Battery cut off switch

[Quart Meg Miles]

Don't think Positive/Negative but Live and Ground (chassis). Anything not bolted to Ground via an unbreakable connection should be booted (and I'd boot the ground too to reduce corrosion, if that is a possibility). Never give a gremlin an even break.

[billwill]

And where am I wrong?

Sorry but you are mistaken Bud. If there is voltage (12 volt you say) at the other side of isolation switch, the fuse would blow the fuse. Dead short. How would the 12v get there?

James

The above bit was the bit that was wrong and caused all the confusion. It assumes that there could only be 12v on the "other" side of the isolator via a low resistance circuit, but Bud was describing the situation where then resistance between the live battery circuit and the "Other side of the isolator" has a high "resistance" eg due to the small amount of power used by a radio to maintain its memory state.

A radio takes maybe about 10 milliamps to maintain its memory so as it normally operates from 12v it has an equivalent resistance of about 1200 ohms.

A volt meter takes maybe 120 microamps to show 12 volts on its dial, say 1 million ohms. So if you draw the circuit of a 12v battery in series with a 1200 ohm resistor and a 1,000,000 resistor and calculate the voltage at each junction you will see that there is indeed about 12volts across the 1,000,000 resistor, i.e. across the open isolator switch.

[holywood3645]

Ok Last attempt. See Cartoon attached, I hoooope this helps. The radio 'keep alive' circuit requires 1 to 3 ma to maintain memory. It is always supposed to have 12Vdc supplied to maintain memory. I can send you a link as reference if anyone requires.

My suggestion is a fuse across the isolation switch on the negative side of the battery as shown. I'm saying in this configuration you will not see voltage (as voltmeter is connected on the diagram) across the isolation switch, opened or closed. That?s where i believe the disagreement comes in. Voltage is developed, across a resistance if current is flowing. No resistance (fuse) = No voltage.
VD-I R

VD is the voltage read by voltmeter
I = current 1 to 10ma
R= 0

3 things to think about by anyone worried about this isolation switch with exposed terminals close to a gas tank in the trunk of a car.
You have an electrical fuel sensor in the gas tank!......

If someone tried to steal you elan, and the isolation switch caused a spark that ignited the gas tank. Would that be a bad thing! I don?t really mean that, but it's a though that brings a smile to my face.

If you don?t want lose your memory on your radio and want to isolate the car battery see the 9v cig lighter radio option in attachments.

I installed my switch more as an anti theft device, and to isolated in case of an electrical short/fire.

James

[billwill]

OK, OK, you are both right, but Bud was basically talking about the case when the fuse is not there and you are talking about the case when the fuse is there.

In both cases it is desirable to insulate the exposed terminals, because if a tool or metal suitcase etc bridges across the exposed terminals it will behave as if the isolation switch is not there, so there would be no point in turning if off.

Or worse still if the starter-motor is activated and you forgot to close the isolation switch, current flowing through a flimsy metal object shorted across the contacts in your boot/trunk could cause a fire in your boot next to the petrol tank.

I used to have one of those isolators of the type which is fastened to a battery terminal and has a short removable key and a fuse. I was forever blowing the fuse by forgetting to put the connector key in.

[Bud English]

James- First of all I totally agree with your suggestion to add the fuse for supplying the keep alive voltage for the radio. Always did, always will, never was a difference of opinion there. Putting the switch in the ground leg is, as you suggest, the best option.

I also agree with you ...

Well, Bill finished it for me as I was typing. That's exactly the difference in opinion.

Just remember that the circuit with the fuse becomes the same as the circuit without the fuse when the fuse blows. ....and the 12V will magically appear on the ungrounded terminal.

[tcsoar]

Hi Guys,

Just been reading throut this thread with intrest as I have just fitted a battery disconnect into my +2. With the +2 we dont have the bulkhead behind the drivers seat to mount a manually operated switch so I used a relay fitted on the ground next to the battery, operated from a switch under the dash. This enables me to isolate the battery with out going into the boot, very handy if you need to use it in an emergency.

With out wanting to inflame the 'disccussion' that a few of you had towards the end of the thread I think that the main problem you have with each other is some of the terminology that is being used. I have found people local to each other useing different terms for the same thing, a problem that becomes more apparent when looking at a internet site with comments from around the globe.

Chris.

[Quart Meg Miles]

Chris,

Are you really isolating the battery through a relay, including the starter current? Post details of the relay, please!

