how to connect to positive battery post

They’re “bedfellows”. I’d go with “A”, it’s less likely to get crusty, and the connection would be undisturbed if you were swapping to a new battery. It looks to be purpose-built for connecting accessories like yours.

 

"B" holds the clamp to the post. "A" is for Main power and accessories....

 

B will set off tiny sparks as you loosen and tighten it. Been there done that, both A and B on mine are used for other connections.

 

Neither one is ideal. If you loosen that clamp and lift it off the battery post, you will notice it drapes down over the side of the battery and contains a 140 amp fuse, and there is another place you can connect—let's call it C—at the bottom end of that fuse.

C, for the win, for two reasons. First, your added wiring is now protected from the battery by the 140 amp fuse. You still need your own fuse sized appropriately for your sub and your added wiring, but the 140 amp 'feeder' fuse helps limit what your added fuse and wiring can be exposed to.

Second, most of the time, as you listen to music with your car in READY, the power isn't coming from the battery, it's coming from the inverter in the front of the car, and connecting at C, you have shortened that path by not having the 140 amp battery fuse in it. (There is still a 125 amp fuse at the front of that cable, protecting the inverter output, so by connecting at C, you have a tap point that is protected that way from both ends.)

It's a little tough to see in this pic because of so much other stuff around, but you can see the solar charger kind of velcro'd on where that fuse hangs down over the side of the battery, and the cable tapping off to the added fuse box is attached at the bottom end there.

Point A is purpose-built to give you a way to change the 140 amp fuse if you have to.

 

We can create an option and not use the given?

Id go with D, buy a battery clamp that has multiple bolts to provide better and safer connections.

D takes the cake and champagne bottle.

 

D is exactly as bad as A and B, for the same reasons.

 

Start by working out the maximum current in amps you expect your inverter to draw from the 12V battery.

Because the inverter's output is nominally 120 V and the input is nominally 12, for a quick estimate you can just take the total amps you expect your 120 V appliances to draw, and multiply by 10.

Suppose you end up, say, determining your inverter will draw 85 amps. Now do two things:

1. Make sure you choose a type and gauge of wire with adequate ampacity, say 20% higher (85 ✕ 1.20 = 102). You can look that right up in a published ampacity table. It doesn't depend on the run length.
2. Make sure your wiring will not have excessive voltage drop. You probably want it to drop no more than a volt or so from the battery to the inverter, while carrying your predicted current. This does depend on the length of the run, so you start by choosing where your inverter will be, and add up the length of the + wire run and the length of the − wire run to your chosen ground point. Look up the resistance per foot of different wire gauges, multiply by the total length of your + and − runs, and multiply by your maximum expected current. The result is your voltage drop. Choose a wire gauge heavy enough to keep the voltage drop small.

In 12-volt systems, often step (2) is what dominates; the voltage drop will drive you to a heavier wire than you would need just according to an ampacity table.

 

I'll take ground loop for $1

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Aha. While that big silver box as a whole is called the "inverter assembly", there are different sections in it, and the section that produces the 12 volt supply is called the "DC/DC converter". Then I'd have known just what you meant.

Interesting ... I might be remembering wrong, but 4 ga sounds about right for the wire I added at position C and I don't remember much trouble with it.

It sounds, though, as if you hear the whine even at low volume levels or with all the RCA inputs disconnected—times when the music amps aren't drawing much at all, so it's hard to see how an undersized supply wire would be the hypothesis.

It seems more likely to me that, well, all of the sections of the inverter assembly: the DC/DC converter, the boost converter, the MG1 inverter, and the MG2 inverter, do their work by switching currents at frequencies that may be in the audible range. The MG1 and MG2 switching frequencies also vary with MG1 and MG2 RPM, which fit well with your observation of pitch change with go pedal input.

If you are hearing switching noise conducted to your amps over the power supply, I wouldn't expect a fatter wire to carry the noise any less. You might get a better result just by improving the supply noise filtering ahead of your amps.

 

I am less inclined to think it is ground-loop if you still hear it with no audio ground coupling.

@ChapmanF has just suggested the next thing on my list- input filtering.

Get a ferrite bead or three and introduce that to the power line feeding the amp. Sometimes you can use just one clamp-on style, sometimes you need to get a real ring type and pass your power wire through it more than once.

Plenty of youtubes out there to explain theory and practice of installation.