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Hooking Up Your Power Supply in the Car

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  • Hooking Up Your Power Supply in the Car

    Connecting Your Power Supply in the Car

    WARNING!FROM THE FACTORY, THE POWER WIRES ARE NOT ALWAYS RED. The statements made in this FAQ are made as a general statement. ALWAYS double check your wire polarity before making any connections.
    The stock head unit connector on your car has the following:
    • 12v line from the battery
    • 12v acc line
    • Ground
    • Lines out to the speakers (usually 4 pair or more)
    • Other miscellaneous wires for various other functions

    You cannot draw your power from this connector because the wire is not thick enough for the amount of current you will have to draw. If you do, you will end up burning up the wire and possibly causing a fire in your car. This is why you must run large cable (8 to 10 gauge) from your battery. You only need to run the positive wire (+)(12v)(red) because the negative (-)(ground)(black) is connected directly to the chassis of the vehicle, so the entire metal frame of the car acts as one big ground wire. This is why you can connect the ground to (almost) any bare metal on your car.
    If you are going to be powering more than just your computer off of this wire, it is wise to increase the size of the wire as necessary for the additional components.

    It is important to note that in general, a cars chassis is only equivilant to a 4 gauge wire. If you are using something bigger than a 4 gauge wire (2 gauge or 1/0), you need to run a wire directly to the battery. It should also be noted that many of today's cars are made out of unique materials. These materials are designed to keep road noise to a minimum in these newer vehicles. These new materials do not conduct electricity as well as previous materials. As such, many install shops have run into problems with equipment not working properly. If you run into this issue, it is recomended to test resistance between the ground on the battery and the ground on the car. It probably shouldn't be more than a few ohms of resistance. Another way to test for this is to test voltage drop. If the voltage at the battery is 12 volts between the two terminals and the voltage between the terminals on the amplifier is, say, less than 11.5 volts, then you are having a resistance issue and should check the construction of the chassis and the materials used.

    Determining The Proper Size Wire For Your Install

    There are some important things that you need to know before you buy wire:
    1. The length of your cable run (approximate is okay, just overestimate)
    2. The total load on the cable in amps (figure out the most youŽll use at one time)
    3. The acceptable voltage drop for your components (There will be a voltage drop)
      • It simply depends on how much drop is acceptable. Considering your car's voltage will be fluxuating from <7V on a cold start to >14.5V while running, you need to make that decision for yourself, this is the hard part. Usually it's determined from the minimum supply voltage listed on the spec sheets, but if that minimum voltage is below your car's minimum starting output then either the spec sheet is lying, or you need a new battery with lower internal resistance.
    Values 2 and 3 will give you a number of acceptable ohms for the cable, and cable resistance is based on cross-sectional area, length and conductor material. You'll probably be using oxygen free copper (OFC), you know the approximate length and the acceptable resistance, from that you can determine the cross-sectional area and therefore the wire gague.

    The chart below can be used to determine the proper gauge wire for the distance you are going to be running:

    When installing your carputer, you will need to identify the acc line behind the factory radio. You will have to either:
    • Tap this lines
    • Find a corresponding plug
    • Cut the connector off altogether
    You must make sure that you TAPE OFF ANY UNUSED WIRES (with black electrical tape) and LEAVE NO WIRE EXPOSED. This is absolutely critical.

    In order to identify which line is which, you will need either a volt-meter, or multi-meter, or a 12v test light. If you are technologically uninclined, it is suggested to use the test light. They only cost a dollar, and their operation is simple. You connect one end to a ground and the other end is a probe with a light in it. When you probe a voltage source, the light lights up. You could also connect the wire to a positive line and probe for a ground, but you will not be doing that.

    Now, with the test light, connect the wire end (usually has an alligator clip) to some bare metal someplace on the body of the car. Turn the ignition on (engine does not have to be running), and begin probing the wires if you find one that lights up the light, then shut the ignition off, if the light goes dark, you have found the acc line. If the light stays on, it is the 12v (positive) line, but you won't be using it anyway. If it is not the acc line, turn the ignition back on and move onto a new wire, and so on, until you identify the acc (accessory) wire.
    Red wires are almost always 12v positive wires while black wires are almost always negative ground wires.

    Make sure you identify the 12v positive, the negative ground, and the acc lines. Tape off the 12v positive and ground wires because you will not need them, as you will be running new ones.

    Determining Proper Fuse Size

    Use the chart below to determine what size fuse you should use under the hood on the wire that goes from the front of the car to the back of the car.

    The reason for fusing the wire under the hood is to protect the wire as it travels through the car. This fuse is not necessarially there to protect the equipment, it's there to prevent the wire from getting too hot and catching on fire. If this happens you will probably have a major fire on your hands if you have not properly fused the power wire for your equipment under the hood.

    Do I Need to Upgrade My Battery and/or Alternator

    One of the first questions asked by individuals looking to install a CarPC in ther car is if they need to upgrade their battery and/or alternator.

    The chart below will tell you if you need to upgrade your battery:

    The chart below will tell you how much spare amperage you will need for the draw you are adding to your system:

    Article written by da_kooz, RedGTiVR6.