# Why does shorting two cells in series heat them up?

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?

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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
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

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