Battery wiring question

[Moke]

I’m planning on adding a shunt into my leisure bank wiring but looking at it the other day it seems to be set up as in the “picture” below. The starter battery (charged from its own alternator has a negative that’s connected to the leisure bank. Will this setup work ? Or should the negative return to the starter battery with no connection to the leisure bank?

    Please ignore the shunt, I just used the picture as it’s easier than trying to explain or draw my own picture

 

[David Mack]

The negatives of both the starter system and the domestic system should be grounded to the boat shell. In your diagram the starter battery should also have a negative cable going to the negative terminal or a mounting bolt of the starter motor (with the engine block then grounded to the hull, especially if it is on rubber mounts). The grounding of the domestic system is then achieved by linking the two battery negatives. Not the only way of doing it but looks fine to me.

[Tony Brooks]

What David Mack said. Without seeing the way the engine electrics negative is or proposed to be wired, one can't be sure you will not put the starter current through the shunt, which is not a very good idea.

[nicknorman]

11 minutes ago, Moke said:

I’m planning on adding a shunt into my leisure bank wiring but looking at it the other day it seems to be set up as in the “picture” below. The starter battery (charged from its own alternator has a negative that’s connected to the leisure bank. Will this setup work ? Or should the negative return to the starter battery with no connection to the leisure bank?

    Please ignore the shunt, I just used the picture as it’s easier than trying to explain or draw my own picture

 

You must have only one thing connected to the leisure battery negative, and that is the battery side of the shunt. The load side of the shunt is connected to the starter battery negative, the boat systems negative (for lights, pumps etc) and a single connection to hull. A connection to hull is likely to already exist via the engine (via exhaust, control cables, prop shaft etc) so in order to avoid current paths through the hull, just make a good quality connection between engine and hull.

 

That is, unless the engine is isolated from starter negative, alternator negative (unlikely, but possible) when you can make a connection from eg starter battery negative to hull.

8 minutes ago, David Mack said:

The negatives of both the starter system and the domestic system should be grounded to the boat shell. In your diagram the starter battery should also have a negative cable going to the negative terminal or a mounting bolt of the starter motor (with the engine block then grounded to the hull, especially if it is on rubber mounts). The grounding of the domestic system is then achieved by linking the two battery negatives. Not the only way of doing it but looks fine to me.

No sorry this is wrong. If you link the battery negatives directly the shunt will not work properly. If you have multiple negative connections to hull, this creates a current path through the hull which can cause corrosion.

[Moke]

As I say, the picture is somewhat of a simplified version of reality. But I would have thought the starter battery earth should return directly to the starter battery and have no connection to the other batteries

[nicknorman]

10 minutes ago, Moke said:

As I say, the picture is somewhat of a simplified version of reality. But I would have thought the starter battery earth should return directly to the starter battery and have no connection to the other batteries

That is one way of doing it, but topologically it is exactly the same whether you have one connection to the engine going to the starter battery and with the shunt connected between starter and leisure battery, vs having one wire from shunt load side to engine and another wire from starter battery negative to engine. You just physically move the point where the two negatives are connected without any effect on the circuit. The two negatives are now connected at the engine instead of being connected at the shunt load side.

Edited February 21 by nicknorman

[David Mack]

10 minutes ago, nicknorman said:

No sorry this is wrong. If you link the battery negatives directly the shunt will not work properly. If you have multiple negative connections to hull, this creates a current path through the hull which can cause corrosion.

If all the domestic charging and loads go through the shunt, it shouldn't matter which side of the shunt is connected to the starter battery negative and from there to the hull ground. The only difference will be that the potential of the domestic battery negative terminal will be a few mV different in the two cases. The rest of the domestic system should be fully isolated from the hull, so where could any additional negative connections to the hull arise?

[nicknorman]

2 minutes ago, David Mack said:

If all the domestic charging and loads go through the shunt, it shouldn't matter which side of the shunt is connected to the starter battery negative and from there to the hull ground. The only difference will be that the potential of the domestic battery negative terminal will be a few mV different in the two cases. The rest of the domestic system should be fully isolated from the hull, so where could any additional negative connections to the hull arise?

No. It creates an alternative current path for the domestic negative so the shunt only registers about 1/2 the charge current but all the discharge current.

[Moke]

I’ve finally worked out how to draw so,   The picture below shows what’s actually there if that’s any clearer, it just strikes me (probably wrongly) that if the shunt were placed between the junction box and the leisure battery then wouldn’t the starter be caught up in the readings?

[dmr]

The rule is that there should be one thick wire going from the "battery" terminal of the shunt directly to the domestic battery negative, and Nothing else should connect to this, nothing at all. Hence no matter what is going on elewhere you can be sure that ALL of the current from the domestic batteries is going through the shunt.

