Battery Isolation Switch for Caravan

thalls01 · Veteran Member

Battery Isolation Switch for Caravan

Ivan 01 · Guru

If it were my van I would use a marine type rotary isolator or even a simple key type isolator on the power or positive lead as close to the battery as physically possible. The isolator will need have the capability to handle the current.

A quality isolator is important as some are of poor build quality.

I can not see the value in isolating the earth on a caravan. There are too many variables with earths. Just the fact that the van has a mechanical connection with the car via the coupling could cause problems. There is another battery in the car to consider as well.

The earth may have been isolated so as to maintain the charging ability when connected to car, solar or charger. I cant see why though.

Those isolators indicated are ok for isolating a battery but just remember that the battery will not receive a charge if it is isolated. I would not rely on one for isolating while travelling as they could be subject to movement or vibration. They are generally ok for isolation while stationary.

-- Edited by Ivan 01 on Friday 24th of June 2022 10:11:23 AM

jegog · Senior Member

Isolation switches normally have spring loaded contacts so vibration is not going to cause any problems.
As you are switching direct current you should have an arc suppression capacitor across the switch contacts.
Usually there is only one connection to the negative pole of the battery whereas the positive pole could have several leads connected so attaching to the negative pole is simpler.

thalls01 · Veteran Member

Ivan, good points, thank-you.

An isolator is not a complex piece of technology however you want to do it once and do it right.

Cheers, Tim

thalls01 · Veteran Member

jegog wrote:
Isolation switches normally have spring loaded contacts so vibration is not going to cause any problems.
As you are switching direct current you should have an arc suppression capacitor across the switch contacts.
Usually there is only one connection to the negative pole of the battery whereas the positive pole could have several leads connected so attaching to the negative pole is simpler.

Thanks for your response.

I doubt that either isolation device that I have has an arc suppression capacitor. There is room for me to add one to the standalone isolator. Can you advise of the rating of the non-electrolytic capacitor that is typically used?

Would consider purchasing one and adding it if I knew what to purchase.

Cheers, Tim

Mike Harding · Guru

In your application I would not bother with arc suppression. The switch will be used very rarely and generally with only a light electrical load, even in the event it is used in an emergency under heavy load it will withstand many cycles of such before the contracts become burnt.

DMaxer · Guru

I know this might sound dumb and I understand that some things like lights operate on 12 volt whilst other things operate on 240 volts but why do we have an isolation switch and should it be left in the on position. Please be gentle with my stupidity as you know what a sensitive soul I am Mike.

dorian · Guru

If you leave your caravan in storage for any length of time, you don't want to come back to a flat battery. Also, an isolation switch facilitates a quick disconnection in the event of an emergency.

I guess one more consideration would depend on whether the battery can be charged from the towing vehicle.

I'm wondering whether the recommendation that the switch interrupt the earth connection is due to a consideration of what would happen if either battery terminal were accidentally connected to an earth point.

-- Edited by dorian on Saturday 25th of June 2022 08:53:28 AM

Mike Harding · Guru

Hi Dmaxer

Isolation switches on any electrical circuit have one basic purpose:

To allow the whole circuit and any and all sub circuits to be de-energized by a single switch which should be located in an obvious and easily accessible position. Some isolation switches incorporated provision to be padlocked in their off position.

The two main reasons to use an isolation switch are:

If the circuit is to undergo maintenance (I'd remove the fuses too)

or

In the event of an emergency - fire, electric shock and such

They are also useful for ensuring a system (eg a caravan in storage) does not constantly use a small amount of electricity and perhaps flatten batteries and the like.

They are well worth having in battery systems as large batteries can produce thousands of amps of current under fault conditions and this will readily start a fire.

DMaxer · Guru

Thank you very much Dorian and Mike.

Hylife · Guru

The reason you should always switch DC from the negative is to stop what is generically termed inrush current.
Inrush current occurs because of momentary voltage sag when devices first power up.
If your battery momentarily sags to say 9 volts for a fraction of a second that will result in devices getting a momentary current increase of 30%

Same reason you always connect the negative jumper lead last and disconnect the negative lead first.

Unlike AC, DC flows in a single direction from positive to negative.

If the positive is the last connection then the inrush current flows through all the equipment that is turned on, to ground/negative, and can kill sensitive electronics.

If the positive is the first connected then the power flows though the equipment that is turned on but without current as there is no ground connection.
Then, when the ground/negative is connected last the inrush current only occurs at the negative switch to negative terminal of the battery.

dorian · Guru

Hylife wrote:
The reason you should always switch DC from the negative is to stop what is generically termed inrush current.
Inrush current occurs because of momentary voltage sag when devices first power up.
If your battery momentarily sags to say 9 volts for a fraction of a second that will result in devices getting a momentary current increase of 30%

Same reason you always connect the negative jumper lead last and disconnect the negative lead first.

