Common negative busbar for multiple batteries not wired in series or parallel?

[fergycool]

This is my first post on any boat forum so be gentle…..!

I’ve recently acquired my first boat (with an engine, or two) and I’m acquainting myself with its workings!

The boat has twin engines with a starter battery each along with a single domestic battery.

The wiring is a bit of a hotchpotch with any old colour used so I’m having a difficult time making a wiring diagram (which of course does not exist!).

The -VE side of all three batteries appear to be connected. These are not wired in parallel as the +VE side is completely separate. However, if I take a reading with a voltmeter across the +VE post of each battery to the -VE busbar in the distribution panel I see 12/14 volts with the correct polarity.

I understand electrically that this is fine. However, in terms of best practice of boats is this correct?

I understand that the earth of the 240v AC circuit should be connected to the -VE of the DC circuit. Is it correct practice to bond the -VE of the starter batteries as well, or with a common negative is it assumed that this is the case? I guess that if the -VE of all three batteries are bonded to AC earth then they will be connected to a common -VE. However, whereas I understand the safety aspect of bonding the -VE of the domestic 12v system to the earth of the 240V AC system (to provide a route to earth if any 240v current crosses to the 12v circuit) I do not understand the need to bond the -VE of the starter batteries as no 240V AC circuit would be anywhere near there!

Incidentally it’s a simple AC circuit with 3 x 13 amp sockets from a RCD protected 16A breaker. The earth is broken between the shoreside input and the earth bonding point in the breaker with a galvanic isolator. The steel hull is also bonded from the boat side of the galvanic isolator.

I am working my way through the Nigel Calder book (cracking read) but this point is one that I’m still unsure about. Google has proven elusive on a definitive answer and so I ask here?

Any advice would be gratefully received!

Cheers
Chris

 

[kashurst]

not easy to answer specifically as everyboat is a little different. So more clues than answers
each engine has its own starter battery
the domestic battery will probably be connected to either the port or starboard alternator via a split diode charging pack or a three way switch so it can be connected/disconnected. This is often done to reduce the chances of flattening the starter batteries if you leave something domestic switched on. To determine which alternator is charging the domestic battery, start one of your engines and watch the starter battery voltage rise to@ 14.5 ish Volts. If the domestic battery voltage is rising too then its charged via that engine. Try the same with the other engine. You may find the domestic battery can be charged from either engine or from just one.
Just a note of caution - don't disconnect any batteries whilst the engines are running - you will wreck the alternators.
If the domestic battery voltage doesn't change at all, look for a big round battery switch somewhere on the boat that allows you to link the domestic battery into the charging system.

the AC supply earth is usually connected to the boat -ve (and battery -ves) so any fault can earth to water. It sounds like you have a galvanic isolator fitted so the supply earth goes to one side of the GI and the other side of the GI to the -ve. The GI protects your steel boat from stray electrical currents in the water. This stops the steel from dissolving away, and your anodes will last a lot longer too.

 

[Forty_Two]

Yes it would be normal to have a common 12v negative. Allows charging or consumers to be used with any battery by just switching the positives as required. Charging of several battery banks from a single source is common using diode splitters or relays.

 

[fergycool]

Many thanks for both of your answers. They are helpful.

That's useful about the charging of the domestic from the alternator. It's not something I've yet considered. From what I can now gather the boat has a relay to a single engine's alternator for the domestic battery.

 

[fergycool]

Apologies I thought that I had replied to thank both of you. I had not. Both of your replies were very helpful.

The domestic battery is charged via a relay (it had never even occurred to me to check how it was charged other than the 240v AC charger). It's only by the port engine.

The -VE side is a complete mess really with multiple wires twisted together and jammed into a small connector. I've bought a proper -VE busbar and I think my first step is to rewire the -VE returns to that properly. All wires are labelled now. As far as I can tell there is no cable from the -VE to the 240v earth. I'm unsure as to whether this should be to the same bonding point on the hull or to the earth bonding point inside the 240v AC breaker. I'd appreciate help on the point.

Thanks!