Well, how about installing a variable resistor in the circuit which triggers the Buzzer/LED? when the voltage falls below certain
Well about the Cooled helmet - The principal remains the same, using a perltier cooler. But we need to make a cut out in the helmet, and install fans at both the ends of the TEC (peltier).
Night Vision - Made use of Camcorder, IR/Night Vision. And the live feed from the camera was sent to a 5 inch LCD screen, either as Heads Up Display or Helmet Mounted Display. Installed 2 arrays of IR LED, consisting of 30 IR LEDs compared to 4 in camcorder. BTW each IR LED 2 years back costed me about 200 bucks!
Usage - See without Headlights, ultra low power consumption, and even a small light is magnified. Never went ahead in actually making full fledged system, but tested with a Handy Cam and it is insanely wicked!!
The best part is that.. it's as good as Night Vision Goggles, used by the military. And you can actually walk in pitch dark jungles without anybody knowing.