if you want to run all the batt's as one big unit then I would check into something like this:
http://www.balmar.net/centerfielder.html

it requires external regulators on the alt's but then you can run all the batt's in parallel and the centerfielder would regulate how the Alts work in the charging mode....
I would still have one battery on a switch you can separate from the rest to have as a backup..

I dont want to run ALL my battery's as one. just 2 (of the 4) for a house bank. the other 2 will be seprated, one for genny, and one for starbaord motor.

and I dont NEED to combine the 2 altenators on one bank, I just wanted to know if this could cause damage. and it can from the responses.

From your original question....yes...it will hurt the alt if you run both together charging at the same time.
As far as adding another battery, you can set it up in parallel.... but like I mentioned before...IF one of the batteries has a bad cell develop....it can damage the other battery along with it so you could be potentially out two batteries at once. Not that it happens all the time but the potential is there....I think a smarter way would be to use a battery isolator for charging so the alternator is charging one battery at a time as they need to be topped off....and then add a 4th on/off switch between this new battery and the original one so when you need full capacity you can turn it on.
It just all depends on how much you want to invest in the setup. If you don't plan on keeping the boat for any length of time then I would just add the new battery in parallel and go buy some Wallymart batteries and change them out if they go bad....they have a decent warranty exchange deal for their batteries

Not trying to be a pain here. but I can take this both ways.

So for clarification.


2 battery's in parallel on one motor/altenator. will hurt not hurt the altenator, but if one battery dies, its going to take out the other with it.

2 alternators/motors, charging the same battery will hurt the alternators.



OR

are you saying that 2 batterys in Parallel will hurt the one alternator charging them?

awesome reply Thanks for the clarification!

BTW, since you have the genny; it is tempting to run the genny when house batteries deplete and charge while still running the stereo.
This WILL burn up a battery charger. A buddy burns up a 40a charger every year running only a 700w Kicker amp. He gets it warrantied but that is pretty much abuse of the warranty because he is causing the failure. Most chargers try to use an ideal charge profile which is no condusive while a load is still on the battery bank.

A Power Supply like Iota or Kinetik can keep up with the draw and not burn up. http://www.iotaengineering.com/12vdc.htm
I know it was not the direct question but usually follows.........

That is a Very weird statement to me. As the boat is set up to use the battery charger with genny or Shore power. and while its charging you are using the boats electrical system, lights, stereo, vacuflush, what ever. the battery charger is what keeps the boat alive for ever with the systems running at the doc or what ever. so I think thats more of a VERY large draw in short bursts that the chager is not liking. he might could as a big Capacitor to help take the shock load off the battyer/charger.

If you put in one of these for your switching needs it will eliminate the possibility of a bad battery killing a good one.

http://www.bepmarine.com/home-mainme...lusters-manual

They have lots of configurations or you can put your own set up together. I have two of the single engine 2 battery bank clusters in my boat one for each motor and the house and starting batteries it charges. so if one of the banks gets below 13.4 or 13.8 volts it disconnects them, so they are only connected when being charge by and alternator or battery charger and you set it up so the starting battery is hooked up to the alternator so it always gets charged first. In practice they combine almost as soon as you start the engines at least for me. Ideally the start battery has no loads on it when off so you can never have a dead start battery and if for some reason it goes bad you can use the emergency parallel switch to start from the house battery.

I also have a blue sea systems remote switch which allows me to manually combine both house banks when I need the extra capacity when the engines are off.

http://www.bluesea.com/products/cate...ttery_Switches

So in my boat each alternator (80A) first charges its own start battery (group 27), then when it reaches the voltage setting of the voltage sensing relay (VSR) then it is also charging the two house batteries (group 24's).

For at the dock charging I have two dual bank chargers in the boat that charge all 4 banks at the same time, the VSR's do combine when they are on the charger but still works fine.

See I DON'T want it to cut off the extra battery, as stated, this was not my worry. I WANT the 2 in parallel to DIE TOGETHER. and charge together. like one big battery. all the time.

just wanted to make sure it was not a problem for the alternator.