Double-Wiring Electronics?

[Jonah]

Hi everyone,

As I mentioned in another thread recently, I have discovered that my previous owner left wires running from the DC panel to the starter batteries at the stern, but also to a house battery that was placed under the aft cabin. This is nice, because it enables me to choose from among the three batteries on my boat for the house electronics, if one should fail.

For now, of course, I have unplugged the wire to the starter, so that my electronics are wired only to the house. But just out of curiosity, I’m wondering what would happen if I were to leave the DC panel wired to both the house battery, and also to one of the two starter batteries? Would my DC electronics draw from both batteries simultaneously? Would the electronics draw from one, and then from the other? Would the system self-destruct and burn up my boat?

I don’t know the proper way to draw an electronic diagram, but the attached photo should illustrate what I mean. Note that the starter battery gets recharged by the engine, whereas the house battery stands alone.

It’s not that I think this seems like a good idea. I think it seems like a bad idea, but I am eager to learn WHY it would be a bad idea.

 

[ocnslr]

If you feed both house and starter to a common panel, without any switching, you are essentially putting those batteries in parallel. That's why it would be a BAD idea.

Lots of good books on basic 12vdc boat wiring. Google Nigel Calder books.

 

[glacierbaze]

https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&field-keywords=boat+wiring+book

 

[Jonah]

Got it, thanks both. Will look into the books you suggest.

For now, my understanding is that this arrangement (which you point out is effectively 'parallel') would not double the voltage. So, I take it that the 'bad idea' part is just that one battery would drain the other?

 

[SkunkBoat]

Yes, one will drain the other and/or they will not charge correctly.
You should not run different battery types in parallel except as a temporary emergency starting combination.
In the very simplest two battery circuit you need an isolator/VSR to connect the House to the charging circuit.
Normally you would also have a 1-2-both switch.

You have asked a lot of questions about batteries and still seem to be uncomfortable with the answers.
Forums like this are great tools but remember, we can't see what you are looking at.
I'm beginning to think your best direction would be to have someone with experience look at your boat and get you on the right path.

 

[Jonah]

Thanks Skunkboat. Yes, I have learned a lot on these threads, and still have some reading to do about wiring, etc. Hard to find reliable (and affordable) help here in Nassau, so am enjoying the opportunity to do more myself.

This last question was more theoretical than practical—just curious really. But I do now feel much more comfortable with my battery setup, as I mentioned at the end of the ‘dying battery’ thread.

Thanks again!

 

[Jonah]

p.s. The friend delivering batteries to me later this month has been working on boats for most of his life, and I'll be going over the wiring with him closely. Again, a good chance to learn more.