One of the biggest obstacles to mass electric vehicle (EV) adoption is the difficulty of charging up the car. Sure, you can plug it in at home overnight, but if you're low on juice while on the road, you'll quickly find the public charging infrastructure is woefully underdeveloped, even in places like San Francisco and Silicon Valley.

That's why everyone from the automakers (like BMW and Volkswagen) to the electric company is building networks of chargers across the country. After all, it's a lot easier to convince people to pay a premium for an electric car when they can, you know, use it.

One of those companies is Volta, a San Francisco-based startup that approaches the game with a different business model. It doesn't provide free charging as a perk of buying an EV (like Tesla), or charge the customer for the electricity she takes (like Chargepoint). Instead, it installs chargers at places like upscale malls and grocery stores—at no cost—then offers their use, for free, to any driver who wants to plug in. It's all good news for property owners, who want to lure in customers with the money for things like electric cars, and those customers get to power up gratis.

The catch? The chargers, which are located in primo locations for walking traffic, have ads on them. Volta calls it a "sponsored service" model, with advertising paying for electric vehicle chargers. In other words, they're billboards.

The company has 110 chargers in San Francisco, Los Angeles, San Diego, Phoenix, and Honolulu, and hopes to expand the network, both within those cities and beyond them, to 400 units, using $12.5 million in equity and debt financing.

"We're giving away a service that meaningfully shifts a community forward," says Volta CEO Scott Mercer. For advertisers, "it can have a positive impact on the brand."

The association with EV charging provides an interesting twist on the traditional billboard, and the opportunity for puns. The Monterey Bay Aquarium used an ad urging passers-by to "Go with the current." (Get it?)

Volta isn't trying to follow the "put chargers everywhere" model used by companies like ChargePoint, and Mercer believes those competitors are complimentary to his model. Even if no one ever charges at Volta's chargers, it still gets paid by the advertisers. The company looks to maximize viewership of the ads on the chargers by choosing good locations, but Mercer says it is a "frictionless pitch" to property owners because of the near-zero cost to them.

Like many in the electric car business, Mercer is enthusiastic about the future of electric cars and believes Volta is doing its part to stimulate their acceptance as a mode of transportation. It's definitely not a charitable exercise, however—he says all the currently-installed chargers are revenue-positive and that the $12.5 million funding raise is solely focused on growth. More power to them.