Last week, Tesla announced it had managed to increase the range of the Model S to 402 miles thanks to a range of vehicle improvements. However, the next big range increase looks set to come from battery technology, and Elon Musk plans to reveal all on Sept. 15.

As Reuters reports, the so-called "Battery Day" was originally planned for May, followed by a shareholder meeting on July 7. However, the coronavirus pandemic has meant both events were postponed and rescheduled (tentatively) for Sept. 15. Musk describes Battery Day as, "one of the most of exciting days in Tesla's history," but has said little about what will be revealed beyond confirming in a tweet that it will include a tour of the company's cell production system

Earlier this month, Chinese battery manufacturer Contemporary Amperex Technology Co. Ltd. (CATL) revealed that it had developed a battery with a lifespan of 16 years and capable of powering a vehicle for 1.24 million miles. The cost of production is only 10 percent higher than the batteries being used in electric vehicles today. The one missing piece of the million-mile battery? Someone to place an order.

Seeing as Tesla is working with CATL, it seems likely that Musk is preparing to reveal some variant of this million-mile battery that will be used exclusively in Tesla's vehicles. Combine new battery tech with ongoing efficiency gains in the design of vehicle components, and we may hear a surprisingly high range being announced by Musk come September.