I'm sure this is probably related to heat, but I'm not sure how to alleviate it. I have an ATMEGA64 driving a whole bunch of darlington arrays (ULQ2003). When using my wall wart, this circuit works fine, when using a 12V car jumpstart box, it works for about a minute before I start getting odd behavior. After the first time the circuit resets on its own, it starts resetting more and more frequently.
There are four separate 7805 regulators (each is decoupled w/ a 0.1 uF ceramic cap) on the board. Each one drives a separate piece (they're not in parallel). The inputs of all the switched loads and the ULQ2003 darlington arrays are tied in to one of three 7805s, the fourth 7805 drives the microcontroller circuit. All grounds are connected. To turn on one item in the circuit, an output pin of the MEGA64 drives the base of a darlington in one of the arrays.
While the 7805s get hot to the touch, I can still touch them without burning myself. When on the wall wart, I measure 7-8 volts at the 7805's inputs and about 4.4 at their output. On the battery (I don't think it's very good), it's about 10V at the input, and about 4.5V at the output. These voltages seem to remain consistent, even when the mcu starts resetting, so I don't know if it's due to a brownout or not.
Like I said, it works flawlessly on my wall wart, but craps out on the battery. I assume it's that the battery can supply more current, and things are going into thermal shut-down, but the voltages that I'm measuring don't seem to indicate this.
If anyone has any suggestions, I would greatly appreciate it.