Battery Disconnect Switch

[Chewbacca]

I am converting to 12v Neg Grnd. I already have a battery disconnect switch installed under my seat from the old positive ground system. Question: Should the disconnect switch interrupt the positive side like it used to with the old system, or can it be on the ground side - which it would be in the new system if I don't move it? Or doesn't it matter? Thanks.

[Tom Wesenberg]

Polarity doesn't matter, but if I used one it would be on the ground cable. That is for safety reasons.

[BILL WILLIAMSON]

Leave it like it is, switch has only a 2 word vocabulary---OFF & ON!!!!! LOL, LOL, LOL, (ONLY me & my DOG laughs at my "humor") As Charlie Chaplin once said, "I don't get NO respect!" Bill W.

[BILL WILLIAMSON]

Originally Posted by Tom Wesenberg

Polarity doesn't matter, but if I used one it would be on the ground cable. That is for safety reasons.

Explain why. Bill W. Never mind, Tom, Buster the Dog figured it out. He said if it was on the feed side & something inside fell apart & shorted out, it could burn your car! He also said if it was on the feed side, it would be carrying a bigger amperage load during cranking and since the starter uses lots of that amperage, the switch would be handling less amperage load if mounted on the ground cable,DUMMY!! (You think he's RIGHT?) Bill W.

[Tom Wesenberg]

It's the same amperage on either side, but you stated why it's safer.

[Mike V. Florida]

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Originally Posted by Tom Wesenberg

Polarity doesn't matter, but if I used one it would be on the ground cable. That is for safety reasons.

I don't get it? Let's assume the electricity goes out the pos side of the battery and the neg it attached to the frame.
If we place the switch on the positive side of the battery nothing goes out the battery to the car so if you were to touch anything in the car nothing would happen because nothing is leaving the battery.

If we place the switch on the neg side of the battery the nothing comes into the battery from the chassis. So if we were to once again touch something nothing would happen because the neg of the battery is not connected to anything.

Where am I going wrong in my thinking? How is a switch on one side of the battery better than another?

[Tom Wesenberg]

If the switch is on the ground side and one of the switch terminals accidentally touches ground, NOTHING happens. Or if the switch is plastic and melts (cheap junk) nothing happens because you are still on the ground side.

If the same bad stuff happens with the switch on the "hot" side of the battery, you will have sparks or fire when the terminal or cable touches ground.

[V4F]

neg side ! no arching that way . always use the neg side for less damage ........

[Tom Wesenberg]

Originally Posted by steve norcal

neg side ! no arching that way . always use the neg side for less damage ........

How do you get less arcing on the negative side?

Actually when you turn off the master switch there should be NO arcing because everything should already be off.

[MikeK]

Originally Posted by steve norcal

neg side ! no arching that way . always use the neg side for less damage ........

A switch in a DC circuit will always have one positive polarity and one negative polarity terminal. It does not matter on which side of the battery you use the frame as one of the conductors or mount the switch, arc characteristics will be the same. HOWEVER, as stated and explained earlier, the SAFE place for the switch is in the ground/earth/frame lead.

If you want to quell arcing on a DC switch to extend its number of MTBF cycles, place a diode across the terminals. This is done often on DC relays to keep the contacts from welding/sticking with inductive loads. For a battery cutoff you would need a rather big (100A ?) diode if you wanted to interrupt a highly inductive stuck-on starter without any arc pitting of the cutoff switch.

FWIW, if there was no arcing on the 'neg side', arc welders would not work when you flip the 'reverse polarity' switch!! What changes is which side gets the most heat energy.

In a switch circuit (battery cutoff) both internal terminals are identical, one or the other will get the same heat energy work (damage) if you swap terminal polarity. The arc will be the same, just reversed like looking in a mirror. No more, no less. Geek stuff-- Actually, simple 1st law of thermodynamics. Also the classic Carnot heat engine, with switch terminal arc damage being W, the work.

[P.S.]

Originally Posted by Mike V. Florida

Let's assume the electricity goes out the pos side of the battery and the neg it attached to the frame.

Nope, you got it backwards. Current flows from the battery negative terminal to the positive terminal.

But either way, it really doesn't matter. When you interrupt a current flow by opening a circuit, it doesn't matter where you open it.

