I think I just bricked my Arduino. Any ideas how? I think I have a short circuit in my Arduino Uno R3. The symptoms are:


It does not seem to be responding as before. 
The orange "L" LED does not blink on boot-up, but just stays on continuously.
If I plug in a 12V source, then the voltage between the 5V and GND pins starts out around 5V, but then starts dropping rapidly. In the meantime, the underside of the board heats up a lot. If I plug in the USB source, the voltage between the 5V and GND pins stabilizes at around 4.28V ... but again, nothing seems to work.
After all this, I also checked continuity between the 5V and GND pins, and heard a beep. However, subsequent reading told me that this is not a reliable (or well-advised) test.


It seems like I have a short on my Arduino, right?  Any idea of how this could have happened?  I can only think of the following possibilities:


I was running the Arduino off a lead-acid 12V battery. At some point, the negative terminal wire disconnected and there was some sparking at the battery when I reconnected it (the pattery was also powering other things). Could this have damaged the Arduino?
I am using the Arduino to run 3 IR LED . To minimize wiring, I had wires these LED in series, rather than in parallel, which (at least at first) seemed to work. Is there any possibility this would have damaged it?


Thanks for any help.
See my answer here. A 12V battery doesn't necessarily provide you with 12V any more than a 1.5V battery provides 1.5V.

See here for example. A 12V lead-acid battery may well provide 12.7V which is out of spec for the Uno.

See the page about the Uno.

You have too much voltage going into the voltage regulator, it tries to dissipate it, it gets hot and either shuts down or fails.

I suggested in the answer above to use a buck converter to down-convert the voltage, and my answer (which was accepted) said that "it is working like butter".