View ASCII in a FIXED width format.


               D1         D2
12V+ (car) >--->|----o----|<------------------< (AA pack) 6V+ 
                     |
                     |  ----------
                     |  |  7805  |
                     |  |        |
                     o--|In   Out|--o---------> reg +5V out
                     |  |        |  | 
              1000uF |  |  GND   |  | 10uF 
              O--||--O  ----------  o--||--o
              |    +        |         +    |
 GND (car) >--O-------------o--------------O--< (AA Pack) GND




Note:
1) As long as the voltage at the Anode of D1 is greater than or equal
   to the Voltage at the Anode of D2, there will be no drain from
   your AA battery pack.

2) In normal conditions, D1 is forward biased and conducts, D2 is in reverse
   biased and acts like a capacitor. (A very small Capacitor)  When the power
   source from D1 is removed or falls below D2's voltage, D2 becomes forward
   biased and conducts.
 