Deciphering my wiring...

[yourmomm]

I'm trying to get to grips with the old wiring in my boat. There are two battery banks (isolated from each other, with completely separate circuits on each). They seem to have earthed one battery bank to the engine block, but for the other, they've just run a connection from its negative terminal to the negative terminal of the battery which is earthed.

Is this is a thing? Or would it be better to earth the second battery bank directly to the engine block, with a new run of 35mm cable?

 

[yourmomm]

I should add that there is no positive connection between the battery banks at all.

 

[pvb]

Doesn't really matter, as long as the cable is appropriately sized.

But why do you have two completely separate battery banks?

 

[VicS]

It is uaual to link the battery negatives.

Use the same size cable as the connection from what is presumably the engine start battery to the engine block. Then it is fit for starter loads if you use the second battery ( presumably the domestic services battery ?) for emergency engine starting

 

[yourmomm]

Doesn't really matter, as long as the cable is appropriately sized.

But why do you have two completely separate battery banks?

Honestly, this is the least of my worries on this boat. I've been working on it for a solid year, and it's still nowhere near seaworthy. I can only imagine that this is the way things were done in 1972...but i do want to install a "off", "1", "2", "both" battery switch, and was wondering whether the joined negatives would impact upon this installation. I'm going to rewire the whole thing at some point, but, right now, given all the other jobs I need to do, I just want to make sure it's safe (it had dead batteries for past year, and I've just bought 3x120ah AGM's, which I dont want to wreck with a dodgy, and/or incorrectly wired electrical system).

 

[yourmomm]

It is uaual to link the battery negatives.

Use the same size cable as the connection from what is presumably the engine start battery to the engine block. Then it is fit for starter loads if you use the second battery ( presumably the domestic services battery ?) for emergency engine starting

Yep it's all appropriately sized. All batteries were dead. I've replaced the house batteries with 3 x 120ah AGM's, but the engine battery is still dead (cant afford to replace after buying the agm's!), and I dont want it asserting any parasitic load on my nice new agm's...

 

[pvb]

Honestly, this is the least of my worries on this boat. I've been working on it for a solid year, and it's still nowhere near seaworthy. I can only imagine that this is the way things were done in 1972...but i do want to install a "off", "1", "2", "both" battery switch, and was wondering whether the joined negatives would impact upon this installation. I'm going to rewire the whole thing at some point, but, right now, given all the other jobs I need to do, I just want to make sure it's safe (it had dead batteries for past year, and I've just bought 3x120ah AGM's, which I dont want to wreck with a dodgy, and/or incorrectly wired electrical system).

Please reconsider; the 1-2-both switch is pure 1970s, and there are much better solutions today. Most importantly, these solutions don't risk you ending up with a flat starting battery, which the 1-2-both switch does. Look instead at having a simple on-off switch for each bank, and a VSR or low-loss splitter to isolate the banks. Much easier to use, and fail-safe.

 

[Refueler]

deleted

 

[JumbleDuck]

But why do you have two completely separate battery banks?

Why not? I have separate house and engine circuits, with joined earths, and I gather that it's not uncommon.

 

[pvb]

Why not? I have separate house and engine circuits, with joined earths, and I gather that it's not uncommon.

You have a bank of batteries for your engine?

 

[VicS]

Honestly, this is the least of my worries on this boat. I've been working on it for a solid year, and it's still nowhere near seaworthy. I can only imagine that this is the way things were done in 1972...but i do want to install a "off", "1", "2", "both" battery switch, and was wondering whether the joined negatives would impact upon this installation. I'm going to rewire the whole thing at some point, but, right now, given all the other jobs I need to do, I just want to make sure it's safe (it had dead batteries for past year, and I've just bought 3x120ah AGM's, which I dont want to wreck with a dodgy, and/or incorrectly wired electrical system).

No problem with the joined negatives if you have a 1,2 both switch

If you have a 1,2 both switch at present keep it.

BUT

If you are completely rewiring fit separate isolation switches to the two battery banks... (The new AGMs feeding all the domestic stuff and an ordinary starter battery for engine starting)
Add an emergency linking switch between the switched sides of the two isolators and you have a good reliable and versatile system with provision for emergency engine starting. This system will also be more appropriate if you add any other battery charging systems ( mains charger, solar etc) and a VSR to automatically keep both battery banks charged without manual intervention

 

[Refueler]

Why not? I have separate house and engine circuits, with joined earths, and I gather that it's not uncommon.

Ditto ... and a 1-2-off switch ...

