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Food for thought:
A 12v generator has trouble keeping up with headlights and windscreen
wipers and is closer to 12v than a modern alternator.
Recently I purchased a hunting 12v spotlight...the instructions said '
do not shorten the battery lead". I assume for the reason you have
alluded to.
My airplane has 2 12v batteries, and runs 28v lamps and everything is
labelled "28v' not 24.

Tom Becker wrote:

> While PWMing a 12v automotive headlamp I've encountered a
> specification problem. Mildly OT, just what is the skinny on "12v"?
>
> An automotive starting battery typically shows ~13.2v after a fresh
> full charge from a ~13.6-14.5 volt alternator or charger, and ~12.6v
> when fully charged in storage. Such a battery produces 12 volts only
> when depleted and under no load. Surely, this is not what 1960s auto
> system designers had in mind when they described these as "12-volt"
> systems.
>
> The lamp I'm controlling specifies 12.0 volts and its life expectancy
> is significantly shortened at both lower and higher voltages, so it
> apparently truly needs 12 volts when operating at full intensity. The
> only way that seems possible in a running automobile is to expect the
> wiring harness to lose an appropriate voltage drop. Assuming the
> battery is charged and the alternator produces 13.8v, that's 1.8v lost
> to resistive wiring.
>
> Is that right?
>
> Tom
>
>------------------------------------------------------------------------
>
>Internal Virus Database is out-of-date.
>Checked by AVG Free Edition.
>Version: 7.5.485 / Virus Database: 269.13.3/986 - Release Date: 3/09/2007 9:31 a.m.
>
>


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> ... the alternator produces 13.8v, that's 1.8v lost...

I found some answers.

Sylvania technical notes show that headlamps are engineered to a
"Design Voltage" of 12.0, 12.8, and 24.0 volts. The Design Voltage is
that which produces the specified characteristics of the bulb - like
color, luminance, power, current and lifetime. Although it happens
that the specific lamp I'm working with is a 12.0volt lamp, the Design
Voltage of most headlamps is 12.8 volts.

Elsewhere, I read that the typical automotive wiring voltage drop to
conventional headlamps is one volt - or 0.1 volt for high-performance
lighting systems, like on off-road vehicles. One volt down from a
13.8volt alternator is 12.8 volts, so that makes sense. That, though,
would overvoltage the lamp I'm working with by 0.8 volt, almost 7%.

[Interesting but off-the-path, dual-filament lamps like stop/turn
lights can have both 12.8volt and 14.0volt filaments in the same bulb;
the 14.0volt filament draws less current so it sees a higher operating
voltage from system wiring.]

Sylvania and others show that a tungsten/halogen lamp lifetime is
dramatically affected by the operating voltage. There are charts to
show extremes, but close to 100% they offer a rule of thumb:

5% overvoltage ->
* Half the life
* 15% higher luminous flux
* 8% higher power consumption
* 3% higher current

5% undervoltage ->
* Twice the life
* 15% lower luminous flux
* 8% lower power consumption
* 3% lower current

So, the 12.0volt lamp in 12.8volt service should be expected to fail
very early but not be significantly brighter. Fortunately, I can
choose a PWM "full-on" value to be less than 100%, essentially
regulating the lamp voltage to 12.0 volts. This exercise has shown me
that I need to measure and maintain the lamp voltage - ideally, right
at the lamp pins.
Tom

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