Yeah, I read some comment that regulating replaceable batteries would be difficult, and something about following voluntary something or other. Here's the quote:
FDA agrees that it is not necessary to regulate batteries that are not intended or reasonably expected to be used with a tobacco product under its tobacco product authorities. However, it is important that batteries that are co-packaged with other parts of an ENDS (e.g., cartridges and tanks) or otherwise intended or reasonably expected to be used with ENDS are components subject to FDA's tobacco product authorities. FDA remains concerned about reports of exploding e-cigarette batteries and finds that regulating them can help address these problems. Toward that end, elsewhere in this issue of the Federal Register, FDA has made available draft guidance, which when final will describe FDA's current thinking regarding some appropriate means of addressing the premarket authorization requirements for newly deemed ENDS products, including compliance with existing voluntary standards for ENDS batteries.