Help Wanted: Battery Not Working, Arduino Nano Warm to Touch

Plugging in my Arduino Nano with a USB cord works fine, but when I plug in my 9V battery to the battery terminal I soldered to it (attached to Vin and Ground) it doesn't work, and the Arduino heats up in the area of red LEDs near the white button. If this helps, measured voltage drops from 7.8 to 2 when I connect the battery.