Roadster62
Well-known member
How good? Can the uneducated understand it? Does it agree with the info posted in this thread? Bob
Battery Disconnect Switch
- Thread starter Chewbacca
- Start date
Napa Skip
Senior Member
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!
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
Well-known member
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
JoeWay
Well-known member
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
Well-known member
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
Well-known member
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
Chris in WNC
Well-known member
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.....
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
Well-known member
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
Well-known member
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
Senior Member
The switch shown can be ordered from Speedway.
JoeWay
Well-known member
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
Senior Member
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...
Last edited:
Roadster62
Well-known member
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
JoeWay
Well-known member
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
Well-known member
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
dumb person
Well-known member
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
Well-known member
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
Well-known member
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
Well-known member
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
Senior Member
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.