Tesla Motors has been opening its Supercharger DC fast-charging stations at a dizzying pace lately, with 175 U.S. sites operating, and almost two dozen more either in construction or permitted.

Globally that total is almost 400 sites, which likely represents more than 2,000 separate outlets.

But Tesla battery swapping? Ah, that's a different story.

DON'T MISS: About That (One) Tesla Battery Swapping Station: An Update

Tesla Motors CEO Elon Musk presided over a flashy event in June 2013 that purported to show the battery of a Model S electric sport sedan being swapped out and replaced in about 90 seconds.

The company would open swapping stations, he said, giving owners on road trips a quicker way to "recharge" their cars.

Tesla Battery Swapping sign, Harris Ranch, California, March 2015 [photo: Lenn Goodside]

Tesla Battery Swapping sign, Harris Ranch, California, March 2015 [photo: Lenn Goodside]

More than a year and a half later, progress has been slow: The one "experimental" Tesla swapping site in Harris Ranch, California, is now nominally open.

But not just any Tesla driver can show up and get a fully charged battery pack.

Instead, the service is limited to a "small number" of California Tesla owners who have been personally invited by the company to test out the service.

They must make an appointment in advance to get their pack swapped, and they will pay a fee said to be roughly equivalent to the cost of a full tank of gasoline--perhaps $50?

And the swapping process isn't fully automated, unlike the one supposedly presented by Musk 21 months ago.

The company never released details on how that system worked, or answered questions on how the battery's liquid-cooling system could be disconnected and reconnected, and any lost coolant replaced, within a 90-second swap.

But the slow pace of the single swap station differs markedly from the fast-growing Supercharger network, where any Tesla owner can drive up, charge for 20 to 40 minutes, and drive away with a battery recharged to about 80 percent of capacity.

Oh, and the Supercharger network is free to all current Tesla owners.

ALSO SEE: Tesla Battery-Swap Pilot Program To Launch Next Week In California (Dec 2014)

It's long been assumed that Tesla opened its sole experimental swapping station solely to fulfill the promise that caused the powerful California Air Resources Board (CARB) to grant the Model S extra zero-emission vehicle credits for "fast refueling" capability.

That translates to providing the car's full range in 10 minutes or less--which gasoline, diesel, natural gas, and hydrogen fuel-cell cars can achieve, but even a Supercharging Tesla cannot.

A Tuesday article in The Los Angeles Times lays out the whole story, which has been previously covered in numerous articles on Green Car Reports and other outlets.

Photo of Tesla Model S battery swapping versus 'fastest gas pump in LA' tweeted by Elon Musk

Photo of Tesla Model S battery swapping versus 'fastest gas pump in LA' tweeted by Elon Musk

The LA Times piece is worth reading for its concise summary of the regulatory requirements and benefits that led Tesla to set up this minimal swapping capability to comply with the letter of the law.

Tesla's main business is building and selling battery-electric cars; it sold roughly 33,000 last year, and hopes to be selling about 2,000 per week globally by the end of this year.

But it earned $66 million in the fourth quarter of last year alone from the sale of zero-emission vehicle credits to other carmakers.

That's the equivalent of an additional 660 Model S sales at $100,000 apiece.

MORE: Why Tesla Launched Battery Swapping (Jul 2013)

Musk has said that Tesla isn't relying on revenue from sales of ZEV credits (and certain other allowances) going forward.

But it has to be nice for the company and its financial staff to get it while it's there.

And the cost of a single, limited-access swapping station and a mechanic or two is a small price to pay for what's now totaled nine figures of income on the company's books.


Follow GreenCarReports on Facebook, Twitter, and Google+.