Battery Disconnect Switch

[Bob Bidonde]

I am not technically convinced that the battery cutoff switch should be in the grounded battery cable. My Model A's have alternators and solid state devices. Both have 12V negative ground electrical systems with their cutoff switch in the positive battery cable that runs to the starter. For many years I have run these cars this way without any electrical issues with alternator regulators and solid state devices.

I doubt there is a technical argument that supports putting the switch in the grounded cable is a better way than putting the switch in the cable to the starter, so I will maintain my status quo which has been successful. I am not saying putting the switch in the grounded cable is wrong.



Thanks to all who responded to my question.

[1928 Sport Coupe]

I used to work for a company that modified off road cranes for special uses and the disconnect switches were always in the positive side.
I installed a disconnect switch in my 1928 and intend to install one in the 1930 as I restore it also. I mounted the switch just in front of the motor mount with a bracket I had that was marked Ford. There was already a threaded hole in the motor mount to bolt it to. To mark on and off I used a Model A ON-Off plate #A-11556 & a Switch plate ring # A-11557.
I have attached a picture, I have replaced all of the wiring with a correct harness since the picture was taken.

[ModelAArmy]

I installed the disconnect switch that vendors sell that mounts to the starter so with this device, the positive wire is used. I have a 12v negative ground vehicle.

[rotorwrench]

A battery disconnect isn't like an inter-circuit on-off switch. If no systems will energize when a battery is reconnected then there will be little or no arcing at all. This is basically a battery switch. Now if several loads are connected live in the car's electrical system then it will draw a load when the battery is turned on and can arc at the switch contacts.

Most battery switches are designed to give a rapid hard contact close when switched ON which will minimize arcing but it may still have a little if there are any loads ON in the car's load circuits.

[Big hammer]

I use the battery disconnect that originally came with my A, I flip the floor mat up, remove the metal cover over the battery, remove the ground terminal from the battery! Hint I leave the terminal loose enough to wiggle and twist it off, R.I.P. Tom Wesenberg also used this method !

[Oldgearz]

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And keep in mind that many millions of cars and trucks for many decades were iinstalled with starter solenoid switches connected to a positive (hot) source which were essentiall cutoff/on switches.

[rotorwrench]

Aircraft have a relay that closes the circuit to the battery. Some just used a very large manual contacter switch on the instrument panel but most use a relay now days.

Ford used foot and hand operated switches for a long time before they started using a starter relay mounted close to the battery. The starter switch button on the dash just grounds the relay to start.

[Bob Bidonde]

Still no scientific explanation of why the switch should be in the ground cable.

[Keith True]

I don't know of any scientific explanation of why it should go in the ground cable,or any scientific explanation why it should go in the hot cable.They need to go in the ground cabls because of a simple mechanical reason,if there is a problem in the cable between the battery and the switch,or in the switch itself the chance of anything catastrophic happening in minimal.Things will just quit,and you won't get the arc welder thing going on like you would with a hot cable that has a problem.I have never seen a reason to put it in the hot line.

[CT Jack]

I believe rotorwrench has identified the key point which is disconnect switch arcing only occurs when there is a load on the circuit. I have solid state ignition in my car and use a separate line connected directly to the battery. Solid state devices do not like line current that can contain spikes of momentary high current which can occur with intermittent faulty lights, toggle switches, poor wiring, etc. Therefore, keeping all accessory devices on a separate fused line minimizes damage.

[Y-Blockhead]

Originally Posted by Bob Bidonde

Still no scientific explanation of why the switch should be in the ground cable.

I think it comes from the reasoning why, when removing a battery, you should remove the ground first. This eliminates the chance of inadvertently arcing the hot side with the wrench to ground.

Following this reasoning, I always put my disconnect on the ground side.

Which brings up a safety point. Don't wear jewelry when working around batteries. Years ago a friend's watchband made contact between Hot and ground. Gold gets HOT real fast. You can imagine the outcome...

[johnneilson]

It really doesn't matter which side of the battery you disconnect, you just need to interrupt the circuit. I see, the mention of race car specifications listed, it depends on the sanctioning body what you are trying to accomplish, trust me, a mechanically injected and magneto ignited motor does not care if the battery is connected or not.

The SCTA requires a battery disconnect to only remove the battery circuit in case of a serious situation where the chassis might need be nipped.

Years ago, in the industrial automation field, some researcher found that digital inputs could react faster if the neg side was switched, in reality it makes no difference.

Years ago my friend pulled a customers Mod "A" into his shop and locked it up for the night. Some time later that night, the wiring caught fire and the place burned down pretty far. If there had been a disconnect switch, this would not have happened, and it would not have mattered which side of the battery was disconnected.

If someone wanted to do an experiment, you could measure the amount of amperage flowing thru each side of the battery. Then potentially use a smaller disconnect switch.

Best, John

[Bob Bidonde]

John Neilson, I agree with your position. It does not matter which battery cable the switch is in so long as it breaks the circuit when it is OFF.

[Y-Blockhead]

Originally Posted by Bob Bidonde

John Neilson, I agree with your position. It does not matter which battery cable the switch is in so long as it breaks the circuit when it is OFF.

Unless like I pointed out, something gets between the hot battery terminal and ground. If the ground side is switched off there would be no complete circuit.

[bbrocksr]

Originally Posted by Bob Bidonde

John Neilson, I agree with your position. It does not matter which battery cable the switch is in so long as it breaks the circuit when it is OFF.

If you put it in the ground side nothing is hot , If you put it in the hot side the hot post and the cable going to the switch are still hot when turned off.
Bill

[bbrocksr]

Originally Posted by Bob Bidonde

John Neilson, I agree with your position. It does not matter which battery cable the switch is in so long as it breaks the circuit when it is OFF.

If you break the ground the complete system is dead. Not so if you break the hot.
Bill

[duke36]

Originally Posted by bbrocksr

If you break the ground the complete system is dead. Not so if you break the hot.
Bill

Same experience

[Flathead]

With a positive ground system you can put the switch on the ground side but it will also be the positive terminal! Everybody is happy!

[Gene F]

Mine disconnects the starter cable too. Been that way since 1994.

[Y-Blockhead]

Please read Model "A" News, September-October 2022 issue, page 13. I wholeheartily believe if he had a Shut Off switch on the ground cable he could have prevented much of the outcome of this mishap.

If the switch was at the starter, it would not have helped... at all!