I use the Kanger Emow Mega and Tesla Sidewinder 2 with mine and they sit perfectly flush. Not to mention they're great batteries. The Tesla is longer, but it's a 2000 mah and the Emow Mega is 1600. The Emow Mega is the shortest battery I've found at that high a mah and they're both exactly 19mm, which is exactly what the Nautilus Mini is. I think the extra .1mm you're seeing is the piece on the airflow adjustment that sticks out slightly for you to grip.