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What happens is that the domestic and starter batteries are permanently connected together, ie there Is now just one battery.

 

Why it happens is usually because the contacts are welded together from passing too much current with some arcing and or melting.

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Thanks.

 

so what would be a quick fix this evening to stop my domestic batteries running down my starter.

 

what wire would I unhook?

would removing the spade terminals from the relay be enough to break the circuit?

 

I don’t really understand what’s  going where on this relay, but it’s something I’ve been looking at because I was about to up to a 100A from a 60 to allow for a larger alternator. 

Or would simply turning the isolator off on my starter do the job?

 

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3 minutes ago, Goliath said:

Or would simply turning the isolator off on my starter do the job?

 

That depends on if it is strictly RCD compliant, many aren't.

 

Take off either of the large blade or stud terminals, but immediately insulate the free terminal.  Or take the negative starter battery terminal and wires off.

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10 minutes ago, Goliath said:

Thanks it’s all I need when I’ve settled for the evening.

 

am I understanding correctly just taking off one of the spade terminals will do the job?

 

Unlikely unless the terminal is one of the bigger ones connecting the batteries. 

 

 

 

 

Edited by jonathanA
Phone throwing wobbly...
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14 minutes ago, jonathanA said:

 

Unlikely unless the terminal is one of the bigger ones connecting the batteries. 

 

 

 

 

Right,

thanks,

yes one connects to starter battery neg, would that do the trick?

 

ive just pulled the other one off that goes to alternator

...which’ll do no good at will it.

Edited by Goliath
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35 minutes ago, Goliath said:

Right,

thanks,

yes one connects to starter battery neg, would that do the trick?

 

ive just pulled the other one off that goes to alternator

...which’ll do no good at will it.

No it won’t. You need to disconnect one of the wires from one of the battery positives. And be careful to insulate it. The ones going to the battery negative and the alternator go to the relay coil. But the coil only pulls the relay in, the relay relaxes (opens) due to spring pressure, which doesn’t work if the contacts are welded.

Edited by nicknorman
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19 minutes ago, nicknorman said:

No it won’t. You need to disconnect one of the wires from one of the battery positives. And be careful to insulate it. The ones going to the battery negative and the alternator go to the relay coil. But the coil only pulls the relay in, the relay relaxes (opens) due to spring pressure, which doesn’t work if the contacts are welded.

Yes, thank you 👍

Ive been and removed the relay positive from the battery terminal.

 

The relay is no longer connected, either through the negative or positive to the starter.


 

 

 

I’ll go and find a replacement tomorrow morning and as I said I’ll swop this old 60A for a 100A.

Which Id I’ve had to do anyway when I change the alternator. 

 

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Things appear to be ok, so thanks again for the help.

 

now it may be that the relay had just come to the end of its life,

 But what other reasons could cause a relay to burn out or stick?

Can over charging cause it?

 

I’ve never had an issue with it over the past 10 years.

I assume it simply has come to the end of it’s life.

But are there things I could look out for in future, or check before fitting a new one?

 

 

 

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1 hour ago, MtB said:

Relays are endless trouble in my experience and best avoided if possible, unless massively over-specced.

 

 

I bet its the "normal" alternator to starter battery wiring and relay to the domestics where all the current is going

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9 hours ago, Goliath said:

Things appear to be ok, so thanks again for the help.

 

now it may be that the relay had just come to the end of its life,

 But what other reasons could cause a relay to burn out or stick?

Can over charging cause it?

 

I’ve never had an issue with it over the past 10 years.

I assume it simply has come to the end of it’s life.

But are there things I could look out for in future, or check before fitting a new one?

 

 

 

 

First of all buy the highest current relay you can, certainly 100 amps or more. The way you are talking, it sounds as if you have an old, small relay with two small and two large blade terminals. They were for caravans, the ones more suitable for large boat battery banks had two studs instead of large blades, so when you fit the new one you may well have to change from large blades to eye terminals in the cables.

 

As Brian says, it has probably never been optimally wired because doing so often differs from the maker's instructions, and it takes a little extra work.  Typically, the alternator output goes to the ENGINE battery via the main starter terminal because this is how the engine is wired from new. This is fine for a typical "car sized" caravan battery but large marine domestic banks allow virtually the full alternator output, possibly plus a bit extra from the engine battery which is normally fully charged, to flow through the relay contacts. That means the relay contacts carry high currents for longer.  To overcome this wire the alternator output to the DOMESTIC battery positive, then all the flows through the relay contact is the small current demanded by the usually fully charged engine battery.

 

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I had one of these yars ago which welded itself. A blue package with blade terminals. I think it was 60a. 

 

 

Probably incorrectly wired. The above post is very valuable information as I believe these devices are very often problematic. 

 

 

 

I wonder if there may be a fire risk associated with a welded one of these as you could find cranking amps going through it in certain circumstances which it definitely isn't capable of handling.

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4 minutes ago, magnetman said:

I wonder if there may be a fire risk associated with a welded one of these as you could find cranking amps going through it in certain circumstances which it definitely isn't capable of handling.

 

I doubt it. I would expect the metal strip that connects the moving contact would melt before the thicker metal parts glowed red.

