# 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?

• 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. Sep 5, 2013 at 19:49
• What's a blue ball? Sep 6, 2013 at 4:33
• The wrench turned into plasma. Sep 6, 2013 at 12:18
• They heat up less if the resistance is high and the other way around. Lithium batteries have low internal resistance and if shorted they become so hot that they often explode. Watch out before shorting stuff! Aug 20, 2019 at 16:10

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