Battery Disconnect Switch

[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]

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]

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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.

[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!

[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.

[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!

[WHN]

Put the disconnect on the ground cable. That is the end of the story.

Once ground is disconnected, there can be “NO” sparks or discharge.

When you install or reconnect a battery. Connect the ground cable LAST!

Without a ground, electricity will not work. Think of birds on a power line, their not grounded that’s why they don’t get killed or shocked.

Enjoy.

[Rob Doe]

Y-Block, I don't have access to the magazine, but I assume something happened between the - neg battery post and the shutoff switch? Can you summarize what happened for us?

[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

[Y-Blockhead]

Originally Posted by Rob Doe

Y-Block, I don't have access to the magazine, but I assume something happened between the - neg battery post and the shutoff switch? Can you summarize what happened for us?

Rob, the brake rod wore thru the insulation, causing a direct short. He pulled the battery cover in the floor off and pulled the negative cable off of the battery, causing a spark. The side of the battery then blew out. Thankfully he was not hurt badly but did get acid on his arms. His wife was able to pour water on his arms.

If he had a shut off on the ground side he could have shut all power off even before removing the battery cover. In this instance, he did not have a shut off switch installed at all!

[Ruth]

I seen that article. Must have got really hot to melt the negative battery post and the connector!!

[DJ S]

I have always been told to switch the "ground" side the of the battery... The only advantage that I can see is in the event the battery disconnect switch "shorts", the "ground" is simply re-established.

If the battery disconnect switch controls the "hot" side and shorts... well you've probably got yourself a fire.

I use my battery disconnect switch every time I park the car, so it is being cycled quite often... just like everything, someday it will wear out.

[Phil Brown]

All valid points for switching the ground side, no argument but why is it that all of the other electric components are switched on the non-ground side ? Headlights, cowl lights, dash lights even generator cut out