[Art Bjornestad]

Tom mentioned something very important, stay well away from those cheap plastic on/off switches......they are not rated to handle starter loads and will melt, usually at a bad time or place.

[Roadster62]

Could we please get this topic with a pole? As with ALL electrical questions there are no CORRECT answers, there are opposing answers. If we get a poll, one would think the ones with the most answers would be correct, not that it would matter as you watch your car burn to the ground. Bob......MikeK, I didn't understand a thing you wrote, but thanks for adding to the confusion.

[BillLee/Chandler, TX]

If you put the switch in the ground line (regardless of + or - ground), you protect EVERYTHING on the car.

If you put it on the "hot" side of the battery, you protect everything downstream but NOT the switch itself.

And chances are, a switch on the "hot" side has been installed after the starter. Ever hear of a stuck starter switch and a burned up starter? If the switch is in the ground line, even the starter is protected.

[Roadster62]

IF a battery "shut off" switch is the SAME AS a race car "KILL SWITCH" are they both placed on the same side of the battery? Or is this a totally different question? Bob

[H. L. Chauvin]

Humble opinion:

When specifying installation instructions, a battery disconnect manufacturer can be held liable for any type of vehicle accident one could ever imagine -- the buck stops at his company -- today, an attorney usually reviews instructions & warranties of most prudent manufacturers to avoid such accidents.

This sentence is taken from one disconnect manufacturer regarding exactly where to provide the disconnect switch:

"Due to the many different applications and variations of components, it is the responsibility of the installer to verify correct connections."

Some manufacturers further state that the purpose of the disconnect is to stop the alternator from charging when the disconnect switch is turned "OFF".

In some cases Model A owners are interested in "killing" the entire electrical system, "including" the battery, when the car is not in use; hence, they disconnect the battery ground -- others have different preferences & opinions to suit what they feel is a safe installation.

With a little research by "first" reading manufacturer's cautious "written" installation instructons, reading individual preferences of vehicle owners, & a little common sense, one can easily decide what he feels is comfortable for "his/her" Model A.

[MikeK]

Originally Posted by Roadster62

Could we please get this topic with a pole? As with ALL electrical questions there are no CORRECT answers, there are opposing answers. If we get a poll, one would think the ones with the most answers would be correct, not that it would matter as you watch your car burn to the ground. Bob......MikeK, I didn't understand a thing you wrote, but thanks for adding to the confusion.

Originally Posted by Roadster62

IF a battery "shut off" switch is the SAME AS a race car "KILL SWITCH" are they both placed on the same side of the battery? Or is this a totally different question? Bob

#1. Baloney. Correct answers are based on, and obtained through research, science, and engineering. If you don't understand it, that does not invalidate it.

#2. NOT the same.

A battery "shut off" does exactly that. It disconnects, or "shuts off" the battery from the electrical system. The engine will continue to run, with ignition powered by a still turning generator, alternator, or magneto.

A "Kill switch" is specifically wired to stop the engine. It may do so by disabling the ignition, fuel supply, or anything necessary to maintain engine locomotion. It does not necessarily disconnect the battery.

If you want both functions in one switch, you need a DPST switch like this, that disconnects the battery AND interrupts an essential circuit in the engine, like the gen/alt, the fuel pump, or the ignition.

Some racing rules require BOTH, a switch like the one below-
The large terminals are for battery disconnect, the two smaller for interrupting an essential "run" function as explained.

This is actually the ideal way to put a switch in an A, but, like most things electrical, misunderstood (including by vendors and their advertising) and thusly not used.

[Roadster62]

Mike I've seen the above switch, and it looks like a far safer unit than the green knob style. I understand the large studs are for battery cable, are the smaller ones with nuts for mounting the switch or do they connect to some gizmo inside and carry electricity? Bob

[MikeK]

Roadster62, the smaller nuts are terminals for a second switch. Run either the gen/alt output or the ign circuit through them and you will have a switch that disconnects the battery AND kills a running engine. Most guys don't realize that switching off a simple battery cutoff will NOT stop a running engine, as the gen/alt will be powering the ign independent of the battery.

