Alternator and charging questions!

[nicknorman]

26 minutes ago, DShK said:

My understanding was it was good practise to have the negative post grounded to the hull in case of something shorting to the hull - so it flows back to the battery - trying to minimize galvanic corrosion. In theory there should be nothing flowing through this, if nothing is awry?

 

 

 

B2B for the leisure battery end goes through the busbar - which goes through the shunt.

 

The alternator is naturally connected to the engine through it's casing I guess? I thought the engine was isolated from the hull because it sits on wood, but thinking about it, I think there are bolts that connect the engine to the hull.

 

So is it a mistake to ground the batteries to the hull then? I was under the impression this was good practise (while still using return cables for everything).

 

You need to have a single connection between battery negative and hull. Typically this will happen somewhere on the engine even if it only via the exhaust, prop shaft, control cables etc. Because the aforementioned things may be highish resistance, best practice is to have a dedicated cable from the engine block to the hull. But definitely nothing from the domestic battery negative to the hull because as I explained, it creates an alternative current path from engine negative to battery negative. This is bad on two counts, firstly it screws up the shunt reading (because current is bypassing the shunt) and secondly current is flowing through the hull which might be a source of corrosion.

So delete the direct connection from battery negative to hull and replace it with a connection from engine casing to hull.

 

Remember, with 2 or more connections to the hull a circuit is made. With 1 connection to the hull, there is no circuit. No circuit means no current can flow through the hull.

Edited June 1, 2023 by nicknorman

[David Mack]

37 minutes ago, DShK said:

I thought the engine was isolated from the hull because it sits on wood, but thinking about it, I think there are bolts that connect the engine to the hull.

The engine may also be connected electrically to the hull via the propshaft and stern gland, exhaust and control cables.

Edited June 1, 2023 by David Mack

[nicknorman]

By way of illustration, see diagram (yes I know I can’t draw!) current path 1 is the desired path, through the shunt. But the connections to hull via the prop shaft, exhaust and cables, plus your battery negative to hull wire, create a second current path 2. Which is bad. You can’t delete the the inherent connections between engine and hull so the only way to break circuit 2 is to remove the wire from battery negative to hull.

 

[DShK]

This makes sense! Funnily enough I only connected the hull to the batteries because the last installation did it. I wouldn't have done it otherwise. Then I read it was a thing somewhere. It makes sense that the alternator gives a route back to the batteries... I was just using it's casing as a negative point after all! I guess I can do a continuity check between the hull and alternator just to confirm.

 

I'll remove the earthing cable and see what the shunt says. Thanks for the help!

[nicknorman]

30 minutes ago, DShK said:

This makes sense! Funnily enough I only connected the hull to the batteries because the last installation did it. I wouldn't have done it otherwise. Then I read it was a thing somewhere. It makes sense that the alternator gives a route back to the batteries... I was just using it's casing as a negative point after all! I guess I can do a continuity check between the hull and alternator just to confirm.

 

I'll remove the earthing cable and see what the shunt says. Thanks for the help!

Well, ideally remove the cable from battery negative to hull, and install a cable between engine casing and hull. That way, the hull is properly connected to battery negative, not just relying on some possibly dodgy connections via exhaust etc.

[DShK]

Confirmed that this solved the issue! Thanks everyone, definitely learnt something from this.