Grounded alternator case, shunts and bonding

[nigelmercier]

I have 12V and 24V on board, with two 12V banks as normal, plus a 24V supply for the bow prop. The 12V alternator (which feeds both 12V banks via a Voltage Controlled Relay) has the casing connected to its negative terminal. As this is physically connected to the engine, this must be my bonding potential.

I also have a shunt for an ammeter (Cruz Pro VAH-65), which is connected to the negative of the house battery.

I'm now in a quandary as to where the various negative leads (battery, alternators, bonding) should go. I only need the ammeter to read the house bank (and certainly not the 24V) I think at present the 24V system is totally isolated, so this may be a pink fish.

Any ideas? Where is Nigel Calder when you need him /forums/images/graemlins/smile.gif

 

[lw395]

If the shunt must go in the negative lead, then one end of it goes directly to the battery, the other end is your chassis, common, 'gnd', engine. Only the ammeter sense wire needs to be connected between the battery -ve and the shunt, drawing any current from this node will fool the ammeter.

 

[saltwater_gypsy]

I have exactly the same system and all my negatives are bonded together and connected to the engine block.. The only problem I have had is from a bad negative starter connection when the circuit was then completed by current through the alternator negative. Smelly but no harm done

 

[nigelmercier]

[ QUOTE ]
If the shunt must go in the negative lead, then one end of it goes directly to the battery, the other end is your chassis, common, 'gnd', engine. Only the ammeter sense wire needs to be connected between the battery -ve and the shunt, drawing any current from this node will fool the ammeter.

[/ QUOTE ]

Diagram added for clarity:

But what about the 12V cranking battery: should I connect direct to house battery, or far side of shunt? If it goes on the far side of the shunt, won't it short out the shunt (with the internal resistance of the batteries) when the relay closes?

At present I think the 24V system is isolated, I'd like it also to be bonded. So again, connect direct to house battery, or far side of shunt?

 

[lw395]

The cranking battery should be grounded as directly as possible to the starter motor, in practise this may mean via the main negative bus or other fat cables shared with other circuits. You certainly do not want the return cranking current going through the shunt.
The 24V system can be isolated as shown, or ground to your 0V bus bar /gnd node. If isolated, then make sure it really is isolated, not grounded or connected elsewhere by low current relay commons, warning lights etc. Such lines could see a lot of current in a fault condition so maybe think about fuses carefully.
Hope that helps?

 

[nigelmercier]

[ QUOTE ]
At present I think the 24V system is isolated, I'd like it also to be bonded. So again, connect direct to house battery, or far side of shunt?

[/ QUOTE ]

I've done some further investigations since my last post. I couldn't figure out how the 24V rail was getting a reading on my volt display unit, as the neutral seemed to be isolated. Then I discovered that there was a second negative wire from the display unit to the 24V battery - so these two systems were joined by a wire of 0.22 sq mm (24 gauge)!!!

I think it would be a good idea to use a larger cable between the batteries! Under normal conditions, there would be no current flow, but what about a fault?

What suggestions on cable sizing from the team.