From Daniel Stern's web site
http://www.danielsternlighting.com/tech/relays/relays.htmlRelays:
WHY AND HOW TO UPGRADE YOUR HEADLAMP CIRCUIT
By Daniel Stern with special assistance from Steve Lacker
Click here for a printer-friendly PDF version of this article.
THOUGHTFUL CARE & CAREFUL THOUGHT REQUIRED
The success or failure of your lighting upgrade efforts rides on the quality of your parts and the quality of your work. It matters how carefully you route wires to avoid chafing insulation. It matters how well you solder connections (crimps and sloppy or 'cold' solder joints corrode and die). It matters how well you shield added wiring from road spray. It matters that you use fuses in the new wiring to protect against vehicle damage due to a new or old electrical fault. It matters that you use high-quality parts that are designed to stand up to the rigors of automotive usage. Such components must be resistant to a wide range of temperatures, road splash, fumes found under the hood of every car, severe and prolonged vibration, etc. It will pay you to select only the products of companies with well established reputations for quality and durability; your $2.25 bargain no-name relay could easily kill you when it fails on a dark road somewhere, leaving you with no lights. Do not purchase vehicle components based solely on price!
The techniques described in this article will yield excellent results only if the work is carried out carefully and to a high standard, with quality parts and materials and without corner-cutting or sloppy work.
I personally wouldn't perform this upgrade on a really collectible car without taking care to hide all the new wiring. Actually, there's probably not much need to go to high-powered Cibie (or other European-specification) headlamps on a true collector car that is not driven at night. But on a hard-working daily (and nightly) driver like mine, powerful headlights are a real blessing, and keeping the wiring out in the open where it can be seen and inspected helps avoid failures!! Also keep in mind that this article focusses on the general principle behind headlamp wiring. There are many variations in original-equipment headlamp circuit design, and it will be worth your while to examine your vehicle's setup thoroughly, preferably with the aid of wiring diagrams applicable to your specific vehicle.
WHY USE RELAYS?
Power for the headlights is controlled by a switch on the dash. This is *not* a great place to tap into the system, for two reasons: The headlamp switch uses tiny, high-resistance contacts to complete circuits, and the wire lengths required to run from the battery to the dashboard and all the way out to the headlamps creates excessive resistive voltage drop, especially with the thin wires used in most factory installations.
In many cases, the thin factory wires are inadequate even for the stock headlamp equipment. Headlamp bulb light output is severely compromised with decreased voltage. The drop in light output is not linear, it is exponential with the power 3.4. For example, let's consider a 9006 low beam bulb rated 1000 lumens at 12.8 Volts and plug in different voltages:
10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens
The Europeans take a slightly more realistic with their voltage ratings; they consider output at 13.2v to be "100%". The loss curve is the same, though. When operating voltage drops to 95 percent (12.54v), headlamp bulbs produce only 83 percent of their rated light output. When voltage drops to 90 percent (11.88v), bulb output is only 67 percent of what it should be. And when voltage drops to 85 percent (11.22v), bulb output is a paltry 53 percent of normal! It is much more common than you might think for factory headlamp wiring/switch setups to produce this kind of voltage drop, especially once they're no longer brand new and the connections have accumulated some corrosion and dirt.
