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I tried this with two 9V batteries that you can interlock easily and they heat up a lot. Why does this happen? The internal resistance of the batteries would be quite small so why would they still heat up a lot due to Joule heating?

Circuit

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War story. A crescent wrench has low resistance. It was dropped between the contacts of a switch in an electrical distribution yard. Blue ball killed the guy. –  Mike Dunlavey Sep 5 '13 at 19:49
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What's a blue ball? –  user80551 Sep 6 '13 at 4:33
    
The wrench turned into plasma. –  Mike Dunlavey Sep 6 '13 at 12:18

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up vote 4 down vote accepted

The short circuit current for a fresh 9V battery is around 4A. With the series connection, the emfs add so the power dissipated by the total internal resistance is roughly:

$P \approx 18V \cdot 4A = 72W$

In other words, you'd expect the combination to produce, for a relatively short time, about as much heat as a 75W light bulb.

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Could there also be some additional heat due to the exothermic reaction of the battery occurring so fast? –  aglassman Sep 5 '13 at 19:29

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