Wiring Battery Disconnect

[aarvig]

So...it seems there is much debate about this. Do I wire the battery disconnect to the negative or positive terminal? With my setup, it would be easier to wire it through the negative terminal. Thoughts?

[Nigel Allen]

Negative is safer. Then the exposed studs on the rear of the switch are at the same electrical potential as the surrounding metal.

Hope that makes sense.

Cheers, Nigel

[edwardb]

Agreed. Lots of debate on the subject. Some sanctioning bodies say positive. Some say negative. Either will get the job done. For cars I've done, where the battery is some distance from the switch (front Breeze mount, switch in cockpit) that would have meant a long negative cable run versus grounding by the battery. So I switched the positive side. If your situation is different, then you have a choice of course. Regardless, don't leave the studs exposed. The Ron Francis MS-1 switches I use come with boots to cover things up.

[RoadRacer]

If you're looking for votes, I chose negative. To me it makes more sense, but at the core both will work.

[GoDadGo]

I did the HOT side only because my battery is in the trunk and my Marine Keyed Switch is in the transmission tunnel just below the parking brake handle.
Since the solenoid is on my starter, it's a crazy clean installation.

https://www.westmarine.com/buy/hella...switch--598375

If you go down this path just understand that the All Keyed Switches Are Not Created Equal.
The 1st one I purchased (Cheepo/Cheepo) from my local auto parts store looked like this switch, but the amp ratings didn't come close.

Had I seen this item when I was doing my build, I would have likely gone with this unit instead.

https://www.summitracing.com/parts/s...UaAnVDEALw_wcB

[RoadRacer]

I did the HOT side only because my battery is in the trunk and my Marine Keyed Switch is in the transmission tunnel just below the parking brake handle.

So typical of GoDadGo to be different First it's the chevy.. now this.

[rich grsc]

Here's how I look at, what does a computer use to control a device, it switched the ground side, not the hot side. Also if you switch the hot side, the positive post at the battery can still short to the frame.

[JohnK]

Correct me if I'm wrong (I often am) but switching the ground side would mean that everything that you want to be able to switch off would have to be grounded back to a single point at the switch. I know that a lot of folks add auxiliary grounds in multiple locations throughout the car. This would prevent anything grounded to any of those grounding points from being switched off, would it not?

[CDXXVII]

I chose to disconnect the positive. Given the location it made disconnecting the negative difficult. One benefit is that it gives you a good connection spot.

Apologies about the sideways pic. I can not seem to fix it.

Disconnect.jpg

Here it is all covered up and protected. The cover has an insulator on the back side to prevent any short circuits.

IMG_0153.JPG

[RoadRacer]

Correct me if I'm wrong (I often am) but switching the ground side would mean that everything that you want to be able to switch off would have to be grounded back to a single point at the switch. I know that a lot of folks add auxiliary grounds in multiple locations throughout the car. This would prevent anything grounded to any of those grounding points from being switched off, would it not?

No. You're disconnecting the battery from the chassis, so you can still ground to anywhere on the chassis.. the circuit is still broken.

But, the same is true for positive side - you just need to break the circuit somehow

[JohnK]

No. You're disconnecting the battery from the chassis, so you can still ground to anywhere on the chassis.. the circuit is still broken.

But, the same is true for positive side - you just need to break the circuit somehow

See, I was pretty sure I was wrong.

[NAZ]

First question you should ask yourself is why do you need a battery disconnect. Are you going to race your car? Is it a requirement of a sanctioning body? If so, they will have specific rules dictating design, location, and polarity. If not a requirement, then why do you need one? How many of your personal vehicles have one? What will happen if you don't have one?

Functionally, it doesn't matter which side of the battery you disconnect if simply wanting to shut off the power. But do you have anything that requires constant power?

I have one installed on my car ONLY because NHRA requires it and they dictate it must switch the positive side, cut ALL power, and be located in the rear of the car so the Safety Safari can easily reach it, and it must be clearly marked which way is off, if a push/pull type, push must be off. Lot's of rules around a simple switch. If not for that requirement I'd not have installed one at all as it's simply superfluous and something else to go wrong.

[GoDadGo]

So typical of GoDadGo to be different First it's the chevy.. now this.

Yep, Nearly Everything In My Car Is Different & I Even Moved The Fuse Block & Flipped The Blinkers Too!

..Left Is Right & Right Is Left So Now You Know Why Mr. Kleiner Calls Me Heyoka, The Owner & Driver Of The Redbone Roadster!

[narly1]

First question you should ask yourself is why do you need a battery disconnect.

