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Can I get 18V from two 9V batteries if I join two together?

How will I join them?

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    \$\begingroup\$ In series, positive to negative. \$\endgroup\$ – Roger Rowland Nov 28 '16 at 20:36
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    \$\begingroup\$ 9V batteries have quite a unique feature in that they can be clipped together without needing any extra hardware, the longest 'stack' I've seen developed some 200V or so. \$\endgroup\$ – Sam Nov 28 '16 at 21:21
  • \$\begingroup\$ @Sam - Here's a video of some crazy dude who clipped 244 of them and got about 2000V out if it. \$\endgroup\$ – Ricardo Nov 28 '16 at 21:37
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    \$\begingroup\$ @Ricardo Ah that's the one, I knew there was 200-somethings involved, I just couldn't remember if it was batteries or volts \$\endgroup\$ – Sam Nov 28 '16 at 21:39
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    \$\begingroup\$ Look inside the battery door of your TV remote. What do you see? Why do you think they did it that way? \$\endgroup\$ – Loren Pechtel Nov 29 '16 at 0:30
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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.

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    \$\begingroup\$ Also, make sure they are both new batteries of the same brand and type. \$\endgroup\$ – user98663 Nov 28 '16 at 20:46
  • \$\begingroup\$ After using them for a time they wont any longer be new. Should you stop using them in this fashion then? Also I don't see why they must be of the same brand if they are the same type and you trust both brands specifications. \$\endgroup\$ – Anonymous Coward Nov 28 '16 at 23:48
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    \$\begingroup\$ @Jose: What he really meant is that any set of batteries you string together should be matched. For normal consumer batteries, the best way to make that likely is for all the batteries to be of the same model and all fresh. With unmatched batteries, some will deplete before others, making the whole string seem worse than most of the batteries actually are. Worst case a bad battery gets reverse biased, which could cause it to leak or even explode. \$\endgroup\$ – Olin Lathrop Nov 29 '16 at 0:02
  • \$\begingroup\$ @OlinLathrop Off-topic: Heading to your site http://www.embedinc.com/pic/dload.htm, Chrome gives a warning: "The site ahead contains harmful programs" support.google.com/chrome/answer/99020?hl=en \$\endgroup\$ – Jonathon Reinhart Nov 29 '16 at 0:05
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    \$\begingroup\$ On the other hand, consumers have been happily sticking multiple cells of whatever type they found in the kitchen drawer together into battery boxes for decades, without significant harm or explosions. \$\endgroup\$ – Ian Bland Nov 29 '16 at 0:47
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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).

enter image description 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.

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Yes, battery ones negative pin goes to battery twos positive pin. Then the positive of battery one is your 18 volts, negative of battery two is your ground.

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