[1931 flamingo]

Very good and timely article in the latest Rod and Custom Magazine about disc sw's.
Paul in CT

[Roadster62]

How good? Can the uneducated understand it? Does it agree with the info posted in this thread? Bob

Originally Posted by 1931 flamingo

Very good and timely article in the latest Rod and Custom Magazine about disc sw's.
Paul in CT

[Napa Skip]

Whether you believe in electron flow (electrons - the -ive "ions" - leave one battery terminal and travel through a circuit to the other battery terminal) or hole flow (the +ive ions are "flowing") the simple DC electrical devices such as are found on Model A's continue to work (as do Ohm's Law, Kirchoff's Laws, and all the other "laws" that engineers have devised in order to "explain" what we observe actually happening).

This includes Doofus' Law, which states that shorting the non-grounded terminal of an installed battery to ground with your handy-dandy crescent-hammer usually results in an attention-grabbing incident, a moderate amount of light (even more light if there's an explosive atmosphere in the vicinity) and perhaps a new notch in said wrench (or in whatever you shorted the battery to; see electron vs hole flow above).

This is probably why the recommended sequence to disconnect a battery is to remove the grounded cable first; if you ground your wrench while doing so it's no big deal whereas - if you removed the non-grounded cable first - you risk the attention-grabbing-incident routine. (The recommended reinstallation sequence is the reverse; non-grounded cable first, grounded last.)

All of which argues (imho and as some have stated above, much more succinctly than I) towards connecting a battery disconnect switch between the grounded battery terminal and ground. In this scenario, shorting of the disconnect switch to ground has fewer consequences than if the disconnect were in the un-grounded cable.

ZZZZAAAAPPPP!

[Roadster62]

So, IF your Model A is running you can switch the battery disconnect switch "OFF" if connceted to the ground side, and it will continue to run. IF you have a "KILL SWITCH" on the other cable you need to turn that to the "OFF" or "KILL" side to stop everything? Correct? Bob

Originally Posted by Napa Skip

All of which argues (imho and as some have stated above, much more succinctly than I) towards connecting a battery disconnect switch between the grounded battery terminal and ground. In this scenario, shorting of the disconnect switch to ground has fewer consequences than if the disconnect were in the un-grounded cable.

ZZZZAAAAPPPP!

[JoeWay]

Originally Posted by Roadster62

So, IF your Model A is running you can switch the battery disconnect switch "OFF" if connceted to the ground side, and it will continue to run. IF you have a "KILL SWITCH" on the other cable you need to turn that to the "OFF" or "KILL" side to stop everything? Correct? Bob

Go back to the first page and re-read MikeK's post #18. That explains the difference.

The Model A will continue to run with the battery disconnect switch turned OFF whether it is in the hot or ground battery cable. The kill switch must be in the ignition circuit or in the hot feed from the generator/alternator to the terminal box in order to shut off the engine.

The switch MikeK shows in his post has both functions built into it. The heavy terminals disconnect the battery, and the small terminals disconnect the ignition or other run-required circuit.

Joe

[springerpete]

I have one of the switches with the green plastic knob that screws a contact down onto the neg battery terminal or lifts it off the terminal to disconnect the battery from the circuit in my car. Never had an issue with it and see no evidence of arcing or burning or melting. Is this the one that is called junk and considered unsafe ?

[Roadster62]

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Now that I think about it that switch mounts through a hole under the switch. From what I understand that would be the BEST switch to have in a car, or is there a better one on the market to kill the flames? Who makes it, and is it Made in the USA? Bob

Originally Posted by JoeWay

The switch MikeK shows in his post has both functions built into it. The heavy terminals disconnect the battery, and the small terminals disconnect the ignition or other run-required circuit.

Joe

[Chris in WNC]

no comment on where to put the switch, but it's a timely question.

when I tried to start our Coupe today after church the starter switch stuck.
the engine didn't stay running but the starter kept spinning.
got out the tools (quickly) and disconnected the ground strap.
then at a more liesurely pace I unscrewed the starter switch. when lifting it off the starter I heard it "click" when whatever was hung up let go- the switch spring was caught on the big round terminal, I guess.

put it all back together and drove home- no damage, just an inconvenience.
I believe that after 26 years of Model As I'm ready to install disconnect switches.....

[Tom Wesenberg]

As far as turning off the ignition, why not just use the ignition switch? I've never heard of one failing in the ON position. If they fail, it's always in the OFF position.

[Roadster62]

Mike, If the switch in your photo is in fact the best on the market, and I wanted to use it on a Hot Rod do I have the electrical system designed around it, or is it added later. Yes, I'm clueless, terrorfied of things electrical. Bob

[Art Bjornestad]

The switch shown can be ordered from Speedway.

