40A? Holy cats that seems a bit excessive, I would be interested to know what the wire gauge is.
I think the 12v line is 12 AWG and the ground is 10. I'm not 100% certain (the wire is probably original with the 2001 truck, so no chance of reading the labeling, and without cutting it, no way to be sure just how thick the insulation is - I supposed they might be 10 and 8). But it looks like 12 and 10 to me.
And I guess a 40A fuse is appropriate (per wire ampacity ratings like
Max Ampacity Chart - Non-Bundled Wire by Compass Marine How To). I.e., it will keep the wire from burning (as a fuse ought to do). But you'd sure never want to pull that much current through it! Thus my attempt to find a <10A charger.
To elaborate further, most installations that I do use the Orion DC-DC 12/12-30 charger, which draws a little over 30A at 25°C. Given a 60' round trip cable length between the vehicle battery and the charger, you'd want 2 AWG cable. There's no way that the 7-pin socket can be used safely.
2 AWG! Could you elaborate again? For 60' round-trip, 14.4v input, 2 AWG, 30A, I get 1.94% voltage drop. For this application, with a DC-DC charger to bump the voltage back up, can we not afford a larger drop than that? E.g., something like 10% (AWG 8 or so)? I based that on advice like "...items which affect the safe operation of the boat and its passengers...use 3%; all other loads use 10%..." (from
Blue Sea, who seems pretty reliable).
I know 10% is wasteful, but we won't be pushing the full amperage for long, and the efficiency will improve as the amperage drops. Or so says my logic, but I'm happy to be corrected by someone with more experience.
I did similar calculations for the 18A charger; 75' total, 14.4v, 10 AWG, 18A: and I get a 9.3% voltage drop. That seemed marginal to me, but possibly doable. But I might be missing something.
PS - And yes, I have some AWG 2 on my little boat (for sustained 40A to an electric outboard with minimal loss), but it seemed overkill in this application.