TOKYO -- Panasonic is beginning to reassess its relationship with U.S. automaker Tesla and its freewheeling CEO, Elon Musk.

The U.S. Securities and Exchange Commission filed charges against Musk on Thursday, alleging he made "false and misleading statements" when he tweeted in August about plans to take his carmaker private.

Tesla's stock price fell by almost 12% in after-hours trading in New York on Thursday. Panasonic shares fell by close to 1% at one point during Friday morning trading on the Tokyo Stock Exchange.

Panasonic's management is uneasy about having its vital car battery business hinge on a man widely regarded as a visionary, but who has also made a series of head-scratching moves of late. Panasonic has invested heavily in the business, which it views as the main driver of its future growth.

The Japanese company's perceptions of Musk started changing when the entrepreneur abruptly tweeted that he had funding for the privatization plan on Aug. 7.

About two weeks later, facing strong opposition from shareholders, Musk withdrew the plan to go private.

A senior Panasonic executive described Musk's announcement, which could have triggered a class-action lawsuit if the plan had been pursued, as "unimaginable."

In early September, the Panasonic executive was stunned to watch the CEO, appearing on a webcast, taking a deep drag on what was apparently marijuana.

In addition to supplying batteries manufactured in Japan to Tesla, Panasonic is also operating a huge battery manufacturing plant in the U.S. state of Nevada jointly with Musk's company. The plant started full-scale operation in 2017 and is expected to ramp up its annual production capacity by 30%. So by the end of March 2019, it will be able to make 35 gigawatt-hours of lithium-ion batteries.

Panasonic is shelling out some 200 billion yen ($1.78 billion) of the estimated total investment of 500 billion yen in the plant.

Sales to businesses account for about 70% of Panasonic's overall group revenue of around 8 trillion yen. Car batteries are the mainstay of its business-to-business operations.

Although it was dethroned from the top spot on the list of the world's leading makers of car batteries by Chinese rivals in 2017, Panasonic is still the No. 1 supplier for passenger vehicles, according to an executive in charge of the business. Japanese, European and American automakers make about 70 models powered by Panasonic batteries.

Tesla is by far the largest customer for Panasonic's car battery business, with each of its electric vehicles using thousands of cylindrical battery cells.

Less than two months before Musk's buyout bombshell, Panasonic felt reassured about its partnership with Tesla. The partner had pledged to raise the production rate of its new, more affordable Model 3 to 5,000 units per week by the end of June.

In mid-June, Panasonic President Kazuhiro Tsuga flew to the U.S. in a private jet and visited the plant in Nevada, known as the Gigafactory. Although many observers were skeptical about the Model 3 production hike, Musk asked Tsuga to make as many batteries as possible to meet strong demand.

Panasonic was considering new plans to boost its battery production capacity to supply Tesla, including expansion of the Gigafactory and a new plant in China, when Musk tweeted the privatization proposal. Then, Tesla's chief accounting officer suddenly resigned in early September after only a month on the job -- one of many recent departures of senior Tesla executives.

All these developments have fueled concerns about Musk's managerial fitness.

Tesla is still losing money and is suffering from negative free cash flow. Panasonic will decide on future investments in its business with Tesla after assessing whether the Model 3 will generate enough profits to allow the company to expand its operations, according to a senior executive.

The success of the Model 3 is crucial for Tesla, if it is to remain the leader in the electric vehicle race. Rivals such as Mercedes-Benz are beginning to bring multiple models into the luxury electric segment.

Panasonic, for its part, is on a solid financial footing. The company's own capital ratio is a healthy 30% or so, and it boasts over 200 billion yen in net profit every year.

The future of its car battery business seems to be bright, given tremendous potential demand as the auto industry goes electric. But Panasonic has spent an amount roughly equivalent to its annual net profit on its joint battery plant with Tesla. If the U.S. partner's business runs into trouble, that would spell trouble for Panasonic as well.

Panasonic has rebounded from the losses of over 750 billion yen it posted for each of the two consecutive years to the end of March 2013, due mainly to a slump in its TV business. The company made a huge but misguided bet on plasma display technology in the early 2000s, investing a total of 600 billion yen to build plants to manufacture plasma TVs at home and abroad.

The market for plasma TVs never took off, as consumers opted for LCD sets instead.

Tsuga, who took the helm in 2012, pushed through a radical overhaul to shift the group's strategic focus to businesses catering to corporate clients. Panasonic's cash reserves surged to 1.3 trillion yen at the end of March 2015, up from about 500 billion yen two years earlier.

The Japanese company struck a deal on the joint battery manufacturing venture with Tesla in 2014 as it regained financial health. Its partnership with Musk's electric vehicle scheme is symbolic of Tsuga's structural reform, and also of his vision for the company's future.

Tsuga, who has reinvented Panasonic's business portfolio without resistance from the old guard, may have seen a kindred spirit in Musk, a radically innovative entrepreneur pushing to revolutionize the auto industry.

But the eccentric Tesla CEO has also become a risk factor. The Justice Department, too, has reportedly launched an investigation into Musk's tweet about delisting Tesla.

Tesla's performance, and Musk's behavior, through the rest of the year will determine the future of the partnership, a senior Panasonic executive said.