[JoeWay]

Originally Posted by Roadster62

Now that I think about it that switch mounts through a hole under the switch. From what I understand that would be the BEST switch to have in a car, or is there a better one on the market to kill the flames? Who makes it, and is it Made in the USA? Bob

Last time MikeK posted about that switch, I went to autorewire.com and bought one. Here's the link: Disconnect Switch

Total cost including shipping was $32.44.

I'm going to mount mine in the floor behind the battery, with the handle accessible through a hole in the floorboard.

Joe

[Napa Skip]

Originally Posted by Roadster62

So, IF your Model A is running you can switch the battery disconnect switch "OFF" if connceted to the ground side, and it will continue to run. IF you have a "KILL SWITCH" on the other cable you need to turn that to the "OFF" or "KILL" side to stop everything? Correct? Bob

Question: will placing the battery kill switch in the ground battery cable enable the Model A to continue running when the switch is opened (placed in the “OFF” or “KILL” position)?

Background & Observation: Attempting to hand crank and start a Model A without an installed battery has been (at least for me) unsuccessful.

Hypothesis: Interrupting the battery ground connection, whether by lifting the battery cable or placing a batter kill switch in the “OFF” or “KILL” position will cause the Model A to stop running.

Experiment: Start a Model A and interrupt (disconnect) the battery ground circuit and observe whether or not the Model A stops.

[There will be a momentary pause here while I stroll (rigorous scientific investigation cannot be rushed) to the garage and perform the above experiment…]

Results:
Test #1: Model A kept running with battery ground circuit interrupted (“open”)
Test #2: Model A kept running with battery ground circuit interrupted (“open”)
Test #3: Model A kept running with battery ground circuit interrupted (“open”)

Interim Conclusion: Hypothesis is false; interrupting the battery ground circuit on a running Model A will not “kill” the engine.

Retest (to observe the effects of opening the ungrounded battery connection):
Test #4: Model A kept running with ungrounded battery circuit interrupted (“open”)
Test #5: Model A kept running with ungrounded battery circuit interrupted (“open”)
Test #6: [Test #6 was interrupted by the Model A co-owner sticking her head into the garage and asking “What the heck are you doing with our Model A?” Sigh…]

Final Conclusion: Once the Model A is running, interrupting the battery circuit (whether grounded or ungrounded) will not cause the engine to stop (see Note 1). The purpose of a battery or “master” disconnect (kill) switch is to keep the automobile from being able to be started (or to provide an easy method of electrically safeing a vehicle while working on it), not to stop it once running.

Note 1: I'm probably the only Model A owner - engineer or not - who did not know this.
Note 2: This Model A is equipped with an alternator.

Sic transit gloria engineerus...

[Roadster62]

Thanks for the link, but true to form nothing electrical said by one person is the same from the second. The Switch in the catalog/link is different, two poles not 4 and there is sone white wired gizmo in one photo. I do like the looks of the red caps though. Bob

Originally Posted by JoeWay

Last time MikeK posted about that switch, I went to autorewire.com and bought one. Here's the link: Disconnect Switch

Total cost including shipping was $32.44.

Joe

[JoeWay]

Originally Posted by Roadster62

Thanks for the link, but true to form nothing electrical said by one person is the same from the second. The Switch in the catalog/link is different, two poles not 4 and there is sone white wired gizmo in one photo. I do like the looks of the red caps though. Bob

The top photo is the one of the switch with four terminals. The other photos are of accessories that can be purchased with this or other switches.

Joe

[Roadster62]

Just checked and noted that the text below the photos explains things, not that I know how many jigawatts are good or bad, too many or not enough. I may buy one to sit on the shelf for a few years as I work up to the stage when wire needs to be added to the project. Thanks. Bob

Originally Posted by JoeWay

The top photo is the one of the switch with four terminals. The other photos are of accessories that can be purchased with this or other switches.

Joe

[dumb person]

Originally Posted by springerpete

I have one of the switches with the green plastic knob that screws a contact down onto the neg battery terminal or lifts it off the terminal to disconnect the battery from the circuit in my car. Never had an issue with it and see no evidence of arcing or burning or melting. Is this the one that is called junk and considered unsafe ?

i have a green one, it looks like it would only be a problem if the plastic in it was damaged in a strong accident.