Because not every builder has the knowledge and experience to safely wire an entire car. Not just the electrical fundamentals aspects (wire gauges, fusing, etc.) but also taking into consideration things like environmental and mechanical effects (think heat melting or humidity/salts causing corrosion), vibration/flexing/chafing, etc all effects which could cause insulation failure and shorting.

A cut off switch is the last line of defense for the novice auto electrician and gives them some piece of mind.

[aarvig]

Because not every builder has the knowledge and experience to safely wire an entire car. Not just the electrical fundamentals aspects (wire gauges, fusing, etc.) but also taking into consideration things like environmental and mechanical effects (think heat melting or humidity/salts causing corrosion), vibration/flexing/chafing, etc all effects which could cause insulation failure and shorting.

A cut off switch is the last line of defense for the novice auto electrician and gives them some piece of mind.

My purpose is primarily anti-theft. These cars are easy to Hotwire and this would be an additional ounce of prevention. Also, what is mentioned here about novice wiring...yep, that’s me!!
I did end up doing the negative side tonight. Worked like a charm. Thanks for your help guys!!

[GoDadGo]

My purpose is primarily anti-theft. These cars are easy to Hotwire and this would be an additional ounce of prevention. Also, what is mentioned here about novice wiring...yep, that’s me!!
I did end up doing the negative side tonight. Worked like a charm. Thanks for your help guys!!

If you want an additional switch to deter possible theft, then consider wiring this into your clutch safety switch circuit.

https://www.amazon.com/Philmore-Swit.../dp/B00B7N8FOW

I've got one on my car and it works fine and is in a hidden location.

[rich grsc]

I think these are one of the least likely car that anyone would steal. They are so unique that they would be easily spotted and very hard to sell, so loud you can't sneak off with it, and has no value to any chop shop. No joy rider will steal it because they can't drive a manual transmission. JMO

[AC Bill]

When I built my car, I used the ground circuit on the cut-off switch. It wasn't the most common way to do it, but anytime you do almost any work on a vehicle, every service manual say's, first remove negative cable from battery. Made sense to use the switch for cutting the ground.

[FF33rod]

Negative is safer. Then the exposed studs on the rear of the switch are at the same electrical potential as the surrounding metal.

Hope that makes sense.

Cheers, Nigel

Actually not true Nigel. If the negative of the batter is disconnected from the chassis then there is actually no reference for the chassis at all now. So in the end, due to paths from "always on" loads, the whole chassis is now at +12V.

In any case, I would suggest switching +12V is the easiest because in this way you can still have a separate (fused) +12V wire for things like radio memory (items needing always on) and yet switch the main +12V for the starter etc...

Steve

[NAZ]

Using a battery cutoff switch to prevent electrical fires and for an anti-theft device are mutually exclusive goals. One should be immediately accessible to the driver and the other should be hidden from thieves.

If you want to cut battery power a more convenient way is to use a continuous solenoid such as this: Solenoid. Of course, the coil has to be powered by a switched constant hot such as a MASTER switch like used in aircraft https://www.summitracing.com/parts/nxo-00-51025

If you want to add a level of anti-theft security there is a better way. Install a NO relay in the power wire from the starter switch to the starter solenoid. The relay will open the starter switch circuit until the relay coil is energized. Energize that relay coil using power from another unrelated switch, such as a dome light, parking light, dash lights, fog lights, or other accessory switch that would normally have nothing to do with starting the vehicle. For example, to start the car you turn on the switch for the fog lights then turn the ignition switch to start. If you fail to turn on the fog lights first the car won’t start. You could use a hidden switch dedicated to this but an experienced thief will likely find it eventually but piggy back to a switch used for a different purpose is more difficult to find. If using a dedicated switch make it a momentary contact and label it something like “passing lights” or ???, use you imagination.

[egchewy79]

FWIW, I added a kill switch to my electric fuel pump ground. You need to have this on to get the fuel pump on prior to ignition. Also allows me to quickly kill the fuel if there's an issue. Between this, my battery cut off switch (wired to pos wire), clutch safety switch, I think I'm good from a theft deterrent standpoint.

[Just puttering]

When I built my car, I used the ground circuit on the cut-off switch. It wasn't the most common way to do it, but anytime you do almost any work on a vehicle, every service manual say's, first remove negative cable from battery. Made sense to use the switch for cutting the ground.

From my experience, you remove the negative because the wrench will not complete a circuit from the negative to metal around the batttery! While disconnecting the positive and touching metal around the battery with the wrench and you get sparks and the possibility of welding the wrench to what ever you touched. Otherwise neg or pos does the same thing!

