Battery Isolator

kordo

Any electrical experts out there?  Conventional wisdom has it that you should not turn off a battery if the engines are running because you will damage your alternator.  What happens in the following situation?  I have a port alternator and a starboard alternator connected to a battery isolator.  On the output side of the isolator, I have a port battery, a starboard battery and 2 house batteries.  What happens if the output wire to the house batteries comes off while the engines are running?  I think this is the same as turning off the battery switch.  Will this harm the alternators?  I don't think so since the isolator protects the current from going back into the other batteries so, the alternators would not know that the house batteries were disconnected.  Am I missing something?  Thanks.

Grahamu

It will not harm your alternators but the wire that becomes disconnected will be live and can cause a fire if it touches a ground point.

Dream_Inn

I agree with Grahamu, it will not harm the alternators, but it is something you need to caution with the loose wires.

Liberty44140

Yep I leave my battery switches on all summer and I have the battery charger on while running my engines and genny. No harm or issues 

kordo

Ok, if my alternators won't be harmed, then is there a problem with using 2 110ah lifep04 batteries as my house bank.  Other than the charging profile issue, if the bms (battery mgmt system on lifep04 batteries) disconnects them, my alternators should be fine.  Just fiddling with some low-cost ways of adding more battery capacity.  Thanks.

Grahamu

Your alternators will never know, the sense wire for them is connected to your engine batteries and their state determines what the alternators will put out. Assuming nobody has messed with the wiring.

kordo

Grahamu, do you happen to have a pic of the sense wires you referred to?  My boat is put away for the winter so I can't check for myself.  Thanks.

Grahamu

No I don't, But I doubt anyone would have changed the engine harness. I will look in the engine manual to see if there are any diagrams or pics.

Grahamu

It's the red/purple and purple wires.

kordo

Thanks.  Looking at the isolator side of the drawing, there must be another wire going to the cranking battery at location 1.  I'll take a good look at my boat in the Spring or maybe the winter to better understand my setup.  Thanks again.

YYZRC

Edit: @Grahamu is correct there was an additional connector. 

Grahamu

@YYZRC, you should also have a sense wire to control the output of the alternator

YYZRC

The connector that @Grahamu is referring to is top center.

Grahamu

@kordo,kordo said:

Thanks.  Looking at the isolator side of the drawing, there must be another wire going to the cranking battery at location 1.  I'll take a good look at my boat in the Spring or maybe the winter to better understand my setup.  Thanks again.

The wire from B1 (Isolator) to the starter relay (d) is the connection to the cranking battery as shown on the lower left side of the drawings.

kordo

I think we're saying the same thing although I don't get your comment regarding the lower left side of the drawings.  There is only one drawing attached.  Only page 4c-14 came through in the post.  Thanks.

Grahamu

There are 2 circuit diagrams within the drawing, I was referring to the left schematic as how the isolator is connected to your cranking battery 

kordo

Ok and not to belabor the point, but there is no isolator in the left schematic.  In any case I understand what you are saying.  Thanks.