Doesn't take much of a bad connection to make a big difference.

The starter pulls 200A. That means every 0.005 Ω resistance will be a loss of a volt.

Considering the contacts on a switch are typically at least that much or double that a loose terminal ring could easily be 0.1 Ω. 0.1 Ω times 200A would be 20V loss starting at 12 it's pretty clear that would be enough to inhibit a start.

If just a little loose and 0.02 Ω:

That means 4v lost on the connector and only 8v left to start.

Also 4v x 200a is 800w which will melt things very quickly

One last example: just a tiny bit loose: 0.01 Ω: that means 2v loss and 400w on the connection.

I had a loose nut on my alternator once and it actually fell off. When I stopped to investigate why my car was giving me odd gremlins I looked down in the engine bay and it was self-coreect illuminated from the alternator bolt GLOWING!

(It's an eight mm nut in case anybody else needs to replace it)

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)