[MikeK]

Originally Posted by Roadster62

Just checked and noted that the text below the photos explains things, not that I know how many jigawatts are good or bad, too many or not enough. I may buy one to sit on the shelf for a few years as I work up to the stage when wire needs to be added to the project. Thanks. Bob

Exactly 1.21 gigawatts. You'll also need to get your "A" up to 88 mph. Use the Fordbarn search app to see how to wire in your flux-capacitor.

[MikeK]

Originally Posted by springerpete

I have one of the switches with the green plastic knob that screws a contact down onto the neg battery terminal or lifts it off the terminal to disconnect the battery from the circuit in my car. Never had an issue with it and see no evidence of arcing or burning or melting. Is this the one that is called junk and considered unsafe ?

What renders any type of switch unsafe is the inability to withstand heat. That heat may be from arcing, but in switch failures it generally is from slower heat buildup caused by contact resistance that increases with wear and age. (Heat in watts = Amperes squared x resistance in Ohms. Dont ask!!)

If any structural or insulator part of the switch is made from a thermoplastic (heat meltable plastic) it is not all that safe in many applications. Failure may result in flame and or direct short to ground, or the inability to turn the thing OFF. Even pot-metal bodied switches are less safe, as they melt at a fairly low temp.

Some materials like bakelite and thermosetting plastics do not melt, they eventually smoke and some may burn. Choice of a material that does not support self-combustion and burns very little, with no open flame is a good thing. So is using metals like steel that have a melting point far above the melting point of the copper, brass, or silver(in high-end switches) conductor parts in the switch.

As a general rule, if you lay a hot soldering iron on a switch part and it melts, it is not a reliable choice for continuous use, or for installation in a spot that is not constantly monitored or observed. The same goes for the insulated wire used in automotive apps. Manufacturers use cross-linked plastic insulated wire. If you lay a hot soldering iron on it, it will smoke and charr, but not melt through to the conductor. Try that with the cheapass junk wire sold in autoparts stores! Like a hot knife through butter. You get what you pay for.

[Sparky]

Originally Posted by springerpete

I have one of the switches with the green plastic knob that screws a contact down onto the neg battery terminal or lifts it off the terminal to disconnect the battery from the circuit in my car. Never had an issue with it and see no evidence of arcing or burning or melting. Is this the one that is called junk and considered unsafe ?

Me too; haven't had a problem with it in the three years it has been installed. Of course with this type of switch you need easy access to the top of the battery.

[BillLee/Chandler, TX]

I installed a cut-off switch last winter. Article is at http://www.cedarcreekas.org/TechTopics/CutOffSwitch.pdf Switch is in the ground line as it should be, it is placed so that the lever extends above the carpet just under the front edge of the seat where it is easily accessed but pretty much out of sight.

[1931 flamingo]

BillLee: your link brings me right back to your post. FWIW

Paul in CT

[Roadster62]

Wouldn't cables of the same color be more proper? I'm clueless on things electrical but just think one color per run would be the way to go. Bob

Originally Posted by BillLee/Chandler, TX

I installed a cut-off switch last winter. Article is at http://www.cedarcreekas.org/TechTopics/CutOffSwitch.pdf Switch is in the ground line as it should be, it is placed so that the lever extends above the carpet just under the front edge of the seat where it is easily accessed but pretty much out of sight.

[Bruce Adams]

Originally Posted by springerpete

I have one of the switches with the green plastic knob that screws a contact down onto the neg battery terminal or lifts it off the terminal to disconnect the battery from the circuit in my car. Never had an issue with it and see no evidence of arcing or burning or melting. Is this the one that is called junk and considered unsafe ?

I have had bad luck with those green switches. (two went bad)
Mine were pot metal that breaks after being tightened and loosened too many times and fails open.
I replaced mine with the heavy brass switch through the firewall that I turn off and on with my toe.

[BillLee/Chandler, TX]

Originally Posted by 1931 flamingo

BillLee: your link brings me right back to your post. FWIW

Paul in CT

Paul, it seems to work o.k. for me.

Bill

[BillLee/Chandler, TX]

Originally Posted by Roadster62

Wouldn't cables of the same color be more proper? I'm clueless on things electrical but just think one color per run would be the way to go. Bob

Perhaps, but no real big deal IMHO.

Bill