Question about the negative terminals on battery

So I will be using a dual battery system. I will have each of the positive lines go to a transfer switch, so that is easy enough, but here is the problem.

I am assuming that the proper way to wire the negative side is battery 2 negative to battery 1 negative to engine. I already have a thick negative cable running to the engine. So that means one of the batteries will have TWO negative cables running to it (one going to the engine, and one coming from the other battery).

That said, is there a special terminal that supports two thick gauge battery cables on a single terminal??? I have had a hard time locating such a thing.

You have a few choices.
1) Oftentimes, rather than using the automotive type clamp that fits around the battery post with the wire crimped in, a clamp with a vertical threaded stud is used. This method requires a closed eye end on your negative battery cable, and several can be piled up on the stud and held in place with a hex or wing nut. Examples are here: http://www.marineengine.com/parts/sierra_marine/sierra00054.htm
2) You can run your existing negative wires from their individual batteries to the engine ground.
3) You can run your existing negative wires to a common "bus", and from there run another wire to your engine ground and to any other grounds needed.

Very good advice. For options 1 and 3 I strongly suggest using a hex nut instead of a wing nut. The wings nuts cannot be tightened that well and have a tendency to vibrate loose.

Thanks for the advice. I am new to this.

So I already have the negative wire run and don't want to run any more, so that throws out number 2.
I would consider number one, but my cable doesn't have a closed eye. I know that for the small cables I can buy a closed eye, and crimp it on. Do they make a similar kit for large cables (this cable is VERY heavy gauge).
Similar question for #3. Do they make a common bus that supports such have gauge cable, and one with no eye???

Any way you do it you will need a negative cable with an eye. Check you local auto parts dealer or order online from West Marine or Boats.net.

The negative cable that is already on the boat is HUGE.... I could be wrong, but it looks like it is 4/0 AWG (the wire including the insulation measures about 11mm).... What AWG would be safe in going from battery to battery?

The engine is a 90HP 1985 Mariner.

If you are running 60 amps, you'll need 6 AWG. Let's see if this chart comes through....

i would not crimp a lug on the wire...crimps(especially on a battery wire) will corrode and the expansion rate with heat will be different because they are different materials...the best bet is as chawk-mans recommendation to buy a made up cable from a auto parts store....i assume from your description that you are using the switch to switch batteries and never have them hooked in complete parallel....two batteries in parallel is a no-no because sooner or later one may go bad and draw from the other and you go to use them and they are both dead...there will be some of that anyway even if you start with new matched batteries..they make a splitter if you want to do that...its nothing but a couple diodes...one thing to keep in mind though is that there is a voltage drop across a diode and that drop will increase as the current increases..that may or may not be important in your application.. marine battery terminals are deep enough to support both heavy wires...i agree with getting rid of the wing nuts if you have them...i have a cheap box end wrench that i leave with the batteries to check/tighten the nuts..they will also back off a tad and its part of the charging routine to check them...one reason for that is my batteries are hell to get to once the boat is in the water...

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My two cents for what it's worth.

Milkman00 said:

So I will be using a dual battery system. I will have each of the positive lines go to a transfer switch, so that is easy enough, but here is the problem.
Are you calling this a "transfer" switch?
Many will refer to these as an MBSS (main battery selector switch).
Most will place their Starting battery bank on #1, and their multiple Deep Cycle bank on #2.... although there is no official rule to be followed on this.
(reasoning: #1 is typically synonymous with 1 battery........ and #2 is typically synonymous with 2 or more batteries)

I am assuming that the proper way to wire the negative side is battery 2 negative to battery 1 negative to engine.
We typically want "redundancy" where either battery bank negative cable makes it's own individual "system negative common" connection..... (i.e., the engine block).
Redundancy means that neither knows that the other is there.

That said, is there a special terminal that supports two thick gauge battery cables on a single terminal???
This would not be advisable, even if such a terminal did exist.

I have had a hard time locating such a thing.
Yes, I think that this would be near impossible, unless you were to fabricate something.

Click to expand...

If this is a single engine boat, here's an example schematic of what works fairly well for many.
This is being shown while NO battery bank has been selected via the MBSS.

I'm a real stickler against any direct battery connections other than POS/NEG batt cables themselves!
You'll notice that I'm showing the O/B Charger using terminals #1 and #2 for it's connection, and I'm showing the bilge pump float switch also receiving power from the largest battery bank..... (i.e., the #2 terminal)
This keeps these smaller wire terminals from the corrosion of direct small terminal battery connections.
It also prevents the likelihood of mistakes when we R&R batteries.

Pay no attention to the shunt resistor, unless you plan to install an Amp Hour battery monitoring system.

kimcrwbr1 said:

The cables cannot be too big only too small.

Click to expand...

Amen to that!

And most definitely DO NOT use the run-of-the-mill battery cables from an Auto Parts Store.

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Here's an updated version of my schematic that reads a bit better.