Everyone’s favourite 1. 2. B switch

[steveeasy]

Just fitting a new control panal, well starting and got confussed trying to work out why I’ve got supply from both 1 and 2 for the existing switch Panels.

I was sure I should only have a supply from one of those the other being purely for the battery.

It seams not to be the case. On 1 I have domestic and engine and on 2 I have domestic and the engine. Is this correct ?. Looks incorrect to me.

I’ve an engine battery and domestic battery with the N terminals connected heading off to the engine. Then a positive coming back to the 1 2 b switch. See image. The positive from starter motor is at the bottom of 1 2 B switch. Where the domestic supply is taken from.

makes no sense to me!!

Steveeasy

 

[B27]

When you switch the 1-B-2 to 'Off', do the domestic circuits stop working?

Traditionally, all loads, both engine and domestic were connected to the 'switched' terminal of the 1b2
Alternatively it seems valid to connect the domestic loads always to the house battery, so the switch now has the function of A) deciding which battery starts the engine and b) linking the two batteries

 

[RupertW]

Just fitting a new control panal, well starting and got confussed trying to work out why I’ve got supply from both 1 and 2 for the existing switch Panels.

I was sure I should only have a supply from one of those the other being purely for the battery.

It seams not to be the case. On 1 I have domestic and engine and on 2 I have domestic and the engine. Is this correct ?. Looks incorrect to me.

I’ve an engine battery and domestic battery with the N terminals connected heading off to the engine. Then a positive coming back to the 1 2 b switch. See image. The positive from starter motor is at the bottom of 1 2 B switch. Where the domestic supply is taken from.

makes no sense to me!!

Steveeasy

In a 1:2:B setup there isn’t really an engine battery and a domestic battery which have different connections. Instead the switch is typically connected to everything from domestic load panel to engine start and alternator output. You are choosing between which battery bank is connected to all that and usually you have nominated which bank you want to use normally as engine and which as domestic and sized those accordingly.

So the you have two independent positives (usually) from each battery bank and one positive which goes to absolutely all loads and engine charging source. So you could run domestic off the “engine” bank if you choose - but that’s why 12Bs have fallen out of favour as all that manual switching and risk is a bit daffy and not at all foolproof.

 

[Refueler]

Basically - you should have -ve's common .... then

Starter battery +ve goes to Term 1 ... Domestic battery goes to Term 2 ..... power out goes to the third term.

The power out on the 1-2-B switch powers everything regardless of which battery position you choose.

 

[steveeasy]

When you switch the 1-B-2 to 'Off', do the domestic circuits stop working?

Traditionally, all loads, both engine and domestic were connected to the 'switched' terminal of the 1b2
Alternatively it seems valid to connect the domestic loads always to the house battery, so the switch now has the function of A) deciding which battery starts the engine and b) linking the two batteries

Yes.

Steveeasy

 

[steveeasy]

Bit. Consussed so I’ll leave it untouched. I can use the existing feed for the switch panel and get an electrical engineer to check what is currently in place.

Steveeasy

 

[QBhoy]

@Refueler has it spot on for sure. There is a common post, batt 1 post and batt 2 post. Usually the start battery will go to batt 1. Leisure go to batt 2 and common goes to the starter and in turn, the boats 12v auxiliary supply system. Having a starter battery and leisure battery (referring to two different battery types, high cca starting design and deep cycle leisure design respectively) linked together permanently to supply a combined permanent fixed wired supply is a quick way to do damage..to one or both of the batteries at least. They will be at the detriment of or to each other as a result. Linking together to emergency start is fine, I’d say…but I’m not sure I’d have it set up as she is now. Hope that helps a little perhaps.

 

[steveeasy]

Basically - you should have -ve's common .... then

Starter battery +ve goes to Term 1 ... Domestic battery goes to Term 2 ..... power out goes to the third term.

The power out on the 1-2-B switch powers everything regardless of which battery position you choose.

If you can see the image of the back of my switch. There are 3 terminals. The one at the bottom of the switch has the + from the starter attached to it with the + Aux feed to panel. The 2 terminals at top have the neg from batteries on one and the neg to engine earth. Does this make sense. Both engine and house batt neg are joined together parallel. Sound right ?

Steveeasy

 

[B27]

If your cabin lights go out when you switch the 1B2 to 'off' then they must be connected to the common terminal of the switch.
The other two terminals will each have a battery connected.

 

[VicS]

If you can see the image of the back of my switch. There are 3 terminals. The one at the bottom of the switch has the + from the starter attached to it with the + Aux feed to panel. The 2 terminals at top have the neg from batteries on one and the neg to engine earth. Does this make sense. Both engine and house batt neg are joined together parallel. Sound right ?

Steveeasy

No! It does not make sense.. It does not sound right at all.

BUT it is possible that your switch is wired in the negative supply and that the battery positives are commoned with all the positive supplies taken from that common connection

Refueler describes the normal wiring of a 1, 2, both switch in #4

A couple of diagrams which may help (Click to enlarge)
Negatives omitted from the first for clarity

The labelling above your switch indicates that your engine start battery is no 2 and the domestic battery is number 1 which is the opposite way round to the diagram from Guest

 

[steveeasy]

No! It does not make sense.. It does not sound right at all.

BUT it is possible that your switch is wired in the negative supply and that the battery positives are commoned with all the positive supplies taken from that common connection

Refueler describes the normal wiring of a 1, 2, both switch in #4

A couple of diagrams which may help (Click to enlarge)
Negatives omitted from the first for clarity

The labelling above your switch indicates that your engine start battery is no 2 and the domestic battery is number 1 which is the opposite way round to the diagram from Guest

View attachment 166965 View attachment 166966

Ok I’ll look at the above. Sat in total darkness now but I’ve still got a heater,at the moment.

This is what I’ve got

 

[steveeasy]

Ok both those diagrams are the same then and my wiring seams the same as the above so it seams it’s correct then.

Many thanks
Steveeasy

 

[VicS]

That's OK.
You have the battery positives connected to the switch, not the negatives as you said earlier

 

[pandos]

Alternatively it seems valid to connect the domestic loads always to the house battery, so the switch now has the function of A) deciding which battery starts the engine and b) linking the two batteries.

This is how I have mine wired, it's how my first boat was wired when I bought it. (I kept the actual switch)

I have added a key switch to the house supply which is wired directly to its own trip and onto the house batteries.
Once I turn this off everything is disconnected except the gas alarm and auto bilge pump which are wired together to the house battery via a fuse.

The alternator is permanently wired to splitter diodes and onto both banks, no fear that it can be disconnected whilst engine running.

Solar goes directly to the house bank.

Pretty fool proof...

Everything is close together, short wire runs, good sized cables so very little voltage losses.