Ever since Tesla chief executive Elon Musk offered to give away the firm's electric car patents for free, the rest of the auto industry has leapt on the technology. Among those lining up outside Musk's door? Some of the biggest names in cars, such as BMW and Nissan. But just what is in Musk's patent portfolio that has them so excited? A lot of what you need to know to actually build a Tesla, for starters.

A survey of Tesla's patents by Thomson Reuters offers a bit of insight. Thomson Reuters estimates that the electric vehicle firm has spent more than $9 million amassing a portfolio of nearly 300 U.S. patents and patent applications. Adding in Tesla's various international patents, that figure jumps to 691.

Included in the list are some pretty boring things that any car company would be expected to file for — ornamental designs for car doors, display panels and so on. But even as Tesla has applied for an increasing number of those patents over time, there's also been a substantial uptick in the number of patents relating to battery and charging technologies, according to Bob Stembridge, Thomson Reuters' lead researcher on Tesla's patents.

Stembridge grouped Tesla's 296 U.S. patents and applications into three categories, but you could probably slice it any number of ways depending on how far you drill down into the portfolio.

Here's a cooler example of what's in there: a patent for a ballistic shield guarding the battery that sits underneath the body of the car. The protective barrier is "interposed between the battery pack enclosure and the driving surface."

Here's a patent for a wind-powered generator, one that presumably can take advantage of headwinds to recharge the battery slightly while you're driving.

And the connector for the power cable you plug in to charge the car:

And the gull-wing doors on the coming Tesla Model X SUV:

Then there's all the technology behind Tesla's incredible charging stations. A year ago, Musk demoed a new addition to his charging infrastructure that could, for a fee, automatically swap out a Tesla's depleted battery in a matter of seconds and mount a new one. The entrepreneur told an audience last year that this would be a speedier alternative to having to plug the car into an outlet (which would always be free, in contrast to the battery swap).

Interestingly, Tesla hasn't filed for as many patents this year as in previous years — and likely won't keep up its former pace, Stembridge said in an interview. Maybe Tesla filed less this year in anticipation of opening up its portfolio.

Tesla has also used its patents as collateral to secure financing, Stembridge said. While it's unclear precisely how much Tesla's patents are actually worth — "valuing patents is a bit of a black art," said Stembridge — the company has handed over a good chunk of its portfolio to PNC and its loan servicing company, Midland.

What this behavior says about Tesla's financial condition isn't entirely apparent, either, said Stembridge. But given how costly it is to build and maintain a patent portfolio — and how far Tesla still has to go before electric cars become a real alternative to gasoline-powered autos, Tesla may be justified in putting its long-term future on the line in exchange for a near-term burst of help from its competitors.