At 11:15 AM 6/4/2003, David Brodbeck wrote:
>drops to 11.5 volts with the headlights on, while the battery voltage in
>back stays at 14!  I'm sure my headlights will last forever with this
>condition, they just don't do a heck of a lot of good.

Maybe not -- the halogen-vapor cycle that cleans tungsten off the walls of
the bulb capsule doesn't work if the bulb isn't burning at some minimum
temperature (wall temperature > 250C, typically 10% undervoltage), so the
bulb can  become blackened (but at the lower temperature the filament isn't
evaporating as much tungsten so the effect is lessened).  Typically and
very approximately, running an incandescent bulb at 10% undervoltage will
increase the life by roughly three times and reduce the output (of visible
light) about 30%.  10% overvoltage will give about 30% more light and about
1/3 the life.  However with halogen bulbs running below ratings the
increase in life may be offset by etching of the filament by the halogen
gas which is not occupied as it should be with the halogen cycle; and
running above ratings can lead to bulb blackening because of not enough
halogen to keep up with the tungsten evaporation, electrical failure from
deterioration of the metal in the seal, gas leakage from seal de-bonding,
and capsule failure either from thermal stress or from softening of part of
the capsule wall.

http://www.cecindustries.com/Catalog.pdf has specs for the bulbs they make
-- their H4 bulbs are rated 12.0 volts, 150/300 hours (with the life
ratings halved for the high-wattage bulbs).  Their 9004s are rated 12.8
volts, 150/320 hours.   Philips bulbs are also rated 12v for H4 and 12.8
for 9004.

david

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"