🚀 go-pugleaf

>Help! I am building a device that needs a power hungry halogen bulb
>(5V, 0.8 amperes). With a standard 9V NiCad battery and
>5V rectifier I only get 1.5 minutes of useable light. I could use
>4 1.5 (actually 1.2 V) NiCads, but these would take up more space
>(space is important). There was talk of  "VARTA"  batteries packing

Might not help the space much, but National makes a little 5V switching
regulator with very low external parts count.  You could get a
_regulated_ 5 volts.  If the 9V (~8.5V) battery is about 80 mAH, you
would get about 80mAH * 8.5V/5V * 75%, or about 100 equivalent mAH at 
5 volts.  That should run the bulb for about 1/8 hour, or 7-1/2 minutes.
I would think the regulator would be a big gain in stabilizing the
color temp, which you say is important!  An alternative would be a
step-up switching regulator operating from a single AA cell (or two);
space about the same, maybe a bit more operating time??  ("left as an
exercise for the reader")