"Steel has a conductivity much less than that of copper, with copper at 58.5 Siemens/m and carbon steel at 5.9 Siemens/m.
The other factor is the cross sectional area of the conductor, with larger CSA conductors being able to carry more current. The CSA of the copper wire you run from the battery to the block is well defined by its diameter. The CSA of the steel conductor basically involves every piece of conducting metal that interconnects to any other piece and runs from the battery connection in the rear to the block connection in the front. Frame rails, floor pan, body panels, roof, any metal to metal connection, which makes for a CSA many times larger than the copper cable.
As I have pointed out before, this ground path is most important when you are starting the car as the battery is the source of all the current. Once the alternator voltage exceeds the battery voltage the ground becomes much less crucial, carrying only the charging current of the battery. Bottom line is - if you turn the key and the car starts readily, the ground connection is sufficient.
Found this in an article with wiring expert Ron Francis. Copper Conductivity Directly related to the grounding issues are those induced by poor battery cable connections. Bowers tells us that many installers still ground the negative battery cable to the frame of a vehicle, but there’s one...
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