Here is a diagram of what I am trying to do. I have a bank of 4 batteries that I want to be able to charge with the on board battery charger or the alternators. I have 2 55amp alternators.
Can I connect the alteranators together at the switch and charge the battery bank with out damagiing the alternators

They will fight each other if you run them together. In the end fry them working hard then not working at all. The volt regs. wont know which way is up. You reall should run 2 batts. together and alt to one and the same to the others.
-Mike

So what your saying is just connect 1 of the alternators to the selector switch.
I was thinking that I may be able to double the charging amps by connecting together.

Here is a pic of the battery bank for the stereo. I am wanting to be able to charge this with the genny when on shore, which is most of the time. When under power for a extended period of time I will need to switch it over to charge with the alternator.
I am running this battery bank separate from the 2 boat batteries.

IMO, charging that many batteries at once off the alternators will put too much strain on the alternators.

I agree if they have been run down. the capacity of the 4 batteries together is huge! run them down and 55a alts dont have a chance! My truck has 2 batteries and a 130a alt.

I have ran 4 batteries off of two 55 amp alt. for 5 + years now, No issues. I play the stereo all day,Put the jump box on and start the boat, and have no problem with them charging back up. the battery bank is the source so you don't have to run big alt., A car usually doesn't have the room to run 4 batteries, so you run a big alt. to keep up with the demand.

if they needed the capicity of more batteries.. i can guarantee you the engineers of any car company could find places in any vehicle for more batteries. running batteries until dead is very hard on them. you must have one hell of a jump box to get 4 batteries in parallel to gain enough to start the engines. i have replaced alternator wires on a boat that was wired by a very highly reccomended engine builder and rigger. the batteries went dead. we jumped it and the draw was so high that it burned the wires off. i wouldnt push it. my boat has (at times ) 2 batteries in parallel and i run a 90a alt.

Couple things. My primary style of boating is 5min to the sand/beach, sit most of the day and charge with the battery charger. If I am going for a cruise it is usually 15-20 min. to next stop and sit again for couple hrs.
I don't intend to run this battery bank dead. Stereo Amps require atleast 11 volts to run efficiantly. I'm not able to run the genny when cruising so I just thought I would allow the alternator to help replenish a little juice. If this is a bad thing for the alternator, no big deal. I realize 55amps is not much.
I have looked into rebuilding alternators for High Output, but thought I
would see how it works out before I spend the money.

I use this jewel -

http://www.atkinsonelectronics.com/p...fs/MBCM-12.pdf

look at application 1

[QUOTE=Fixxxer22;3111650] running batteries until dead is very hard on them.

That is why you run deep cycle batteries!!!!!! They are designed to be run all the way down and charged back up.