Can I get 18V from two 9V batteries if I join two together?
How will I join them?
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
Yes, two 9 V batteries in series results in one 18 V battery. You connect the + end of one battery to the - end of the other. The remaining unconnected battery ends are the ends of the overall 18 V battery.
While you get more voltage, you don't get more current. If each battery is capable of 9 V at 300 mA, for example, then the combined battery is capable of 18 V at 300 mA.
Yes, they can be connected in series. You can snap them together (female to male snap) and they will be connected correctly. The voltage is doubled and the Ah life is the same (but twice the voltage, so twice the energy, as you might expect). Or just connect the wires from the snaps as shown below (image from here).
If you choose to clip more than a few together you should take appropriate safety precautions. The 'crazy dude' in the comments who linked 244 together to get ~2kV has an electric chair supply on his hands. The short-circuit current of a fresh alkaline 9V battery is about 3A, meaning that string could supply >1kV at 1.5A.
Even if the current didn't go through your heart, the resulting 1.5kW of heat would quickly cause severe tissue damage. Allegedly DC can be more dangerous than AC because it makes it harder to let go. I don't know that there have ever been detailed experiments done on this (perhaps some organization such as Unit 731 did). In any case, avoid getting shocked.