Nikola, a start-up electric truck company that had made a big splash on Wall Street, announced a broad alliance with General Motors in September that promised to give the company critical technology, financial support and credibility with investors.
But the news was quickly overtaken by claims that Nikola had exaggerated its capabilities, and the firm’s founder resigned. Investors were left to wonder whether Nikola was indeed an automotive innovator and whether G.M. had made an embarrassing mistake in associating with the start-up.
After weeks of talks, the two companies announced Monday that they would work together on a much more limited basis than what they proposed in September.
G.M. still intends to supply hydrogen fuel cells to Nikola for use in the heavy-duty trucks the start-up is developing but has yet to mass produce. But G.M. will no longer make an electric pickup truck for Nikola or take an 11 percent stake in the company — once valued at $2 billion.
“This is better than what it was because it gets us back to what is most important to both of us, which is fuel cells,” Nikola’s chief executive, Mark Russell, said in an interview.
But news of the scaled-down partnership prompted a sell-off in Nikola’s stock. The stock closed down by $7.52 a share, or 27 percent, to $20.41.
The decline lowered Nikola’s market value to less than $8 billion — about a quarter of the value it had just after its market debut in June. At one point, stock market investors valued Nikola at more than they did Ford Motor, one of the world’s largest automakers.
The new agreement gives Nikola access to fuel-cell technology that G.M. has developed but never commercialized. The cells use hydrogen to produce electricity and some experts consider them a better way to eliminate greenhouse-gas emissions from heavy-duty trucks than batteries, which weigh a lot and can take hours to recharge.
“I suspect Nikola needed a deal more than G.M. needed the deal,” said David Whiston, a Morningstar analyst. “Nikola needs fuel cells for their core business. For G.M., one supply deal isn’t a big deal either way.”
G.M. has plans to produce its own electric pickup trucks, sport-utility vehicles and cars. Earlier in November, the company said it intended to spend $27 billion on electric vehicles over the next five years.
Mr. Russell said Nikola had concluded that the pickup it was going to make with G.M., the Badger, was not part of its core focus. “It was difficult to make a business case for it. It was always something we did on the side and would take a partner to do it.”
Nikola was founded in 2014 by Trevor Milton, 39, a college dropout who previously started a security alarm business and a company that sought to convert diesel engines to run on natural gas before it ran into legal trouble and was sold. He envisioned Nikola as the trucking industry’s answer to Tesla, the maker of luxury electric cars and S.U.V.s.
Daimler, Toyota and other companies are also working on fuel-cell trucks, but Nikola’s plan has a twist in that it also intends to build a network of hydrogen fueling stations. The company’s plan to create a full-service hydrogen truck business was compelling to many investors. Nikola went public earlier this year through a merger with a special purpose acquisition company — an increasingly popular kind of venture that owns nothing but a stock listing and a pile of cash from investors hoping to acquire a promising business.
Nikola shares soared when trading began in June, helped by Mr. Milton’s decision make a battery-powered pickup truck, after similar moves by automakers like Ford, G.M., Tesla and Rivian. At the time, Nikola had no concrete plans for making the truck, and some analysts viewed it as only tangentially related to its goal of putting heavy-duty hydrogen trucks on the road.
The September announcement was widely interpreted as a seal of approval from G.M. The automaker was supposed to develop and manufacture the Badger.
But just days after the Sept. 8 partnership announcement, a small investment firm, Hindenburg Research, put out a report asserting that Nikola and Mr. Milton had greatly overstated how much technology the company had developed. The report said that the company produced a video in 2017 in which a truck was rolled down an incline to make it look as if the company had developed a working prototype.
Nikola called the report “false and defamatory,” and said that the company never claimed that the truck was moving under its own power. Later that month, Mr. Milton resigned.
The Justice Department and the Securities and Exchange Commission have begun investigating the company.
Nikola has since developed two working prototypes of its fuel-cell truck. Five test models of a second truck powered only by batteries have nearly been completed, Mr. Russell said. The battery trucks are on track to go into production by the end of 2021, with fuel-cell trucks following in 2022, he added.
Weeks after the Hindenburg report, G.M. said it had not closed its deal with Nikola as planned, and the companies continued to negotiate. With Mr. Milton gone, Nikola replaced two board members who had been close to the founder.
The deal the companies announced on Monday was described as “a nonbinding memorandum of understanding.” G.M. also said that Nikola would have to “pay upfront” for it to expand capacity to produce hydrogen fuel cells.
G.M. and Nikola also said that they would discuss Nikola’s use of a G.M. battery system called Ultium for the fully electric trucks the start-up plans to start producing next year.