BMW i Ventures, the automaker's venture-capital arm, led ONE's $65 million funding round last October.
The official agreement between the battery startup and BMW, which was announced on Tuesday, signals further collaboration ahead.
"We are on a path where every piece of the puzzle that is required in understanding a production program is part of what we are doing and discussing with them," Mujeeb Ijaz, ONE's CEO and founder, told Automotive News.
"This is a matter of how to channel this to market, and what we're seeing in BMW's view, what bridges we have to get across, how the product works on energy, power and cost, and can we put it into production from a manufacturing perspective with known technologies," Ijaz said.
ONE's foray with BMW comes at a time more car owners are considering EVs. One big hindrance with consumers remains range, and ONE's approach to addressing that involves marrying a base lithium iron phosphate battery with a separate one with a chemistry tailored for longer ranges.
In December, the dual-battery technology, which ONE calls Gemini, achieved 752 miles of range in real-world driving in a retrofitted Tesla Model S. The work on the BMW iX serves as another proof point.
Ijaz said ONE is scouting U.S. sites for a cell factory that could make products for both the company's base battery, called Aries, as well as the longer-range Gemini.
That is a separate endeavor than the tie-up with Piston Automotive that ONE unveiled last month, in which Piston will manufacture Aries batteries for ONE at a factory west of Detroit in Van Buren Township, Michigan. Those batteries are intended for customers in the commercial trucking sector.
"We are moving in parallel to put cells into production with LFP chemistry as well as the Gemini range-extender chemistry," Ijaz said. "It's the best way we can approach building our market, doing that in parallel."