View Single Post
Old 11-12-2006, 08:41 AM   #90
Boxer Fan
Joined: Jun 2005
Location: Hampton Roads, Virginia, USA
Oddometer: 23
Why doesn't the battery cable melt?
Originally Posted by zero
You battery cable isn't high resistance wire ;)

This is a quick list of what's happening in the electrics:
Your voltage regulator is going to do it's best to keep the voltage across the battery (or wherever else you start your electrical system) constant - about 12 Volts (V)
Power, measured in watts, at the work load (motor, light bulb, resistance wire) is Watts (W) = Volts (V) x Amps (I)
If the volts are constant, thanks to the voltage regulator, the only way to increase or reduce power (which is equal to heat in your wires) is by changing current (I).
Current is dependent on voltage and resistance (I = V/R). Since voltage is constant, the only thing you can change is resistance. Lowering resistance will increase the current. You can lower resistance by using less wire, or a thicker wire. Think of it as a pipe, with resistance as friction. The less pipe a liquid has to travel through, the less the walls of pipe interfere. Likewise, a bigger pipe restricts flow less. So if you shorten the wires, more heat.

So why doesn't the battery cable heat up? The power in your system is put out in a serial fashion in the line. If you have a light hooked up, it glows brightly, but the more lights you hook up (in series), each glows dimmer. If you have different watt bulbs, each takes a different load. Now the final part - the voltage dropped in total along the line (series) equals the sum of the individual parts. It'd take a lot longer to explain, but basically your battery cable is accounting for almost no voltage drop, while the high resistance wire is dropping almost all the load. If you used a low resistance wire for the heater element, it'd be a whole different story - you'd be blistered and the battery cable would be heating (and your voltage regulator/generator would be toast if you didn't use a fuse) Thats also why a long run or small gauge of power cord to a high power motor may cause the motor to lug, as the voltage drop on the supply line becomes very significant (length of wire drives up it's resistance - same as using a higher resistance wire). If your battery cable had more resistance, it'd drop more voltage, and then it would begin dissipating power (heating up). If you feel such a supply cord trying to handle the load, you'll find it very warm or hot.
If the battery cable was the only thing on the circuit, then it'd have to dissipate all the power - if the cable has almost zero resistance then the watts would be something like Power (watts) = 12 / 0, which is almost unlimited and a very bad thing for your generator to try (user hint, it will try but will never make it - the power generated will roughly correlate to the dollar cost of fixing the damage)
ArbyNav is offline   Reply With Quote