I just did some testing for a similar circuit to see how hard we had to drive a flashing LED.

I used a green led, gave it a trickle of 10 mA, and surprisingly enough there was little change in intensity if I drove it any longer then 20 milliseconds. The human eye is a peak detector, and the short burst is enough to let you see it, even from across the room; I could get about 30 feet away and it was still plainly visible in room ambient light.

Milestag has a good idea, although I wouldn’t bother with a regulator between the battery and the PIC, it’s just a waste of power (which batteries don’t have much of). Drive it direct, and use a low current low voltage reference to reference the A2D. Divide down the battery voltage to put it in the range of the lower reference voltage and just measure.