 

[yourmomm]

No problem with the joined negatives if you have a 1,2 both switch

If you have a 1,2 both switch at present keep it.

BUT

If you are completely rewiring fit separate isolation switches to the two battery banks... (The new AGMs feeding all the domestic stuff and an ordinary starter battery for engine starting)
Add an emergency linking switch between the switched sides of the two isolators and you have a good reliable and versatile system with provision for emergency engine starting. This system will also be more appropriate if you add any other battery charging systems ( mains charger, solar etc) and a VSR to automatically keep both battery banks charged without manual intervention

Bingo! This is EXACTLY what I was considering. I've already got solar and a battery charger (powered by a petrol generator) hooked in to system, with isolation switches already installed on both house and engine start battery circuits. But I wanted to add an "off", "1", "2", "both" switch, upstream of the isolation switches, which could control where the solar regulator and/or battery charger directed their charge, even when the main isolation switches were turned off. I may add a VSR later, but I'm naturally suspicious of anything automatic, preferring manual control at all times

 

[VicS]

Bingo! This is EXACTLY what I was considering. I've already got solar and a battery charger (powered by a petrol generator) hooked in to system, with isolation switches already installed on both house and engine start battery circuits. But I wanted to add an "off", "1", "2", "both" switch, upstream of the isolation switches, which could control where the solar regulator and/or battery charger directed their charge, even when the main isolation switches were turned off. I may add a VSR later, but I'm naturally suspicious of anything automatic, preferring manual control at all times

No. Fit the a two way VSR. Connect the alternator output to the engine start battery and the battery charger, solar etc to the house battery. Then your engine will recharge charge its own start battery as first priority and the others will charge the house bank as their first priority

Consider a Victron Cyrix rather than a bog standard VSR Google for the details and the manual and have a read before splashing the cash

If you want to add a bit of manual control put a switch in the negative connection to the VSR then you can open that to disable it. so that the battery charger and/or solar wont continue to charge an already fully charged starter battery
P Rainbow usually suggests leaving out the "start assist" switch but that's your choice

 

[PaulRainbow]

No. Fit the a two way VSR. Connect the alternator output to the engine start battery and the battery charger, solar etc to the house battery. Then your engine will recharge charge its own start battery as first priority and the others will charge the house bank as their first priority

Consider a Victron Cyrix rather than a bog standard VSR Google for the details and the manual and have a read before splashing the cash

If you want to add a bit of manual control put a switch in the negative connection to the VSR then you can open that to disable it. so that the battery charger and/or solar wont continue to charge an already fully charged starter battery
P Rainbow usually suggests leaving out the "start assist" switch but that's your choice

Good advice on the charge wiring and choice of VSR Vic.

The reason i prefer not to connect he start assist wire is that i'd rather fit a 3rd switch for emergency use. As you know, this not only acts as an emergency parallel switch, bit it also allows a faulty battery/battery bank to be isolated and to run all circuits from the good bank. Such a switch renders the start assist function redundant.

 

[JumbleDuck]

You have a bank of batteries for your engine?

A bank of one, yes. We are not at home to Mr Picky.

No. Fit the a two way VSR. Connect the alternator output to the engine start battery and the battery charger, solar etc to the house battery. Then your engine will recharge charge its own start battery as first priority and the others will charge the house bank as their first priority

Or alternatively fit a twin-output solar controller. Mine is set to direct 90% of its efforts to the house batteries and 10% to the engine one,

If you want to add a bit of manual control put a switch in the negative connection to the VSR then you can open that to disable it. so that the battery charger and/or solar wont continue to charge an already fully charged starter battery

I'm not sure I see the point of that. Any decent charger or controller can't force a battery to take more charge than it wants. In my case the VSR earth is switched by relay so that it only operates when the engine is running, but that was really just to avoid confusing the solar regulator, and I don't know if it was strictly necessary.

The reason i prefer not to connect he start assist wire is that i'd rather fit a 3rd switch for emergency use. As you know, this not only acts as an emergency parallel switch, bit it also allows a faulty battery/battery bank to be isolated and to run all circuits from the good bank. Such a switch renders the start assist function redundant.

I went for a BlueSea dual circuit + combine switch. My batteries all have individual thermal breakers on them, which can be popped to disconnect duds from the system.

 

[PaulRainbow]

I went for a BlueSea dual circuit + combine switch. My batteries all have individual thermal breakers on them, which can be popped to disconnect duds from the system.

It's a good switch. It does miss out on a couple of points over separate switches, such as faulty battery disconnecting, but you have that covered.