Tesla’s long-awaited mass-market electric car will begin rolling off the assembly line this week. But even as it moves ahead, the automaker is encountering challenges to its ambitious plans for growth.

On Monday, it acknowledged that it had experienced a “severe shortfall” in production of 100-kilowatt battery packs that use new technologies and are made on new assembly lines.

As a result, Tesla’s output of 25,708 cars in the second quarter barely exceeded its first-quarter production, though it was a 40 percent increase from a year ago.

Until June, the supply of battery packs was about 40 percent below demand, Tesla said, though supplies improved last month.