Hi.

First of all. You cant use a 5V regulator on a 6V supply. (the regulator needs approx 1.25 volts across it.)

I would use a NPW transistor With the 6V in at the collector, and the approx. 5V out at the emitter. Then you should use a 5.6V (standart) zener diode from ground to the basis. And then you just needs a pull up resistor from the basis to the collector.

This way the voltage to the processor vill be approx 5V (4.95-5.05) when the batterys are fully charged. When the battery voltage falls below 5.6 volts then the processor voltage will allways be 0.6 volts below battery voltage.

Now you just make a voltage devider on the battery side, with 2 resistors, and then when the voltage falls below a desired level, eg. 5.3 V (then you should have approx. 4.7 volts on the processor and it should probably still run :-) The internal A/D converter converts the value and warns the user.

Hope this helps. If you have aditionel problems with this, dont hesitate to ask, and I will make a schematic of the supply for u.

Wisdom is the booby prize - given when you've been unwise.

Regards
Mads