Battery Disconnect Switch

Chewbacca

Chewbacca

Junior Member
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.
 
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.
 
Tom Wesenberg said:
Polarity doesn't matter, but if I used one it would be on the ground cable. That is for safety reasons.
Click to expand...

Explain why. Bill W.:confused: 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.
 
Last edited:
Tom Wesenberg said:
Polarity doesn't matter, but if I used one it would be on the ground cable. That is for safety reasons.
Click to expand...

I don't get it?:confused: 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?:confused:
 
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.
 
neg side ! no arching that way . always use the neg side for less damage ........
 
steve norcal said:
neg side ! no arching that way . always use the neg side for less damage ........
Click to expand...

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.
 
Mike V. Florida said:
Let's assume the electricity goes out the pos side of the battery and the neg it attached to the frame.
Click to expand...


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.
 
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.
 
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.
 
Last edited:
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.
 
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
 
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.​




 
Last edited:
Roadster62 said:
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.
Click to expand...

Roadster62 said:
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
Click to expand...

#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.
51eDyXGGpLL._SL500_.jpg
 
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
 
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.
 
You must log in or register to reply here.
Back
Top