Battery wiring question

[nicknorman]

10 hours ago, Moke said:

If I understand you correctly then a shunt in the cable between the junction box and the first leisure battery (as in the first drawing) would work ?

No it wouldn’t work, because there is likely an alternative current path from the starter negative lead that bypasses the shunt. If there isn’t such a lead and only one negative cable on the load side of the shunt, then all the starter current would pass through the shunt. Hundreds of amps. The shunt is there to track only the leisure battery state of charge, stuff happening with the starter battery needs to be kept separate.

12 hours ago, Tony Brooks said:

I don't think that matters re hull negative bonding because if you only bond one bank a short to negative on the other won't blow the relevant fuse and you end up with a live hull with the attendant danger of corrosion. I think both banks need bonding to the hull but the engine negative needs to join at the battery side on the shunt. Thus, the start current bypasses the shunts, but all domestic negatives pass through it.

You just want one bonding point to hull, otherwise you create circuit paths through the hull. Because the engine is already probably connected to hull via the exhaust, prop shaft, control cables etc it is best to “properly” connect the engine to hull via a short lead. At the engine, the leisure and engine starter battery negatives are intrinsically connected together so any positive short from either starter or leisure battery will blow a fuse (if there is one).

10 hours ago, David Mack said:

Thinking further on this, if you have separate engine and domestic alternators and both have the negative connection through the alternator body and thus connected to the engine block, as is normal, the battery-battery negative link should be connected to the load side of the shunt, as otherwise whenever there is a current flowing through the shunt (in either direction) there will be a few mV potential difference in the circuit comprising the alternator-battery negative cables, the engine block, the battery-battery negative link and the shunt, and that will cause some small stray currents in the cables (but not in the hull unless there is an unintended connection somewhere), although whether that current is big enough to cause any problems I can't say.

Yes this is more or less correct. If there are 2 possible current paths, one via a fat wire from engine to starter battery negative (connected direct to leisure negative) and one fat wire from engine to leisure battery negative via a shunt, the latter will have more resistance (the shunt) and so more than half the leisure alternator current will bypass the shunt. Meanwhile all the leisure battery discharge current will go through the shunt. Net result being that the indicated SoC decreases faster to zero after a short time, or bounces around between zero and 20% or somesuch.

 

A friend’s boat was “professionally” wired like this when they got a battery monitor fitted. It didn’t work, as the indicated SoC was always extremely low no matter how much they charged it - for the reasons mentioned above. I was summoned to correct the wiring error made by the “professional” and the system now works properly.

[blackrose]

14 hours ago, David Mack said:

Do you have one or two alternators?

 

Two.

 

And one DC bonding point to the hull. I think that's connected to the engine block somewhere...

 

And my shunt battery monitor works fine.

Edited February 22 by blackrose

[jonathanA]

I was just reading this thread this morning and had to draw a numpty diagram form myself, so apologies for the p-poor drawing and the forum software rotating it...

 

but if you look at the starter /leisure negative connection you can see if you move it to other side of the Shunt then the point Nick /David are making about the 2nd alternator bypassing the shunt is obvious.  i think it really doesn't matter too much where you put the hull bond so long as you DONT put it on the Leisure negative terminal directly in this situation.

 

Blackrose if your negs really are connected directly then you have the situation Nick describes he had to sort after the professional. maybe you had the same professional 😀  are you sure they aren't connected on the load side of the shunt ?

 

hope that helps. 

 

 

[blackrose]

24 minutes ago, jonathanA said:

Blackrose if your negs really are connected directly then you have the situation Nick describes he had to sort after the professional. maybe you had the same professional 😀  are you sure they aren't connected on the load side of the shunt ?

 

They're definitely connected directly, on the battery side of the shunt. They've been like that for 18 years. I've read posts from electrical experts on the forum before saying this is normal. But I don't think those threads necessarily related to shunts, just people asking if it was normal for start and domestic battery banks to have the negatives connected, and the answer was always yes. I'm not sure why this thread is coming up with a different answer?

 

Here's an example. I've posted a screenshot rather than a link to a thread to highlight the post. It's just an example but there must be hundreds of similar posts on electrical threads in the forum archives.

 

Edited February 22 by blackrose

[jonathanA]

I was just trying to help,  by putting some of the words into  a simple diagram, which helped me so thought it might help others. no where in this thread does anyone say don't connect the negatives together, what is being said is if you are fitting a shunt to monitor the leisure/domestics then you need to think about where the shunt is positioned electrically. 

 

I don't know your boat specifics, it could be for example that your alternator(s) are insulated return, or I think on some betas the whole engine is insulated return,  which would change things a bit. 

 

if it ain't broke don't fix it... 

36 minutes ago, blackrose said:

 

They're definitely connected directly, on the battery side of the shunt. They've been like that for 18 years. I've read posts from electrical experts on the forum before saying this is normal. But I don't think those threads necessarily related to shunts, just people asking if it was normal for start and domestic battery banks to have the negatives connected, and the answer was always yes. I'm not sure why this thread is coming up with a different answer?

 

Here's an example. I've posted a screenshot rather than a link to a thread to highlight the post. It's just an example but there must be hundreds of similar posts on electrical threads in the forum archives.

 

Edited February 22 by jonathanA

[blackrose]

45 minutes ago, jonathanA said:

I was just trying to help,  by putting some of the words into  a simple diagram, which helped me so thought it might help others. no where in this thread does anyone say don't connect the negatives together, what is being said is if you are fitting a shunt to monitor the leisure/domestics then you need to think about where the shunt is positioned electrically. 

 

I don't know your boat specifics, it could be for example that your alternator(s) are insulated return, or I think on some betas the whole engine is insulated return,  which would change things a bit. 

 

if it ain't broke don't fix it... 

 

Ok thanks, apologies if my last response sounded like I was being dismissive. I wasn't. My 12v electrical knowledge isn't great and I accept what you say.

 

My battery monitor works fine so I guess the shunt must have been fitted correctly.

 

Edit: your diagram puzzles me a bit. I assume that's the shunt at the top (or right hand side)? I'm sure on mine it's the negative that runs through the shunt and not the positive side.

Edited February 22 by blackrose

[jonathanA]

3 hours ago, blackrose said:

Edit: your diagram puzzles me a bit. I assume that's the shunt at the top (or right hand side)? I'm sure on mine it's the negative that runs through the shunt and not the positive side.

my diagram ?  top right is the engine, two alternators (AS /AL) shunt is not marked  but has the 'load' connected to the top or right and is in the neg ?,  S is starter batt and L is leisure.  Earth symbol hull bond. 

 

 

[blackrose]

1 hour ago, jonathanA said:

my diagram ?  top right is the engine, two alternators (AS /AL) shunt is not marked  but has the 'load' connected to the top or right and is in the neg ?,  S is starter batt and L is leisure.  Earth symbol hull bond. 

 

Ah, ok I see.

[ditchcrawler]

Sorry its late, I have a lot of posts to catch up with. here is a sketch I posted some time ago.

[Moke]

Thanks for that. I think I’m beginning to see the light at the end of the tunnel now.