So, what happens after #7?
It gets a little confusing.
#8 is for Lead Acid Batteries
#9 is for Alkaline Batteries. But #9 is also the Plastic Resin Code for ABS, those black pipes used for drainage. ABS is the abbreviation for Acrylonitrile Butadiene Styrene – a type of Acrylic Butylated plastic. Lego uses ABS for its bricks. See http://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene
Therefore, #19 is also designated for Alkaline Batteries.