Unlike AC, DC flows in a single direction from positive to negative.

If the positive is the last connection then the inrush current flows through all the equipment that is turned on, to ground/negative, and can kill sensitive electronics.

If the positive is the first connected then the power flows though the equipment that is turned on but without current as there is no ground connection.
Then, when the ground/negative is connected last the inrush current only occurs at the negative switch to negative terminal of the battery.

This makes no sense to me. Then same current flows out of the positive terminal as flows into the negative terminal.

The only reason that one would disconnect the negative terminal first, and reconnect it last, would be to prevent a short if the spanner were to contact an earthed point while disconnecting the positive lead, or if the positive jumper lead were to land on the frame of the other vehicle while you were handling it.

-- Edited by dorian on Monday 27th of June 2022 09:37:06 AM

Ivan 01 · Guru

dorian wrote:
Hylife wrote:
The reason you should always switch DC from the negative is to stop what is generically termed inrush current.
Inrush current occurs because of momentary voltage sag when devices first power up.
If your battery momentarily sags to say 9 volts for a fraction of a second that will result in devices getting a momentary current increase of 30%

Same reason you always connect the negative jumper lead last and disconnect the negative lead first.

Unlike AC, DC flows in a single direction from positive to negative.

If the positive is the last connection then the inrush current flows through all the equipment that is turned on, to ground/negative, and can kill sensitive electronics.

If the positive is the first connected then the power flows though the equipment that is turned on but without current as there is no ground connection.
Then, when the ground/negative is connected last the inrush current only occurs at the negative switch to negative terminal of the battery.
This makes no sense to me. Then same current flows out of the positive terminal as flows into the negative terminal.
The only reason that one would disconnect the negative terminal first, and reconnect it last, would be to prevent a short if the spanner were to contact an earthed point while disconnecting the positive lead, or if the positive jumper lead were to land on the frame of the other vehicle while you were handling it.

-- Edited by dorian on Monday 27th of June 2022 09:37:06 AM

Hi dorian

You are correct. The short circuiting of a spanner is the reason

I think it is more to do with arc suppression as was mentioned above.

The on / off / on in rapid succession may damage sensitive equipment in rare cases. Both Caravan and Marine industries dont appear to be worried with this.

Poor quality isolation switches can be a problem and thank you china.

In the old days with Minespec regulations I think they were required to comply. Eliminating an arc was mandatory.

These regs are under constant change so I am not sure of requirements in some industries but for the sake of the second hand caravan it would be pretty safe and a lot more simple just to place the isolator in the Power or Positive immediately from the battery.

When I was briefly involved in motor racing the isolator was installed on the positive lead as a regulation.

-- Edited by Ivan 01 on Monday 27th of June 2022 10:01:46 AM

dorian · Guru

I've heard that it's not recommended to jump start modern vehicles because sudden changes in the load can result in excessive voltages due to load dump. But that was a while ago, so maybe modern electronics is designed to account for these scenarios?

Mike Harding · Guru

I think Hylife was smoking something, quite a lot of it, when he wrote that.

Ivan is correct about the reason for the automotive habit of connecting positive first, there is no electrical reason to do so.

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.

The electronics which go into automotive applications are very well protected and specified and are not "sensitive" - if we put sensitive electronics in automotive applications the roadsides would be littered with dead cars. Nevertheless it makes good sense to be careful when, say, arc welding on a vehicle. Jump starts will be safe enough, just follow the owner's handbook procedure.

Scubadoo · Veteran Member

My decision was based on a very complex technical theory. The negative battery terminal area was less cluttered. That is where the isolating switch went. All those inrush current electrons will apparently be happy.

Hylife · Guru

No, I wasn't smoking something, I've 40 + years in designing complex electronic circuits.
What I said about switching negative is well known, do a little research and learn something new guys.

dorian · Guru

As I understand it, during sustained starter motor cranking the battery voltage can sag down to 9V. That's hundreds of amps.

https://houseownerssolution.com/how-does-cranking-voltage-compared-with-battery-voltage/

-- Edited by dorian on Tuesday 28th of June 2022 10:30:57 AM

dorian · Guru

Hylife wrote:
No, I wasn't smoking something, I've 40 + years in designing complex electronic circuits.
What I said about switching negative is well known, do a little research and learn something new guys.

Can you point to any references? I can't find anything which addresses your argument.

