enables not only the flow of energy (with a maximum power of 40 kW) but also bidirectional information transfer. It can transmit video signals from the trailer’s cameras and the status of the RET to the towing vehicle. It can also receive commands from the towing vehicle regarding the drive or braking of individual wheels and the control of the rear lights.
The gooseneck allows movements with three degrees of freedom:
- Vertical: approximately ±40 degrees of movement.
- Horizontal: approximately ±70 degrees of movement.
- Torsion: approximately ±50 degrees.
The front end of the gooseneck connects to a detachable connector located at the rear of the electric vehicle.
- Mechanically: The connection between the towing vehicle and the gooseneck is rigid. (Unlike the ball joint of a conventional tow hitch, it does not allow movement.)
- Electronically: The towing vehicle must be equipped with a connector very similar to the NACS standard.
[11] Flexible rubber bellows
[12] BMS + control electronics
[13] “Roof box” lid
[14] 100–150 kWh battery
[15] Rear light
[16] Charging cover
[17] Cooling fin
STRUCTURE OF THE RET:
- The area marked in green is the space for the battery.
- The light blue area (under the openable/closable roof box lid) is the storage space where the renter/owner can freely pack.
- The RET is equipped with an active battery management system and control electronics, which, when connected to the towing vehicle’s electronics, ensure stable driving and steering.
- At the bottom (marked in turquoise), there are cooling fins necessary for the thermal stability of the internal liquid cooling system. This surface allows the trailer to dissipate heat or absorb it from the environment.
- The RET has two active wheels (capable of both driving and braking), equipped with swing arms and damping.
When the charging cover is opened, the charging connectors become accessible (CCS in Europe, NACS in the USA). It would be practical to produce an RET version that supports bidirectional charging. Such a trailer could supply a household with energy for up to a week, enabling power supply for buildings not connected to the electrical grid.