What happens if you connect two 9-volt batteries together? - Quora
badBrick - (3) 9 Volt Battery Case and top for Arduino UNO R3. by badBrick - Thingiverse
LPT: Are you finding yourself paying for expensive power? To save yourself from paying thousands of dollars, just connect two 9-Volt batteries like shown in the picture below. The batteries will continue
Turn an Ordinary 9-Volt Battery into a Secret Safe « MacGyverisms :: WonderHowTo
Can you get 18V from two 9V batteries - Electrical Engineering Stack Exchange