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1 hour ago, Tony Brooks said:

 

First of all buy the highest current relay you can, certainly 100 amps or more. The way you are talking, it sounds as if you have an old, small relay with two small and two large blade terminals. They were for caravans, the ones more suitable for large boat battery banks had two studs instead of large blades, so when you fit the new one you may well have to change from large blades to eye terminals in the cables.

 

As Brian says, it has probably never been optimally wired because doing so often differs from the maker's instructions, and it takes a little extra work.  Typically, the alternator output goes to the ENGINE battery via the main starter terminal because this is how the engine is wired from new. This is fine for a typical "car sized" caravan battery but large marine domestic banks allow virtually the full alternator output, possibly plus a bit extra from the engine battery which is normally fully charged, to flow through the relay contacts. That means the relay contacts carry high currents for longer.  To overcome this wire the alternator output to the DOMESTIC battery positive, then all the flows through the relay contact is the small current demanded by the usually fully charged engine battery.

 

Thanks Tony

my  alternator output does already go to the domestic bank positive. 

My problem is getting a replacement today. 

I’m wondering what to do in the meantime. 
I haven’t started the engine yet because I’m out trying to find a tepecement with the relay in my pocket. 
 

can I put this back on and run it for short periods?

 

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60A

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23 minutes ago, Goliath said:

Thanks Tony

my  alternator output does already go to the domestic bank positive. 

My problem is getting a replacement today. 

I’m wondering what to do in the meantime. 
I haven’t started the engine yet because I’m out trying to find a tepecement with the relay in my pocket. 
 

can I put this back on and run it for short periods?

 

60A

 

 

 

Yes, you can put it back on but as the contacts seem to be welded just attach the two screw (large) connections or if you have a small nut & bolt just join the two thick wires and insulate.  Otherwise, use a jump lead to join the engine and domestic battery positives only while the engine is running. Really, do whichever is easiest.

 

As long as the engine battery is in good condition, then a 40 amp car relay from the likes of Halfords will probably do for now, while you source a better one. Unless you have a local car electrical specialist, it will probably have to be mail order.

 

Now is the ideal time to change it for a bidirectional VSR. Apart from terminals, it should be a direct replacement, but just cut/tape back the THIN positive wire that operated the coil. If you do that, then any solar or mains charger will in effect charge both banks when either has a high enough voltage.

Edited by Tony Brooks
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10 minutes ago, Tony Brooks said:

 

 

Yes, you can put it back on but as the contacts seem to be welded just attach the two screw (large) connections or if you have a small nut & bolt just join the two thick wires and insulate.  Otherwise, use a jump lead to join the engine and domestic battery positives only while the engine is running. Really, do whichever is easiest.

 

As long as the engine battery is in good condition, then a 40 amp car relay from the likes of Halfords will probably do for now, while you source a better one. Unless you have a local car electrical specialist, it will probably have to be mail order.

 

Now is the ideal time to change it for a bidirectional VSR. Apart from terminals, it should be a direct replacement, but just cut/tape back the THIN positive wire that operated the coil. If you do that, then any solar or mains charger will in effect charge both banks when either has a high enough voltage.

That’s great Tony

I’ll let you know how I get on

I really need to get on and move today. 
 

thanks again 👍

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Well nothing caught fire or melted.

If anything my batteries may have got a better charge.

I put the old relay back on, simply so the wires were all back in an order and tidy.

And I’ve unhooked the positives again while engine off. 
 

Yes, I’ve been thinking about better options than a relay, I may well go for the VSR,

In the further future I suspect I’ll get one of them dc/dc chargers.

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5 minutes ago, Goliath said:

Well nothing caught fire or melted.

If anything my batteries may have got a better charge.

I put the old relay back on, simply so the wires were all back in an order and tidy.

And I’ve unhooked the positives again while engine off. 
 

Yes, I’ve been thinking about better options than a relay, I may well go for the VSR,

In the further future I suspect I’ll get one of them dc/dc chargers.

You will be better off with the VSR in my opinion.

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20 minutes ago, Goliath said:

Well nothing caught fire or melted.

If anything my batteries may have got a better charge.

I put the old relay back on, simply so the wires were all back in an order and tidy.

And I’ve unhooked the positives again while engine off. 
 

Yes, I’ve been thinking about better options than a relay, I may well go for the VSR,

In the further future I suspect I’ll get one of them dc/dc chargers.

 

For goodness’s sake, what is this apparently universal wish to spend money on a fancy gadget when a cheaper and simpler device will do the job. What do you think a DC/DC charger (like the Sterling B to B ) will do what a VSR won't? It is different if you have one lead acid battery, a lithium bank but only one alternator.  I had a DECENT split charge relay in service for about 15 years before I changed it to a VSR. You are the second person today that seems to want to spend £100+ for what looks like a marginal benefit, if any.

 

How old is that relay, being a mechanical device it will fail eventually,  especially as it was build down to a price, but that is true for all sorts of things like a lot of domestic electrical equipment.

 

If you intend to go lithium for the domestic bank in the next 5 years, then a programmable DC to DC charger may make sense, but not just for battery bank isolation.

Edited by Tony Brooks
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Ok fair enough 

I thought dcdc chargers were supposed to be the dogs at proper charging?

I can easily be persuaded to save my beer money. 
 

My relay has lasted me 10 years, and how long it was on the boat before that who knows?

I would quite willingly put another on, but bigger.

 but if there’s something better?

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