Relay placement

LightCrazy

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Dec 31, 2011
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400
Location
USA, Western PA
Hello. I am new here, and love anything to do with flashlights and aux. car lights. My question is this. When aux. lights are mounted on a vehicle, the relays are typically mounted by the firewall, which means the main power for the lights is running about 3-4 feet from the battery to the relay, and then another six feet or more back to the actual lights. Would it be more efficient to mount the relay closer to the battery so the actual power wires are shorter, and let the signal power from the switch to the relay be longer? I am just thinking of current loss, etc. Also, how many people ground their aux lights directly to the battery?

Thanks, and keep up the great site!
 

-Virgil-

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Staff member
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Mar 26, 2004
Messages
7,591
Yes, all other factors being equal, the relay-switching circuit can be as long as you want and the load-power circuit should be as short as you can reasonably make it. A good ground is essential. You can go directly to battery negative, to the alternator housing, to one end or the other of an existing ground strap, to the engine block…avoid the temptation to ground to body metal panels, which usually give a ground that can't be called better than "adequate" and sometimes not even that.
 

Echo63

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Apr 26, 2004
Messages
1,778
Location
Perth - West Australia
The Aux lights on my car are wired direct to the battery, no earth on the bodywork, other than the earth for the relay

I used a pretty heavy gauge cable, but the relay is on the firewall, so there will be a bit of voltage drop.
I put the relay back there as there was a nice bolt already there to make mounting easy, and it's up out of the way of puddles etc (my car seems to power wash the engine bay when I go through a puddle)
 

LightCrazy

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Joined
Dec 31, 2011
Messages
400
Location
USA, Western PA
Thanks for the help. When the weather breaks I'll spend a nice day moving my relays closer to the battery and lights. It makes sense to keep the actual "load" wires as short as possible, and just use a longer wire from the switch.

My Bosch fog lights are switched from the parking light circuit, and my Hella 1000FF lights are switched from the high beam circuit. makes more sense for those "signal" circuit wires to be longer and the actual power/load wires to be short.


Bosch fog lights--waiting for Hella to release the newer 500FF Fog lights
Hella 1000FF Driving for off road
Streamlight PolyTac, Stinger, Strion LED, Pelican LAPD and who knows how may more.
 
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