I was installing an amplifier in a car this weekend. From the negative terminal of the battery to the back had a resistance of 1-1.5 ohms which seemed high to me. Is this typical or should it be lower? Would this affect the amp at all?

 

Sounds like a poor ground connection to me...

 

It may be a normal resistance reading. It is the sum of the small resistances from
all conductors, plus
battery post to its terminal resistance,
plus battery terminal to its cable,
plus battery cable to a grounding terminal,
plus grounding terminal to chassis or body,
plus body resistance to trunk area,
plus body ground location at trunk to its terminal,
plus terminal to amplifier connector resistance,
plus resistance of the meter leads,
And keep counting.

And should not affect the amplifier in more than a small decrease of power output at maximum crank.
Just keep connections clean and tight.
Miguel

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Abolish the deciBel !

 

Try reversing the leads of your ohm meter. Do you get the same reading? If you read something very different, or a "negative resistance" then you are getting a false reading due to current being drawn in the circuit causing a small voltage drop. The real resistance is probably lower unless your car is a rust bucket.
If you want to use the car body as the negative lead for high current draw applications you should connect to the frame and not to the body panels.

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Well our car has a battery at the back of the car. The negative is earthed to the bodyshell. There are two positive cables to the front of the car. The one that feeds the starter motor can be cut by an explosive bolt in the event of an accident, and the small one has a 200A fuse.

So BMW reckon that if the small wire is shorted to ground in an accident, there will be plenty of current to blow a 200A fuse. However the starter motor takes a lot more current, so much that a fuse can't be used. I guess that the starter motor current can hit 500 A, so the total resistance, including battery, starter motor, cable and bodyshell, is about 20 mΩ.

I also think that if bodyshell had any significant resistance, they wouldn't use it for earth. That means that the bodyshell is less than 1 mΩ.

If you are seeing 1 - 1.5 Ω ground resistance, you certainly won't be able to start the car. 8 - 12 A will not even make the starter motor click, let alone turn the engine. It would also make any fuse over 10 A completely redundant.

How did you measure the resistance? My meter shows about 0.4 Ω for just its own leads, and if I wanted to measure from the front to the back of the car I would need something to extend the leads and that something would have resistance.