BYD overtook Tesla as the world’s top seller of electric vehicles (EV) at the end of last year, crowning an extraordinary rise for the Chinese carmaker.

It delivered more fully electric cars than Tesla for the first time in the three-month period to December 31, and slashed the sales lead held by Elon Musk’s company over the year as a whole.

So how did a little-known Chinese battery maker grow so quickly to become Tesla’s biggest rival?