Ivan 01 · Guru

dorian wrote:
As I understand it, during sustained starter motor cranking the battery voltage can sag down to 9V. That's hundreds of amps.
https://houseownerssolution.com/how-does-cranking-voltage-compared-with-battery-voltage/

-- Edited by dorian on Tuesday 28th of June 2022 10:30:57 AM

As a real life example, when the model HQ Holden was very first released the cranking of the starter motor in the V8 versions would throw a current spike or surge into the new radio / cassette player that was an genuine accessory and would render to radio cassette unit useless.

My memory fails me as to the exact fix but I am fairly sure it was remedied within the components of the replacement radio unit.

Mike Harding · Guru

Hylife: you are ignoring both Ohm's Law and Kirchhoff's Law.

As I = V/R when the voltage drops the current will drop.

Your concept of "inrush current" (a technically meaningless term) is way off track; inrush current is usually understood as a surge of current during the period immediately following the application of power to a device and only occurs with a few specific loads eg. a capacitive load, most motors and cold filament lamps and heaters.

"If your battery momentarily sags to say 9 volts for a fraction of a second that will result in devices getting a momentary current increase of 30%"

No, no and thrice no! ---- I = V/R

If 12V is applied to a 6R load then 2A will flow - if 9V is applied to that same 6R load then 1.5A will flow.

The *only* way the current can remain the same if the voltage falls is if the load is an active constant current load and adjusts its resistance in order to obtain more current and thus keep the power it receives constant. The only load I can think of in domestic use which does this to a small extent is some 12V fridges.

If you wish to switch the negative in floating systems then go right ahead, it'll work just as well as switching the positive but it will confuse service people and the like. Also, keep in mind other than their reference to one-another there is no negative or positive in a floating system.

dorian · Guru

Ivan 01 wrote:
dorian wrote:
As I understand it, during sustained starter motor cranking the battery voltage can sag down to 9V. That's hundreds of amps.
https://houseownerssolution.com/how-does-cranking-voltage-compared-with-battery-voltage/

-- Edited by dorian on Tuesday 28th of June 2022 10:30:57 AM
As a real life example, when the model HQ Holden was very first released the cranking of the starter motor in the V8 versions would throw a current spike or surge into the new radio / cassette player that was an genuine accessory and would render to radio cassette unit useless.
My memory fails me as to the exact fix but I am fairly sure it was remedied within the components of the replacement radio unit.

That's odd. During cranking the accessories are disconnected from the battery, so any damage would had to have occurred when the ignition switch was released.

Ivan 01 · Guru

dorian wrote:
Ivan 01 wrote:
dorian wrote:
As I understand it, during sustained starter motor cranking the battery voltage can sag down to 9V. That's hundreds of amps.
https://houseownerssolution.com/how-does-cranking-voltage-compared-with-battery-voltage/

-- Edited by dorian on Tuesday 28th of June 2022 10:30:57 AM
As a real life example, when the model HQ Holden was very first released the cranking of the starter motor in the V8 versions would throw a current spike or surge into the new radio / cassette player that was an genuine accessory and would render to radio cassette unit useless.
My memory fails me as to the exact fix but I am fairly sure it was remedied within the components of the replacement radio unit.
That's odd. During cranking the accessories are disconnected from the battery, so any damage would had to have occurred when the ignition switch was released.

Well as odd as it may seem to you it was in fact, a fault.

Considering that at the very moment the engine starts there can be a surge of current from the alternator. We need to remember that voltage regulators were mechanical affairs in those days.

It should be also noted that this was happening on a new model. It may just have been a fault within the radio itself or even the wiring.

As I said I can not remember as to what the fix was. but the instruction was for owners to turn the wireless off until GMH fixed it.

I might point out that I was not working in a Holden Workshop but my boss at the time had just gone out and bought a brand new V8 Monaro and it was from this that we learned that GMH had this fault.

This is probably so far off topic now even the confused are confused.

dorian · Guru

I just remembered that old British cars had a positive earth electrical system, so all the switching was done one the negative side ...

I think for an EE an automotive environment would pose a lot of challenges. In fact electronic components are often separately graded for automotive use. Bosch did have a very high wattage Zener which could be fitted across the supply to absorb voltage transients. I never actually saw one, though. I also saw a case where a diode was installed inside a Chrysler wiring harness, but I think this was only required when electronic ignitions were introduced.

Mike Harding · Guru

Dorian:

Yes, I do remember positive earth vehicles, just. A switch to negative earth was made because there is some advantage regarding corrosion, although I cannot recall the details.

The main semiconductor component used for high voltage/energy spike suppression is the transient voltage suppressor (Transzorb) which is very fast and can handle considerable energy although, physically, they are on the large side in surface mount terms. In this area I did some work for CSIRO and one thing I found which is excellent for high voltage spikes is the humble spark gap constructed simply from PCB traces and located as close as possible to the point on the PCB where external wiring entered.

