Apple worked with Chinese automaker BYD for years, developing batteries that helped lay the groundwork for tech used today, according to people familiar with the situation.

As reported by Bloomberg on Wednesday, Apple has worked closely with Chinese automaker BYD. Both companies reportedly reached an agreement around 2017 to build batteries using lithium iron phosphate cells (LFP). The technology would significantly increase the autonomy of the car and was also considered safer than typical electric vehicle batteries.

Apple (NASDAQ:AAPL) and Chinese automaker BYD (OTCPK:BYDDF) worked secretly for years as part of the technology firm’s now-canceled car project, Bloomberg reported, citing people familiar with the situation.

Before the Project Titan was scrapped, Apple reportedly worked with Chinese automaker BYD on custom batteries for Apple Car for years, but ultimately abandoned that partnership too.