 

.....but for the pedants, there is just one exception, if you have a Smartgage or anything else that needs to accurately sense battery voltage then this can connect to the batery terminal. These devices only take very small currents so bypassing the shunt is acceptable.

 

and a negative busbar is of course fine as long as nothing else connects to it 😀

[Moke]

So this is how it should be?

[David Mack]

Deleted.

Edited February 21 by David Mack

[nicknorman]

23 minutes ago, Moke said:

I’ve finally worked out how to draw so,   The picture below shows what’s actually there if that’s any clearer, it just strikes me (probably wrongly) that if the shunt were placed between the junction box and the leisure battery then wouldn’t the starter be caught up in the readings?

I’m on a ferry from Amsterdam to uk and about to lose signal, but just to clarify, connections to be as follows:

 

From leisure battery to shunt (battery side). From shunt (load side) to engine/alternator negative - either via a connection to starter negative and thence on to engine/alternator negative, or direct to engine/alternator negative (both these options are the same from a circuit point of view. Nothing. Nothing. NOTHING  to be connected to leisure battery negative, apart from the shunt.

Just now, David Mack said:

How? The OP refers to the domestic battery (LA) having its own alternator, and his diagram shows the Li domestic battery as having its own charging (presumably including a second alternator). So no charging current (for either battery) will pass through the lead connecting the battery negatives so there can't be any impact on the current through the shunt.

The alternator negatives are connected at the engine, so some of the leisure alternator negative current will flow via the shunt, some of the leisure alternator negative current will flow direct via the starter battery negative lead bypassing the shunt.

7 minutes ago, Moke said:

So this is how it should be?

Yes. Somewhere in your”junction box” you need a lead to the engine casing (alternator negative). From the engine casing a short lead to the hull.

[blackrose]

1 hour ago, Moke said:

As I say, the picture is somewhat of a simplified version of reality. But I would have thought the starter battery earth should return directly to the starter battery and have no connection to the other batteries

 

I'm pretty crap when it comes to electrics but the negatives of my domestic and start batteries are connected and I understood that this was pretty normal. I also have a shunt on the negative side of my domestic bank for a battery monitor and it works fine.

[David Mack]

1 hour ago, blackrose said:

 

I'm pretty crap when it comes to electrics but the negatives of my domestic and start batteries are connected and I understood that this was pretty normal. I also have a shunt on the negative side of my domestic bank for a battery monitor and it works fine.

Do you have one or two alternators?

[Tony Brooks]

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.

[dmr]

1 hour ago, blackrose said:

 

I'm pretty crap when it comes to electrics but the negatives of my domestic and start batteries are connected and I understood that this was pretty normal. I also have a shunt on the negative side of my domestic bank for a battery monitor and it works fine.

As long as there is no connection between domestic battery negative and the boat hull (or engine) then it will be ok. Trouble is most people reckon its a good idea to bond the negative to the hull (at one location only).  So, your system MIGHT work, but if you do it properly it WILL work.

[GUMPY]

Connect nothing apart from the shunt to the battery, smartgauge excepted.

 

This is how  Loddon was wired with two alternators and two ammeters with shunts. The shunts were big enough to deal with the Beta starter motor, 300a

 

 

12V NEW 2_1.pdf

 

 

Edited February 21 by GUMPY
Spurious letter

[Tony Brooks]

15 minutes ago, dmr said:

As long as there is no connection between domestic battery negative and the boat hull (or engine) then it will be ok. Trouble is most people reckon its a good idea to bond the negative to the hull (at one location only).  So, your system MIGHT work, but if you do it properly it WILL work.

 

So with no domestic negative hull bond, how will a domestic fuse blow if a domestic positive shorts to the hull?

[dmr]

4 minutes ago, Tony Brooks said:

 

So with no domestic negative hull bond, how will a domestic fuse blow if a domestic positive shorts to the hull?

 

It won't blow, thats likely why people think its a good idea to bond the negative to the hull.

So, the starter negative must bond to the engine as it takes the starter current, it should also bond to the hull because it probably does anyway by the exhaust or whatever. The domestic negative bonds to the hull for improved safety, but this must be done from the "non battery" side of the shunt otherwise the alternator current will not go through the shunt (unless you have an isolated ground alternator but thats a whole new discussion.)

[MtB]

9 minutes ago, Tony Brooks said:

 

So with no domestic negative hull bond, how will a domestic fuse blow if a domestic positive shorts to the hull?

 

Would it matter? The hull would rise to 12Vdc potential I suppose...

[Tony Brooks]

3 minutes ago, MtB said:

 

Would it matter? The hull would rise to 12Vdc potential I suppose...

 

Then there is the POSSIBILITY of electrical corrosion.

[dmr]

Also it means that something bad has happened, maybe a wire goes through the hull without a proper gland or whatever and has chaffed. Its better to blow a fuse than for a latent fault to remain undetected.

[David Mack]

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.

[Moke]

2 minutes 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.

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 ?