Headlights on relay anyone? [Archive] - Nova Tech

: Headlights on relay anyone?

Aug 5th, 10, 07:55 PM
If you install a relay for the headlights, will the system voltage increase? I have a good SI10 alt. (13.8volts), but with wipers, lights, & blower running it goes to 12.0 volts in traffic sometimes. I know the lights are the biggest load.

Custom Jim
Aug 5th, 10, 08:23 PM
Does the voltage come back up when you are out of traffic ?. Chances are if it does the alternator is not spinning fast enough. Lower engine RPM's mean lower alternator RPM's and at the low RPM's the alternator will put out a lower voltage than what it would at a higher RPM.
Sometimes you can change the alternator pulley diameter to a smaller size and this will increase the RPM's in relationship to the engines RPM.
Adding a relay to the headlights is just redirecting power through another path. You will still have the same current demands (and a tad more now that a relay also has to be powered up).
If you are concerned about how bright the headlights are sometimes in traffic, then you probably could design a capacitor circuit to charge up to the higher voltage (13.8) and then when the voltage drops to 12.0 then the capacitor could be supplying the higher voltage to the headlights only. Who knows how large of a capacitor you would need but you would also need some diodes to keep the cap from backfeeding into other things in the car and make it drain faster. After all of this work, it might just be better to see if a better alternator could be installed, replacing what you have.


Aug 5th, 10, 08:34 PM
70 amps should be plenty. They came on cars with A/C. Maybe something else is loading it down. I know its worse when raining. I'll blow off a relay though. Thanx Jim

Aug 5th, 10, 08:41 PM
The relay solution will take a portion of the load off of the wiring harness wires that share the same circuit. Instead of the headlites drawing their current thru that leg of the harness, the juice to turn the relay on is all that is added to the harness leg. You wire the make contacts of the relay directly to the "battery" or other source. This will make your lites brighter but has nothing to do with the battery/alternator voltage.

Aug 5th, 10, 09:08 PM
If the sense wire is connected to the battery terminal on the alternator the system voltage will be lower. The alternator needs to see the voltage in the main harness to regulate the system at the proper voltage.
A relay set up will increase the voltage to your lights by providing a shorter path, nothing else.

Aug 5th, 10, 09:09 PM
Sounds right. This is a dumb question: when the starter gets wet in the rain, why doesn't the battery cable ground to the starter case?

Aug 5th, 10, 09:20 PM
Not a dumb question at all, there is not a continuous path between the cable and a ground by the water, unless you drive into a lake :)

Aug 5th, 10, 11:42 PM

I used this theory, and it works GREAT! :thumbsup:

Aug 6th, 10, 08:11 AM
Sounds right. This is a dumb question: when the starter gets wet in the rain, why doesn't the battery cable ground to the starter case?

Rainwater is relatively pure, and pure water is an insulator. The addition of impurities to water make it a conductor. When you conduct current through water, it heats up and evaporates, then the conduction ceases.

Big Dave
Aug 6th, 10, 09:25 AM
Dull headlights can be fixed with a different alternator. The DN and SI series of alternators did not out put full voltage or their rated amperage until the motor is above 2,800 RPM. The DN depended upon an external voltage regulator with mechanical points that often burned and reduced voltage to the system. The SI was internally regulated but the six diodes are very small and burn out as transistors where a brand new invention and they were incredibly expensive when these cars were new.

The CS alternators, the ones advertised as one wire alternators, are what every one should have on their cars (just use three wires instead of only one). They are a direct bolt on, output full Voltage and their rated Amperage at idle and stay cooler due to a better fan up front and better bigger holes in the case to circulate air over the electronic control circuits. (the main complaint people have against them is they don't look like the old inadequate alternator with a non-functional fan). All CS alternators start with an Amperage rating higher than the maximum available in a DN and climb higher than the old Cop alternators of old in a standard sized case.

Back when I was a kid you could always tell when you had a cop tailing you at night because his headlights were twice as bright as every other car on the road due to a larger capacity alternator delivering full voltage to the bulbs. This was because cop cars had alternators off of semi trucks that had huge frames for added windings and double or triple sets of diode bridges to handle the added load of auxiliary lights and radio. Today every one's headlights are bright because of modern electronics allow alternators to work. So just like putting on bigger tires and brakes install a bigger alternator.

Big Dave