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?


  • $\begingroup$ 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. $\endgroup$ Sep 5, 2013 at 19:49
  • 1
    $\begingroup$ What's a blue ball? $\endgroup$
    – user80551
    Sep 6, 2013 at 4:33
  • $\begingroup$ The wrench turned into plasma. $\endgroup$ Sep 6, 2013 at 12:18
  • $\begingroup$ 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! $\endgroup$
    – Dakatine
    Aug 20, 2019 at 16:10

1 Answer 1


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.

  • $\begingroup$ Could there also be some additional heat due to the exothermic reaction of the battery occurring so fast? $\endgroup$
    – aglassman
    Sep 5, 2013 at 19:29
  • $\begingroup$ How did you get that 4A? Thank you $\endgroup$
    – Dakatine
    Aug 20, 2019 at 16:20
  • $\begingroup$ Oh well OK you assumed the internal resistance is 1 ohm I suppose $\endgroup$
    – Dakatine
    Aug 20, 2019 at 16:23
  • $\begingroup$ @Dakatine, I didn't assume an internal resistance, I just looked up the typical short circuit current for a fresh 9V battery. $\endgroup$ Aug 20, 2019 at 18:14

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