Battery Isolation Switch for Caravan

kgarnett · Senior Member

RE: Battery Isolation Switch for Caravan

Ivan 01 · Guru

A caravan by its nature may have many extra accessories fitted.
It may be even more confusing if you are not the original owner or you arent the person that did the fitting of extra accessories.

If you are 100% sure that no one has connected any accessory earth to the chassis and you are sure that all accessories that have to be earthed are wired back to a common terminal block for earth to the battery then go for it.
Fit that sucker in the negative.
I wont even go into the problems that may be experienced with towing vehicle earths as well.
The modern vehicle can be very challenging electrically.
Remember it is many times that the caravan chassis or frame by the nature of the tow ball is earthed to the tow vehicle.
If in doubt I would employ an auto electrician.

Ivan 01 · Guru

Mike Harding said this in a post above which pretty much sums it up.

*With a floating battery supply it doesn't matter in which pole an isolation switch is located although it could be argued it's better to put it in the positive pole as there is a higher possibility of multiple paths to 0V on the negative side.*

If considering putting the isolator in the negative because there is less clutter then maybe you should be triple checking every earth.
Clutter with wiring is usually a fitting or installation problem .

jegog · Senior Member

Arc suppression capacitor, Why should you use one when switching DC?
Do you use gas in your caravan?
If no and there is no likelihood that any gas appliance will ever be used, even portable appliances, then you have no real need to suppress sparks in your isolation switches.
If you use gas I would argue that one is required.
At first I thought Hylife was taking the piss when he poster that "Unlike AC, DC flows in a single direction from positive to negative."
That was the belief once but with the advent of semi-conductors it became obvious that the flow is from negative to positive.
Actually thermionic valves relied upon negative to positive too.

dorian · Guru

jegog wrote:
At first I thought Hylife was taking the piss when he poster that "Unlike AC, DC flows in a single direction from positive to negative."
That was the belief once but with the advent of semi-conductors it became obvious that the flow is from negative to positive.

https://en.wikipedia.org/wiki/Electric_current#Conventions

The direction of conventional current is arbitrarily defined as the direction in which positive charges flow. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.

Whenarewethere · Guru

Ivan 01 wrote:
A caravan by its nature may have many extra accessories fitted.
It may be even more confusing if you are not the original owner or you arent the person that did the fitting of extra accessories.

I once ask the electrician at my work place. What do you do when you have a major rewiring job on a building. He said it is often easier to rip it all out & start again. There are always changes on changes & the wiring is a dog's breakfast.

jegog · Senior Member

dorian wrote:
jegog wrote:
At first I thought Hylife was taking the piss when he poster that "Unlike AC, DC flows in a single direction from positive to negative."
That was the belief once but with the advent of semi-conductors it became obvious that the flow is from negative to positive.
https://en.wikipedia.org/wiki/Electric_current#Conventions
The direction of conventional current is arbitrarily defined as the direction in which positive charges flow. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.

Electrons, which have a negative charge are the only things that move. Well outside of a nuclear reaction. There is no such thing as positive current flow as there is no movement of anything with a positive charge. It is just convenient to use to explain to newbies how semiconductors work.

dorian · Guru

jegog wrote:
dorian wrote:
jegog wrote:
At first I thought Hylife was taking the piss when he poster that "Unlike AC, DC flows in a single direction from positive to negative."
That was the belief once but with the advent of semi-conductors it became obvious that the flow is from negative to positive.
https://en.wikipedia.org/wiki/Electric_current#Conventions
The direction of conventional current is arbitrarily defined as the direction in which positive charges flow. Negatively charged carriers, such as the electrons (the charge carriers in metal wires and many other electronic circuit components), therefore flow in the opposite direction of conventional current flow in an electrical circuit.

Electrons, which have a negative charge are the only things that move. Well outside of a nuclear reaction. There is no such thing as positive current flow as there is no movement of anything with a positive charge. It is just convenient to use to explain to newbies how semiconductors work.