BTW In the original discussion, when the fuse blows or is not fitted the voltage on the ground-isolated terminal only appears when current is demanded by one of the circuits and is the inverse polarity to the battery. Doesn't affect the principle or operation, just mentioning it in case someone unknowledgable gets surprised by it.

[MickG]

Chris,



BTW In the original discussion, when the fuse blows or is not fitted the voltage on the ground-isolated terminal only appears when current is demanded by one of the circuits and is the inverse polarity to the battery. Doesn't affect the principle or operation, just mentioning it in case someone unknowledgable gets surprised by it.

I'm surprised by it. No circuit no voltage or current.
How many volts do you have coming from the positive terminal on the battery? I say none.
Some statements are potentially confusing (clue). I think I know what you are saying but I also think the explanation needs expanding.


Mick G

[billwill]

Oh dear, I was accused of being condescending, for stating a straight fact above and now I have to do the same.

Mick & Meg, you appear to have forgotten your basic electricity lessons from school.

Current doesn't produce voltage, its the other way around. Voltage produces current.

You can't have 'voltage coming out of a terminal' in isolation; voltage can only exist between two points on a circuit i.e initially between the positive terminal and the negative terminal of a battery. But a battery on its own, certainly has voltage between its terminals, it does not 'need' a closing circuit (load circuit) to posses that voltage.

Comparing electricity and mechanics (Hydraulics)

Voltage is the 'equivalent' of pressure
Current (amps) is the equivalent of flow of fluid.

[billwill]

Hi Guys,

Just been reading throut this thread with intrest as I have just fitted a battery disconnect into my +2. With the +2 we dont have the bulkhead behind the drivers seat to mount a manually operated switch so I used a relay fitted on the ground next to the battery, operated from a switch under the dash. This enables me to isolate the battery with out going into the boot, very handy if you need to use it in an emergency.

Chris.

It does seem that what you have done there is strange. To isolate the battery completely including the starting circuit and doing it safely, would require a relay with contacts capable of carrying the full starter motor current around 150 amps?. I.E the relay would be as bulky as the starter solenoid. Admittedly the relay contacts are not making/breaking the starter motor current so are not likely to be sparking so slightly less bulky contacts are possible.

Secondly you can't be completely isolating the battery if you have a live feed to the dashboard to a switch which then comes back to the boot to operate the relay. Admittedly the switch can be on the chassis side of the relay-operating circuit, so the post-crash danger is low, but if the insulation on that wire from boot to switch were to be burned through and if it then shorted to chassis, that would operate the relay and turn the main circuit back on.

[rcraven]

Perhaps Chris has got one of these remotely operated cut-off switches http://www.demon-tweeks.co.uk/motorsport/battery-isolators-master-switches

[billwill]

I've just spent an hour or so searching the Internet for a good appropriate level guide to electric circuits so that we can try to avoid misunderstandings in future.

Eventually a found what I think is a good one and I've posted a link in a topic over in Electrical/Instruments.
electrical-instruments-f38/electric-circuits-cars-general-t27461.html

I like the fact that it contains the comparison between electricity and water Pressure/Flow that I mentioned above, making electricity much easier to understand.

[billwill]

Perhaps Chris has got one of these remotely operated cut-off switches http://www.demon-tweeks.co.uk/motorsport/battery-isolators-master-switches

That's an impressive bunch of heavy current switches. I quite like this one:
http://www.demon-tweeks.co.uk/motorspor ... y-isolator
which compare well with Chris's description.

And of course one could wire a 1 amp fuse across the heavy contacts to keep a radio memory alive.

[Bud English]

Eventually a found what I think is a good one and I've posted a link in a topic over in Electrical/Instruments.

Partial quote

Great brush up on basic DC circuits and free to boot! Page 21 has a diagram of just what has been thrashed about the past couple of days.

Thanks for posting that Bill.

[Quart Meg Miles]

I wrote

BTW In the original discussion, when the fuse blows or is not fitted the voltage on the ground-isolated terminal only appears when current is demanded by one of the circuits and is the inverse polarity to the battery. Doesn't affect the principle or operation, just mentioning it in case someone unknowledgable gets surprised by it.

As stated, "voltage ... only appears when a current is demanded by one of the circuits", i.e. when someone has switched something on. This connects the battery live terminal through the impedance of the selected circuit to the chassis, therefore the usually chassis side of the battery, the isolated terminal, will measure, on a meter, -battery volts. I am talking specifically about the chassis-side isolator system. I think, perhaps, my statement hasn't been read as I intended.

If you don't accept that then beware when you drive your modern car because it may contain Engine Management and ABS etc systems tested by equipment I designed!