Battery Disconnect Switch

[Bob Bidonde]

I have read more then once that a battery cutoff switch should be in the grounded battery cable. I do not have an understanding about why a battery cutoff switch is best put into the ground cable versus the opposite cable that goes to the starter. Somebody please provide a sound engineering rationale for putting the switch in the grounded cable.

[Badpuppy]

Eliminates safety issue should one of the switch contacts inadvertently connect to ground - swinging wrench, etc.

[Oldgearz]

Really a good question. My switch disconnects the starter cable. I've read about "safety issues" when breaking the "hot" cable, but like you I don't really understand why. My rationale for why it is a safety issue when breaking the negative "hot" cable is that if the switch should short out internally, there could be a lot of fireworks with no immediate way to shut the power off. Using an in-line circuit breaker on the negative cable between the battery and the switch could solve this problem, but I haven't seen any cables like this advertised for an A.

[alexiskai]

I've seen a few anecdotal reports that placing the switch on the hot side had a tendency to blow out solid-state components. We don't have many of those, but for example alternators have diodes, or you might have an electronic ignition, etc.

A lot of folks say that if your purpose is solely to reduce parasitic drain or an unattended short, it doesn't really matter which side you put it on. They recommend considering factors like, can you get the switch closer to the battery on one side or the other, or does it make a difference in terms of being able to reach the switch easily.

[Jim Brierley]

It insures that EVERYGTHING is shut off. Many later model cars have some items that are on all the time. Of course many newer cars have things that must remain on, like clock and radio settings, engine computer, etc., so it doesn't apply to everything today as it did year ago.

[SAXBY2]

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Here's one approach from Hot Rod mag

[Oldgearz]

As a follow-up, I've installed an additional ground to the transmission housing which many people suggest. Seems like I would need to have two cut-off switches if I put the switch on the positive (ground) circuit. Do the benefits of the two ground straps from the positive battery terminal outweigh the benefit of putting the cut-off switch on the ground circuit???

[WHN]

Installing battery, negative first.

Removing battery, positive first. This is were your disconnect switch should be installed.

This is for a positive ground car.

Modern cars would be positive first, negative second.

Otherwise you could have sparks flying.

Enjoy.

[alexiskai]

Originally Posted by Oldgearz

As a follow-up, I've installed an additional ground to the transmission housing which many people suggest. Seems like I would need to have two cut-off switches if I put the switch on the positive (ground) circuit. Do the benefits of the two ground straps from the positive battery terminal outweigh the benefit of putting the cut-off switch on the ground circuit???

I guess it depends on how you wired it up, but on mine, the additional cable connects to the bolt on the chassis that the ground strap goes to. So to install a cutoff switch, you would remove the strap from the chassis to the battery, and instead run a wire from the chassis to the switch and then a wire from the switch to the battery.

[Keith True]

The switch should be in the ground side in case of trouble in the cable between the battery and switch.If the cable wears through on something,no bog deal.It is just the ground side.If that happens on the hot side,you are off to the races.I saw a cable pull out of the ring terminal on a switch once,and drop down.The switch was mounted high on the firewall of an A.It arced and bounced,but luckily it only blew the battery up.I saw one mounted in one of those lollipop things on the starter,the vibration broke that one and it dropped and shorted.I watched a John Deere 40 crawler burn up in front of my garage from a bad switch mounted in the hot side.I cannot think of one good reason to mount the switch in the hot side.

[ModelA29]

All the diagrams I've seen show the disconnect in the hot side. Shorts and failures can be traced to bad install or failure to inspect on a regular basis.

[Phil Brown]

Makes more sense to break the ground side, but most racing organizations' rules want the hot side switched
Its the difference between a battery disconnect and a "kill" switch
Remember if you have an alternator the switch will NOT shut down a running engine if needed without breaking the feed power from the alternator
I've done it both ways, both work, lean toward switch in the ground side

[alexiskai]

Originally Posted by Phil Brown

Makes more sense to break the ground side, but most racing organizations' rules want the hot side switched

People cite the racing rules a lot, but those are to enable safety personnel to kill the engine instantly. Different design objective.

[old31]

Originally Posted by Phil Brown

Makes more sense to break the ground side, but most racing organizations' rules want the hot side switched
Its the difference between a battery disconnect and a "kill" switch
Remember if you have an alternator the switch will NOT shut down a running engine if needed without breaking the feed power from the alternator
I've done it both ways, both work, lean toward switch in the ground side

"Remember if you have an alternator the switch will NOT shut down a running engine if needed without breaking the feed power from the alternator"

I have lost you on this. Are you sayin that if the kill switch is on the ground side a running car can not be stopped by flipping the switch? Please explain so the simple minded can understand.

[Phil Brown]

Originally Posted by old31

"Remember if you have an alternator the switch will NOT shut down a running engine if needed without breaking the feed power from the alternator"

I have lost you on this. Are you sayin that if the kill switch is on the ground side a running car can not be stopped by flipping the switch? Please explain so the simple minded can understand.

Yes, an alternator back feeds enough to keep the motor running. Want to try it just go out start the car and pull off the battery cable, it will continue to run.
To stop a motor with an alternator you also need to break the feed from the alternator

Back when we were kids hang out with our first cars inevitably someone had a dead battery and none of us had jumper cables. Easy pull out your dead battery put in your buddys good one, start your car, pull the cables off, your car continues to run, give the battery back to your buddy, put your battery back in and hopefully when you hook it up it starts charging

[Bruce of MN]

Originally Posted by SAXBY2

Here's one approach from Hot Rod mag

Can you post a bigger image, I can't read that.

[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.