Can you use 3 dual battery switches to run 4 batteries?
 

I have 4 batteries. Two for the port motor and two for the starboard motor. There are two dual battery switches, one for the two port batt's and one for the two starboard batt's. Furthermore, all the accesories (lights, stereo, etc.) are wired directly to the port battery switch.

Due to the way the system is currently wired, there is no ability for the port batteries to start the starboard engine if necessary (or vice versa), and no ability for the accessoreis (lights, stereo, etc.) to run off the starboard batt's.

This doesn't make any sense to me. What I was hoping to do is install a third dual battery switch that selects between the current port and starboard battery switches. This would then allow the current port and starboard batt switches to continue to select the individual battery (or both) for each side. I would think this would allow me to start either motor or run the accessories off of any of the four batteries (or any combination of the four batteries).

However, I'm not an electrical guru, and am not sure if this setup will work or if it will cause damage to the alternators or any other portion of the electrical system.

Any advice would be greatly appreciated.

Re: Can you use 3 dual battery switches to run 4 batteries?
 

I believe that you will need 2 switches per side to do what you want. The 1st switch will be to select battery 1 or 2. The output from this switch will feed switch 2. Switch 2 will allow you selective outputs to either port or starboard engine. You will just duplcate these switches and wiring for the other side.

Re: Can you use 3 dual battery switches to run 4 batteries?
 

Yes you can make that idea work....you would need just a simple on off switch like the kind you would have for a single battery setup. You would run an extra battery cables from the output posts of the two original switches you allready have in the boat and hook up to the on/off switch. This way what ever battery you select on a certain side will go to the output side of the other side. Make sense or did i confuse you?

Re: Can you use 3 dual battery switches to run 4 batteries?
 

Viper7/Wally - Thanks for the advice. I think both are great ideas.

Wally - I think your idea to use the added on/off switch as kind of a jumper between the two current switches sounds the easiest. Do you think I would have to rewire anything else? Would there be any alternator problems with this third switch being in the on position while underway? Appreciate your thoughts.

Re: Can you use 3 dual battery switches to run 4 batteries?
 

Keep and eye on total active path length for your positive side with that many switches. Over time the switches can develop an internal resistance that will lead to hard starting. What gauge wire will you be using?

Re: Can you use 3 dual battery switches to run 4 batteries?
 

If you are strapping (two) dual battery circuits together, then you should use a "diesel size" on/off switch between the outputs of the two A/B switches. I assume that this wire will only be 5 or 6 feet long, so use one size heavier than your existing "switch to motor" cables.

There should be no weirdness in that setup, except that you will be able to run ALL 4 batteries dead simultaneously if you want to.

If you only want to charge and use the batteries that are switched ON, then you don't need fancy alternator tricks or isolators. They will charge and discharge together when switched on. One of your alternators will have its voltage regulator set a teeny bit higher than the other. This will allow one alternator to drop out of the circuit when the batteries are near proper float voltage. This is fine, unless you accidentally happen to have voltage regulators that are almost identical in float voltage setting (there's usually a couple tenths of a volt difference which is enough). If they are "too close" then they will sort of fight each other at the "charged" end of the scale. I this is the case, you can simply buy one adjustable regulator and set one slightly lower than the other.

If you want to be able to charge batteries that are "OFF", then you will need an isolator. This allows the charging system to maintain the battery(s) that are out of the loop.