Battery cut off switch

[rcraven]

A tip for the radio guys. Place a =/<1 amp fuse across the terminls. This will limit the current availiable to circuits, and if anyone trys to start it it will pop the fuse.

That's OK provided you always remember to use the key before trying to start. Otherwise you'll keep blowing the fuse yourself. I know I'd do this so I just put up with losing the memorised stations.
And one reason for a cut off switch is to prevent the battery going flat through mysterious current drains.

[oldelanman]

James is suggesting that you leave the circuit live, via the fuse, to maintain the radio memory but if any naughty person tries to use any circuit it will blow the fuse thereby disconnecting everything (including the memory maintenance) completely.

Ah of course! The way I read James' post I thought he was suggesting putting a fuse across the radio terminals which made no sense

I'm not sure that interrupting the ground side is, er, grounds for leaving off the rubber sleeving, any switches left on accidentally will place the "disconnected" terminal at battery voltage though current limited by the circuit's resistance. Looks naked and amateurish without sleeving, especially in the vicinity of the petrol tank.

Good point. I'll get some boots sorted.

Thanks Meg.

[holywood3645]

Meg, Its the ground lead. The petrol tank is gounded, The chassis is grounded, the engine is grounded.... Ground leads do not need insulation.
In fact on alot of cars the Negitive from a battery is not insulated.

The fuse goes across isolation switch terminals. With the isolation switch removed, Everything still has power..... an alarm system, radio, internal lights etc. But all the circuits are limted to the rating of the fuse. The high current drawn by a starter motor will blow the fuse and everthing will be dead.

The red switch handle is attached to a quick disconnect on my keys. Hard to forget. "mysterious current drains" You need to get on top of any current drains. Try putting a multimeter in series with your battery lead. I have never found the drain you are talking about. The battery discharges internally if left long enough. and if its cold, the battery efficency drops. I have a little battery tender in the car also. I leave it plugged in when the car is not going to be driven for a a while.

I know the switch on the negitive side of the battery may sound strange, But I can assure its a mod that is very effective. As for the "mysterious current drains" I honestly don't think they exist, but thats another discussion.

James

[RichardHawkins]

Earnie,
I live in Britain, and buy my electrical component from www.autoelectricsupplies.co.uk or www.vehicleproducts.co.uk

Auto electric supplies show a battery isolation terminal with a small bypass fuse on page 30 of their paper catalogue ?10.85 about $15. I don't think this is quite what you are looking for but the illustration does show how the bypass fuse is wired.

I expect there are similar companies in USA, if not, and if auto electrical supplies do not offer an export service I could post one to you.

Regards,
Richard Hawkins

[Bud English]

James- Meg is totally correct. The frame side of the switch will be at ground (frame) potential. The other side of the switch will be at 12V with the current only limited by any, always on, device that draws from the unswitched 12V leg of the loom. That's why the fuse trick works.

Anything shorting that side of the switch to ground defeats the switch.

[holywood3645]

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?

Current above 1amp to operate for any device, will blow the fuse.

Take the starter circuit. Initally the solenoid is switched it will try to pick up. But the current for the solenoid will blow isolation fuse. The + 12v leg from the battery will be isolated at the solenoid (under the hood)
In other scenerios like interior lights left on or radio,loads <1.0 amp. How can you see a voltage across an isolation fuse that is not blown.

Think about it some more, It's a little confusing because we are switching and isolating the -ve side

James

[Bud English]

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?

Current above 1amp to operate for any device, will blow the fuse.

James

partial quote

Not to beat a dead horse, and my last thoughts on the subject... Edit: I guess they weren't my last thoughts...

How would the 12v get there? Through the very devices you are trying to keep live by using that 1A fuse. The voltage potential passes through these devices even with no current flow. If the 12V wasn't present on that terminal the fuse wouldn't provide a path for the keep alive current to flow.

As your next line states, any current up to 1A will pass and anything drawing more than 1A will blow the fuse. ...and when the fuse blows, the 12V will again be available to the non-grounded terminal.

If you have a cutoff switch, turn it off and check for 12V on the non-grounded side of the switch using a meter (the meter completes the current path and allows the 12V potential to flow to ground). If there is no voltage present, you won't need the fuse because nothing is drawing any power from the circuit. If your radio, or whatever, needs a voltage to stay alive, there will be 12V on that side of the switch. That's a handy check for those "mysterious current drains" as well. Disconnect your negative battery lead and put a volt meter between the neg battery terminal and a good ground point. If you read any voltage at all you've got a leak.

Honest...

[holywood3645]

Bud,one question, are you an electrical engineer?

You would put an ammeter is series to see curent flow,thinis the only way to check for current flow. A voltmeter will only measure votlage. There can be voltage present and no current flowing. Ohm's law and series DC circuit theory.

James

PS, I are 1

[Bud English]

No, but I play one on the internet!

I did work 20+years in aviation electronics and have since successfully wired cars, boats and aircraft.

Tricky edit...
If you r 1, then you know that there can't be current flow without a voltage present. If you have anything in the circuit that is constantly "on" and you put a voltmeter across the open switch you'll read a voltage. If you put an ammeter across it you'll read a current. If there is nothing in the circuit that is "on" you won't read anything on either meter.

The original statement, way back when, was that there would be a voltage present on the ungrounded terminal. So I guess only one rubber boot is in order.

[holywood3645]

Electrical Instrumentation and Controls Engineer
40 years

http://www.thefreelibrary.com/Triconex+ ... 0112678592

James

[Bud English]

Self censored....

Didn't mean to be nasty at all and wasn't looking to "win".

You said, "There can be voltage present and no current flowing. Ohm's law and series DC circuit theory." I agree with you totally and you make my point. The voltage present would be 12V with no current flowing. Add the fuse and enough current would flow to power anything up to 1A. Short that terminal to ground and it would complete the circuit for anything that was "on", just like closing the switch.

[holywood3645]

Ok Bud you have won, just by getting nasty.

James

[billwill]

Bud,one question, are you an electrical engineer?

You would put an ammeter is series to see curent flow,thinis the only way to check for current flow. A voltmeter will only measure votlage. There can be voltage present and no current flowing. Ohm's law and series DC circuit theory.

James

PS, I are 1

I do have an Electrical Engineering Degree and Bud is the one that is right!

[AHM]

Will this work for positive earth?
Or will I need to put a boot on the other side of the switch?

[holywood3645]

And where am I wrong?