If your car runs out of gas, you can call roadside assistance and get a tank of gas delivered to you. But what if you have an electric car? Most of the time you’re gonna be plumb outta luck.

But a young startup out of Germany has a new idea for helping out electric cars that have run out of zip and more broadly wants to bring battery power to cars, instead of bringing cars to outlets.

Nomadic Power, the 2½-year-old startup, has developed a mobile battery that can charge an electric car on demand within about 20 minutes or so. When needed, a vehicle would arrive towing a trailer full of charged lithium ion batteries and swiftly charge up the depleted car.

A rendering of Nomadic Power's mobile battery trailer.
A rendering of Nomadic Power’s mobile battery trailer. Rendering courtesy of Nomadic Power.

Nomadic Power CEO Manfred Baumgärtner tells Fortune he sees the opportunity for mobile electric car charging infrastructure a little like telecom was a decade ago: It’s going mobile. Public wired charging infrastructure can be somewhat expensive to build out, so he thinks there will be a big market for wireless charging services in the future.

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Roadside assistance is just one of the ways that mobile charging could be used, says Baumgärtner. Nomadic Power also envisions that companies with large fleets or many employees with electric cars—picture a Google GOOG or an Apple AAPL —could be interested in paying for a handful of mobile batteries in their parking lots during hours when their permanent electric car chargers are in particular high demand.

Nomadic Power CEO Manfred Baumgärtner. Photo by Ferdinando Iannone.

Another scenario, which Nomadic Power isn’t actively pursuing right now, is that electric vehicle owners could rent the mobile battery and tow it behind them on long trips as a sort of external range extender. A trailer full of 85 kilowatt hours’ worth of batteries could double the distance of one of Tesla’s TSLA cars.

Baumgärtner pictures that roadside assistance customers would be willing to pay about the same amount a tow would cost in Germany, or between 250 and 300 euros, for mobile electric car charging. Roadside assistance companies could work with the startup on various monthly pricing models. For on-demand charging in a company parking lot, Nomadic Power might charge the company, or the individual, the same as it costs to fill up a tank of gas.

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Right now, though, the idea of mobile charging is more of an idea than a sustainable business. Nomadic Power received a grant of 2 million euros from the European Commission to build six of its mobile chargers and test out the idea this year.

The company also opened an office in Palo Alto because it sees California as one of the best markets for electric car charging services. Of course, it doesn’t hurt to be near those venture capitalists on Sand Hill Road, which the company is trying to convince to invest in their first funding round.

The more electric cars on the roads, the more viable mobile charging could be. Tesla has sold 100,000 electric cars to date, and Nissan nsany has sold about 300,000. According to some stats, there are about a million electric cars on the roads across the globe. That’s a small drop in the bucket compared with the number of traditional cars on the roads, and sold every year.