Minutes after stepping off his private jet in Braunschweig, northern Germany, at the start of this month, Elon Musk climbed behind the wheel of a brand new make-or-break electric car: a Volkswagen ID.3. The often abrasive Tesla chief managed to summon up some polite if muted, approval for the vehicle as he drove it up and down the airstrip in the drizzling rain. “Fora non-sporty car, it’s pretty good,” Mr. Musk ventured to VW chief executive Herbert Diess, who sat in the passenger seat.

If there was a trace of condescension in the words, it was hardly surprising. It is eight years since the launch of Tesla’s Model S set a new standard for electric cars, giving the US company a widely acknowledged leader in the technology. Repeated predictions since then that an onslaught of serious electric competition from traditional carmakers was around the corner have, so far, proven unfounded.

VW’s ID. 3, however, could be a sign that things are changing. It is part of a wave of new vehicles that are finally expected to come closer to matching the efficiency of Tesla’s technology.

All of this has guaranteed maximum attention this week for an event where Mr. Musk is expected to reveal the latest advances in Tesla’s battery technology. With customary hyperbole, the master showman of the auto world has promised it will be “one of th e most exciting days in Tesla’s history”. Amid a powerful rally in Tesla’s shares this year, the boasting has been like

throwing gasoline on a raging fire, raising the hopes of Tesla bulls and turning “battery day”, as Mr. Musk has dubbed it, into a hotly-anticipated moment in the company’s calendar. For his weary critics, on the other hand, it is shaping to be the latest example of Mr Musk promising tech breakthroughs long before they arrive.

“One thing is sure: Elon will announce something big which is nowhere near ready for prime time,” says Bob Lutz, a former top executive at America’s three biggest automakers. Beneath the hype, though, the event could provide important clues about whether Tesla can maintain the technology edge that has made it the auto industry’s most envied –  and emulated – innovator.