Automotive has very strict rules for electronic robustness; iirc Mercedes had a standard whereby any module supplied to them had to withstand 120V DC for 300mS on any external connection this in addition to HV tests.

Talking of diodes, you may care to look up the tunnel diode; a most interesting device which operates in the quantum area and exhibits negative resistance for part of its curve, I have witnessed such an effect with small neons too.

KevinJ · Senior Member

I think we had an old Morris of some sort when I was a kid which was positive earth with a 6 volt system and generator, not alternator. It started with a crank handle.

yobarr · Guru

KevinJ wrote:
I think we had an old Morris of some sort when I was a kid which was positive earth with a 6 volt system and generator, not alternator. It started with a crank handle.

Same deal with neighbour's 1948 Ford Prefect but we changed it to 12 volt for him. Before we did that he had to park it on a downhill slope each night to save having to use the crank handle on cold mornings. Pathetically slow and terribly unstable, but our neighbour used to fit his family of 6 into his for the monthly trip to town for supplies. Seatbelts? What seatbelts? Cheers

dorian · Guru

Ivan 01 wrote:
dorian wrote:
Ivan 01 wrote:
dorian wrote:
As I understand it, during sustained starter motor cranking the battery voltage can sag down to 9V. That's hundreds of amps.
https://houseownerssolution.com/how-does-cranking-voltage-compared-with-battery-voltage/

-- Edited by dorian on Tuesday 28th of June 2022 10:30:57 AM
As a real life example, when the model HQ Holden was very first released the cranking of the starter motor in the V8 versions would throw a current spike or surge into the new radio / cassette player that was an genuine accessory and would render to radio cassette unit useless.
My memory fails me as to the exact fix but I am fairly sure it was remedied within the components of the replacement radio unit.
That's odd. During cranking the accessories are disconnected from the battery, so any damage would had to have occurred when the ignition switch was released.
Well as odd as it may seem to you it was in fact, a fault.

I didn't mean to suggest that I didn't believe you. The starter motor and starter solenoid are inductive devices. These generate large voltage spikes when their current is suddenly interrupted.

Ivan 01 · Guru

Ok. It is not the first time it has been suggested for me to explain a previous statement in a topic.

The wiring and primitive electronics in a vehicle manufactured 50 years ago wont be what is available today in the automotive industry.

It could have been caused by several conditions which were not predicted in early production of those vehicles.

There were no recalls as we know them today. The owner was usually notified when the fault occurred or when the vehicle went in for the first service.

I am just going out to flick the isolation switch on the Triton. biggrin

Radar · Guru

thalls01 wrote:
Hi, I have two isolation switches from Ampper (manufacturer) to isolate the van's battery supply. The first is a rotary switch on a bus bar that attaches to the battery's post terminal. By turning the knob it acts to first to connect and further rotation serves to disconnect the circuit from the battery.
In reading the recommendations for this switch they refer to placing it on the negative terminal to isolate the battery and equipment from the earth supply.
The second switch is a standalone rotary switch on bus bar with 10mm terminals underside that requires a lead from the battery to one of its terminals with the usual lead sitting on the battery attached to the other terminal to complete the circuit. Again it specifies placement of the switch between the negative battery terminal and the usual negative/earth lead. Just wondering how much success other Nomads have had with this bus bar switch as an isolator. I have heard they can become weak where the rotary switch is located on the bus bar and lose conductivity. Part number is AM-CBS31 for look-up.
Other query relates to the recommendation that both switch types isolate the negative post of battery and not the positive terminal. Not absolutely sure why this is. My guess is that the negative post offers an earth via the chassis in some instances in which case isolating the negative terminal disconnects everything earthed in the instance welding or transmission of current via the chassis was to be made.
Am I correct? What do others think or recommend?
All comments welcome - Tim

Hi,

I am very interested in this Post as would like to include a battery isolation switch in my simple 12 volt caravan system, been rereading the writings of the Contributors and have at times been totally confused but that is to be expected as I have no formal training and when/why comes into it, I am lost.

I remembered years ago there was a simple green, quick undo thingy add on to the negative battery terminal which was great, was told when I assisted my father to jump start a car about 60 years ago red to red first then the black earth cables, I do not know why but that is what I still practise to this day, "connect the earth last son", not far off 75 and still here to remind my son to connect earth last.

Relooking at my 12 volt electrical system, you know it going to be easier to put the isolator on the negative side of the battery, a no brainer there.

Going to town today to buy a isolator switch.