Apple worked with a Chinese automaker for years to design a battery system for its now-canceled car project.

That’s according to Bloomberg, which reports that Apple was collaborating with BYD Co., a Shenzhen-based car company, to develop batteries using lithium iron phosphate (LFP) cells. Apple contributed engineering expertise, specifically in areas like heat management and advanced battery packs, while BYD supplied its manufacturing resources and proprietary LFP technologies.