A headline making the rounds this week promises that supercapacitors could let electric cars charge in 15 seconds. The number is real. The application is not the one the framing implies. That 15-second figure comes from electric transit buses in Switzerland that sip small amounts of energy at fixed stops, not from any passenger EV that exists or is in credible development. I have spent years watching fast-charging claims get stretched from the narrow case where they hold to the broad case where they collapse, and this is a textbook example.

The physics is not subtle. Supercapacitors charge and discharge in seconds because they store energy on the surface of an electrode rather than inside it. That same property is why they hold a small fraction of the energy a lithium-ion pack holds. You can refill a thimble in 15 seconds. Refilling a bathtub is a different problem, and a car battery is the bathtub.

The energy density gap is the whole story

Commercial supercapacitors store roughly 5 to 10 watt-hours per kilogram. Modern lithium-ion EV cells store somewhere between 150 and 265 watt-hours per kilogram. That is a 20-to-50-fold difference in how much driving range you can pack into a given mass, and it is the reason the 15-second pitch falls apart the moment you apply it to a car that needs to travel 300 miles between stops.

Engineers separate two metrics that casual coverage tends to blur. Power density is how fast you can move energy in and out. Energy density is how much you can store. Supercapacitors win the first by a wide margin and lose the second by an even wider one. Batteries sit in the middle on both, which is exactly why they remain the default for vehicles that need to store a day of driving rather than a few blocks of it. A supercapacitor bus carries enough charge for tens of kilometers, not hundreds, which is fine when a charging bar is waiting at the next stop and useless as a model for a road trip.

The Swiss buses work because the route does the heavy lifting

Flash-charging transit buses recover energy in short bursts at stops along a fixed loop, which works precisely because the route is short, repetitive, and lined with chargers. The system most often cited is ABB’s TOSA, demonstrated in Geneva, where a roof-mounted arm connects to an overhead contact for a brief top-up while passengers board. The vehicle never needs to hold much energy at once because the next refill is always seconds away.

That is the trick the viral framing leaves out. A bus on a 15-stop circuit can run all day on small, frequent top-ups. A passenger car driving from one city to the next has no such luxury of a charger at every corner, so it has to store the whole trip’s worth of energy in the vehicle. Supercapacitors cannot do that at any weight or price that would fit in a car. The buses are a genuine success story for the technology. They are not a preview of your driveway.

Real fast charging is happening, and it is coming from batteries

The frustrating part of the supercapacitor hype is that it distracts from genuine charging progress already shipping on real cars. BYD launched its Super e-Platform in April 2025 with the Han L and Tang L, running a 1,000-volt architecture at a peak of 1,000 kW (1 megawatt) and claiming roughly 400 km of range added in five minutes. We covered that rollout in BYD’s Super E-Platform Redefines EV Charging, and the figures, while still awaiting independent verification, point at the same convenience the supercapacitor story promises, achieved with a battery you can actually drive home.

Tesla is moving the same direction at a lower ceiling. Its V4 Superchargers deliver up to 500 kW and add roughly 168 to 200 miles in 15 minutes, and the company opened that network to other brands in China, as we reported in Tesla’s V4 Superchargers Launch in China. None of this is 15 seconds. All of it is real, durable, and sitting under cars on sale today. The contrast matters: a five-minute battery charge that holds 400 km beats a 15-second supercapacitor charge that holds 40.

Supercapacitors have real automotive jobs, just not this one

Dismissing the technology entirely would be its own error, because supercapacitors already do useful work in cars where bursts of power matter more than total storage. Lamborghini built a 48-volt supercapacitor system into the Sián to feed its hybrid motor during acceleration. Mazda’s i-ELOOP system used a supercapacitor to capture braking energy and run cabin electronics. The pattern holds across these cases: short, intense power demands that a capacitor handles better than a battery, paired with a battery that handles the storage.

That pairing is where the original reporting actually landed, and it is worth separating from the headline built on top of it. Drexel materials scientist Yury Gogotsi, quoted in the Popular Mechanics piece this coverage traces back to, made the modest version of the claim: there is no single best way to store energy, and supercapacitors and batteries complement each other rather than one replacing the other. That is correct. It is also a long way from your car charging in 15 seconds.

EVXL’s Take

This is the same pattern I flagged when BYD’s Denza Z9GT claimed a 1,036 km range on the CLTC cycle: a real number from a narrow test condition gets repackaged as a universal promise, and the caveat that makes it honest stays buried three paragraphs down. The supercapacitor version is more excusable because the underlying science is genuinely interesting, but the result is the same kind of reader walking away believing something that is not true.

I have been writing about battery chemistry and charging claims long enough to have a simple filter for this stuff. When a storage breakthrough promises both faster charging and more range at once, check whether anyone is selling it. Supercapacitors have been around for decades, the Geneva buses have run since the 2010s, and there is still no passenger EV using them as a primary store. That absence is the answer.

My prediction: no production passenger EV will use supercapacitors as its primary energy storage by the end of 2028. The real charging race will keep being won by high-voltage battery architectures like BYD’s megawatt platform, which will reach a 10-to-80-percent charge under 10 minutes on multiple production models before any supercapacitor car gets close to highway range. The thimble is fast. The bathtub is what you actually drive.

Sources: Inc., Chemistry World.

EVXL uses automated tools to support research and source retrieval. All reporting and editorial perspectives are by Haye Kesteloo.