[Andrew Davis]

Hate to be cynical, but aside from teen joy riders, professional thieves would just winch it onto a flatbed and worry about the electricals back at the shop. Andy

[narly1]

I've been re-thinking this thread a bit and I'm thinking that both a fusible link and a cut-off switch would be the best of both worlds. The first to catch any catastrophic high current shorts and the second to isolate the battery from the vehicle's power systems for long term storage and parasitic drain reasons.

Most new cars that I have seen have a high current fusible link on the positive battery terminal.

[JohnK]

Hate to be cynical, but aside from teen joy riders, professional thieves would just winch it onto a flatbed and worry about the electricals back at the shop. Andy

This is where an electric parking brake comes in handy! Kill the power to the parking brake while it's engaged, and the car isn't going anywhere. Sure, they could jack up the rear and roll it on the front wheels, but thieves are opportunists, and chances are that by the time they realize that the power has been killed, they can't Hotwire it, and they can't release the parking brake, they'll likely just move on to another target. If they're that determined to steal it... well, that's where good insurance comes into play.

[rich grsc]

Hate to be cynical, but aside from teen joy riders, professional thieves would just winch it onto a flatbed and worry about the electricals back at the shop. Andy

Well, these are high risk, low value cars to a professional car thieve. Pro's steal cars for the parts, there is very little value in chop shop parts on these cars, they are high risk because they are unique, can't just resell one, and have it 'lost' from sight in the thousands of identical cars.

[Hoooper]

This is where an electric parking brake comes in handy! Kill the power to the parking brake while it's engaged, and the car isn't going anywhere. Sure, they could jack up the rear and roll it on the front wheels, but thieves are opportunists, and chances are that by the time they realize that the power has been killed, they can't Hotwire it, and they can't release the parking brake, they'll likely just move on to another target. If they're that determined to steal it... well, that's where good insurance comes into play.

If they are using a winch it will barely even notice that the rear tires are locked up.

[Jetfuel]

In my current project for theft deterrent and personal safety if no seat belt buckled...no crank...

Jet

[Hoooper]

Mine has a master disconnect, PDM shutdown switch, and an RFID starter interlock that will all keep it from starting, but I have done that mostly because I am not using a keyed ignition. As was mentioned before if pros show up they will be doing the same thing they would be doing to my daily, which is just winch it onto a flatbed and deal with sorting out the electronics later.

For the disconnect I have mine on the positive side as seems to be required by NHRA, NASA, and SCCA all seem to require that the disconnect be able to shut off the engine. You can run the engine with battery completely removed from the car, which means just disconnecting the negative doesnt accomplish the goal of shutting down the engine. Disconnect the negative and you would only kill electrical components which are grounded directly to the battery terminal.

[D Stand]

Even disconnecting the positive after car is started the engine will still run.

[NAZ]

Even disconnecting the positive after car is started the engine will still run.

Correct, in many cases but doesn't have to be that way. If you run the alternator power (+) wire to a point on the battery side of the master switch then when the switch is open the engine will stop running if you have a conventional ignition system (not a magneto).

[D Stand]

Very true NAZ

[FFinisher]

Correct me if I'm wrong (I often am) but switching the ground side would mean that everything that you want to be able to switch off would have to be grounded back to a single point at the switch. I know that a lot of folks add auxiliary grounds in multiple locations throughout the car. This would prevent anything grounded to any of those grounding points from being switched off, would it not?

If the ground from the battery is connected to the frame all components grounded anywhere in the car should work if the rest of thier circuitry is correct.

If the cable to ground from the battery to the chassis is interrupted by a switch, the the chassis is no longer a conductor for the current and becomes a big chunk of metal.


I have done it both ways depending on logistics.

[Bob Cowan]

Correct me if I'm wrong (I often am) but switching the ground side would mean that everything that you want to be able to switch off would have to be grounded back to a single point at the switch. I know that a lot of folks add auxiliary grounds in multiple locations throughout the car. This would prevent anything grounded to any of those grounding points from being switched off, would it not?

No, that is not correct. If you remove the negative terminal from the battery, nothing will work. That's what the switch does.

Ideally, you should use a 4 post switch, and place it in the positive side of the battery. Placing it on the negative side will not shut off the engine when it's running. On a running car, you can completely remove the battery and it will still be running. The 4 post switch disconnects the battery and alternator, shutting off the engine.

[AC Bill]

After they read this thread, hopefully nobody will test their switch, with the engine running..or they will be shopping for a new alternator..

As mentioned, there is a way to wire the switch to avoid this, but I'm betting 70% of the builders with a cut-off switch, haven't wired them that way.

My main reason for installing a cut off switch wasn't so much for an emergency, such as a crash, it was more to do with security, and if working on the car. There would be no reason for me ever, to use it to try and shut off the car.