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Upgrade Headlight Wiring

Discussion in 'Gen 2 Prius Accessories & Modifications' started by jsinclair92, Mar 24, 2017.

  1. jsinclair92

    jsinclair92 Junior Member

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    I have upgraded my headlights and want to add in a headlight relay harness, that draws power directly from the battery.

    On a vehicle with the battery in the engine compartment, this is a breeze. Does anyone have any thoughts as to how I could pull this off? I'll paste a link to a wiring harness similar to the one I would be using (or making) below.


     
  2. ChapmanF

    ChapmanF Senior Member

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    Out of curiosity, what have you upgraded them to?
     
  3. jsinclair92

    jsinclair92 Junior Member

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    100/55w. Well worth the $5. Low beams are to spec. High beams are great for backroads (only). Vast improvement over stock.
     
  4. xliderider

    xliderider Senior Member

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    Sorry, I'm only familiar with the Gen 3. I have HID kits running off of dual relay harnesses in both our Gen 3 and Camry. In the Camry, power is from the positive terminal of the battery. In the Gen 3, power is from the engine fuse box.
     
  5. valde3

    valde3 Senior Member

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    That sort of kit works fine on Prius. Just connect the power to the bolt at positive power terminal in the under hood fuse box. Prius charging system provides constant and stable voltage at that point.

    But please reply back on how your headlight assemblies work with the higher wattage of those bulbs. No problem with “glass” or mirror surface?
     
  6. Sam Spade

    Sam Spade Senior Member

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    Then WHY do you think you need to mess with the wiring ?

    If the wires are not getting hot and the plastic parts of the headlight assembly aren't over heating.........increasing the current flow to the bulbs probably won't result in enough brightness increase that you can see but it might increase the heat dissipation at the bulb enough to melt the housings if you run with the high beams on for very long.
     
  7. jsinclair92

    jsinclair92 Junior Member

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    Don't quote me on this, but the stock headlight wiring looks to be no bigger than 24 gauge, maybe smaller. But that is just my guess after a quick glance at it. I'd feel more comfortable with 16 gauge. Why?

    A headlight at 100w and 11.5v to 14v (I used to voltage variance just to cover my bases) will pull 7 to 9 amps.

    24ga wire is rated at about 3.5 amps. 22ga is 7 amps.

    I would rather us a thicker wire to handle the load. Maybe not quite 16 gauge though.

    As for the glass and the reflector, I am quite pleased with the way it has held up. I have had the headlights in for a few days with no problems.

    As to why I want to upgrade the wiring: With the stock setup, power is transferred through the headlight switch. My knowledge on these particular cars is limited, but this is the case with cars I have had in the past. With one of these headlight relay wiring kits, the power is drawn from the battery straight to the lights (less power loss). This is my understanding of how that works, anyway.
     
  8. valde3

    valde3 Senior Member

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    There are relays for headlights so power doesn’t go through the switch. But stock wiring is pretty thin so I think that’s good idea.

    In a Prius bolt in the front fusebox will give you more stable power than connecting it to 12V battery. And of course the front fusebox is a lot closer to headlights. So just connect the power to it.
     
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  9. jsinclair92

    jsinclair92 Junior Member

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    This is exactly what I needed to know. Thanks for the info!!
     
  10. ChapmanF

    ChapmanF Senior Member

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    In this application, you may want to do more of the math yourself, and rely less on published ampacities. Why?

    Ampacity isn't a fundamental characteristic of a piece of wire. Resistance, yes. Temperature rating of insulation, yes. Ampacity, no.

    When you know the current that will be flowing, you can directly calculate how much the voltage will drop, and you can multiply that by the current to know how much heat will be produced in the wire. What that does to the wire's temperature will depend on non-fundamental things like the surrounding temperature, airflow, and so on. When sources publish ampacities for wire gauges, they are making assumptions about those things (such as, for building codes, the wires will be in enclosed wall spaces and so on). What you really want is to make sure the wire will stay below its rated temperature in the conditions of use.

    I would suggest a close look at the packaging of the bulbs you intend to use, to find their rated voltage. Chances are, it is lower than the normal system voltage of a Prius, which you don't actually have to model as "11.5v to 14v" because in a Prius in READY mode it is very steady and between 13.8 and 14-and-change.

    Also, when you computed the current draw based on 100W over that range of voltages, you were treating an incandescent bulb as more of an active component than it is. If you feed it a lower voltage than it's rated for, it will not compensate by drawing more current to keep the power at 100W. It will just have less output (by about the square of the voltage reduction). Conversely, if you feed it a higher voltage, its output will exceed 100W, again by about the square of the voltage increase. Increased output sounds great, but there is another effect too: its life is shortened, and this by about the thirteenth power of the voltage increase.

    Once you wrap around that idea, you see that the skinniness of Toyota's headlight wiring is part of the engineering of the car, and is about achieving enough voltage drop through the wiring that the bulb is not grossly overvolted by the somewhat high system voltage of the Prius. (They've actually had to issue a TSB for the first couple years of Gen 3 because the wiring didn't drop the voltage enough, and went through headlights like popcorn.)

    It's a crude method, but because the bulbs they designed for are known quantities, it's just a matter of doing the math and choosing the wire size. Since you are changing the bulb specs, you certainly may want to change the wire specs too, but if you haven't thought about what the final voltage at the bulb will be dropped to compared to what the bulb is rated for, you should probably add that to your considerations ... unless you own stock in a bulb company.

    -Chap
     
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  11. jsinclair92

    jsinclair92 Junior Member

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    Thanks for sharing the wisdom, Chap. I'll take that into consideration when I go back to the drawing board and go over the numbers.
     
  12. jsinclair92

    jsinclair92 Junior Member

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    The bulb is rated 100w at 12v. Which means the wattage would actually be higher at 14v, but how much higher?
     
  13. ChapmanF

    ChapmanF Senior Member

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    If light bulbs were strictly ohmic, it would increase by the square of 14/12, which is about 1.36, so the 100 watt bulb would produce about 136 watts (total of visible light and heat; it would also look even brighter than that, with more of its total output shifted into the visible range).

    Their expected lifespan would be divided by about the 13th power of 14/12, which is about 7.4, so if they were rated, say, for 200 hours on the package (I'm making that up, haven't looked), you'd be able to expect maybe 27.

    Light bulbs aren't strictly ohmic, but in that range of voltages should be pretty close.

    -Chap