"Current" is a term which was defined long before electrons were found to be the mechanism by which charges moved in a wire. The direction of current is as I stated above. Electron flow is a negative current. That's the convention that every scientist accepts and understands.

Semiconductors are doped with impurities, and charges flow by means of electrons (negative) or "holes" (positive).

Charges in an electrolyte can be in the form of negative or positive ions, but current still flows out from the positive terminal and returns via the negative.

Essentially you are equating the term "current" with a flow of charge carriers, whether they be positive or negative. These are not the same thing.

-- Edited by dorian on Saturday 2nd of July 2022 10:57:46 AM

jegog · Senior Member

So why should you connect and disconnect with the negative lead?
I'm talking about cars here,
Cars have lots of electronic modules fitted and there is a lot of dependency between them.
And these modules are mostly digital.
When designing digital circuits apart from the run logic you have to also design the initial state, the initialisation and the shutdown logic.
The initial state is to ensure that things that shouldn't happen, don't happen. Like cranking the engine.
The initialisation sets things up for accepting input from the user or other modules.
The shutdown is to set outputs to a known state so dying circuits cannot initiate operations.
What has this to do with which battery lead is disconnected first?
The negative rail is the active rail and the initial state is controlled by inserting components connecting nodes to the negative rail. A node connected to the negative rail with a capacitor will have a Lo(negative rail) state until it charges up.
Connecting the negative pole first ensures that all the nodes that need to be at Lo are at Lo before the other pole is connected.
Shutdown logic senses that the positive supply has gone and uses the residual power in the module to control the state of it's outputs.
Disconnecting and connecting the positive negates the initial state and shutdown systems.

Mamil · Senior Member

Just to pour more fuel on the fire - I use a double pole isolator switch so I'm isolating both positive and negative biggrin

dorian · Guru

jegog wrote:
So why should you connect and disconnect with the negative lead?
I'm talking about cars here,

When you are disconnecting or reconnecting the battery cables, think what would happen if your spanner were to contact the metalwork.

Mike Harding · Guru

jegog wrote:
So why should you connect and disconnect with the negative lead?
I'm talking about cars here,
Cars have lots of electronic modules fitted and there is a lot of dependency between them.
And these modules are mostly digital.
When designing digital circuits apart from the run logic you have to also design the initial state, the initialisation and the shutdown logic.
The initial state is to ensure that things that shouldn't happen, don't happen. Like cranking the engine.
The initialisation sets things up for accepting input from the user or other modules.
The shutdown is to set outputs to a known state so dying circuits cannot initiate operations.
What has this to do with which battery lead is disconnected first?
The negative rail is the active rail and the initial state is controlled by inserting components connecting nodes to the negative rail. A node connected to the negative rail with a capacitor will have a Lo(negative rail) state until it charges up.
Connecting the negative pole first ensures that all the nodes that need to be at Lo are at Lo before the other pole is connected.
Shutdown logic senses that the positive supply has gone and uses the residual power in the module to control the state of it's outputs.
Disconnecting and connecting the positive negates the initial state and shutdown systems.

The originator of this post has next to no understanding of logic systems or electronics and is wrong about every salient point he raises.

KevinJ · Senior Member

The attached is from the user manual of my BMPRO BatteryPlus35 (the nerve centre of my van).

Mike Harding · Guru

KevinJ wrote:
The attached is from the user manual of my BMPRO BatteryPlus35 (the nerve centre of my van).

And that is for the same reason both Dorian and I have highlighted. It is not because the little electrons get confused or the mothership neutron blasters will go off accidentally or any other magical event imagined by idiots who have no *&^$# understanding of electricity.

Edit:

I don't think my blood pressure will withstand any more of this thread, I'm gone.

-- Edited by Mike Harding on Sunday 10th of July 2022 03:57:51 PM

KevinJ · Senior Member

I'm not disagreeing with you mike. It's also what my grandfather taught me when I was a kid. biggrin