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Battery cut off switch

ridenby

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Moving battery redoing cut off switch. Got to wondering about putting switch on negitive side rather than positive. Is this going to be a bad idea?
 

trampas

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it's a good idea

current flow direction is opposite the flow of electrons (from - to +) so I always disco the neg
 

dnsfailure

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Interesting. I've always put cutoff on the positive, though I don't really have a reason why, that's just what I've done.

Negative kinda makes sense though, any time I'm working on the electrical system I always pop off the negative cable, not the positive.
 

ridenby

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Interesting. I've always put cutoff on the positive, though I don't really have a reason why, that's just what I've done.

Negative kinda makes sense though, any time I'm working on the electrical system I always pop off the negative cable, not the positive.

When I was removing the cutoff it occured to me that it would not take much to create a short at the switch. Thinking that having it on negative will prevent that from happening.
 

M92PV4U

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(Quote) stolen but this is what I've heard and have done on drag cars.
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I have mine on the positive side. If i remember right, that is how the NHRA rule book states it to be installed. You must also run your alternator feed between the battery and the switch so when its in the off posission it will kill the power to the hole car, including the engine. If you put the switch on the ground side of the battery the engine would continue to run, as it doesnt need battery ground.​


SEP 27, 2007 (Quote)
 

YROC FAB

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Always disconnected the negative side of the battery if your trying to kill all power. If some of your electronics have capacitors or small battery cells built in they can still flow electricity if the just the positive side of the battery is disconnected.
 

SLOWPOKE693

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In most forms of competitive motorsports the battery switch MUST be tied in to the positive side of the battery and alternator charging wire to meet the rules. As someone stated above, if its tied into the negative side, once the engine is running the battery switch won't do shit to turn off the electrical system. It is installed on the positive side so if there is ever a crash ect and fuel lines get ripped open the pumps and engine will not stay running when the switch it turned off. It's for saftey.

If all your looking to do is add one on to keep a battery from discharging, go on with your bad self and install it on the negative terminal.
 

dnsfailure

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I'm not so good with electronics. If a motor could still keep running without being grounded (I assume it would die pretty quick after though, if it has electronic fuel pump?), could you push-start a car that's not grounded?
 

SLOWPOKE693

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I'm not so good with electronics. If a motor could still keep running without being grounded (I assume it would die pretty quick after though, if it has electronic fuel pump?), could you push-start a car that's not grounded?

Nope. As long as the engine is running and the alternator is charging it will never shut off.

You have an older car or truck? If so, go start it up and take the battery cables off while it's running. I bet you it stays running until it runs out of fuel.

NHRA, NASCAR, ect all require it to be wired in to the posititive side for this reason.
 

dnsfailure

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Nope. As long as the engine is running and the alternator is charging it will never shut off.

You have an older car or truck? If so, go start it up and take the battery cables off while it's running. I bet you it stays running until it runs out of fuel.

NHRA, NASCAR, ect all require it to be wired in to the posititive side for this reason.

Mine are diesel, so they run without a battery regardless :lmao:

That makes sense with the alternator, I guess the distributor doesn't need ground to make spark then? I know magneto distributors will spark on their own.
 

ThePanzerFuhrer

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All my heavy equipment the shut offs are on the negative side. That way when you run the volatage down you don’t run a bijillion Amps through the switch, instead you just burn the cables off at the starter.

fwiw cat makes a awesome keyed shutoff that holds up forever. I got like 40 of them in service and the only reason I had them fail is the keys rusting in them from the lack of use.
 

dnsfailure

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All my heavy equipment the shut offs are on the negative side. That way when you run the volatage down you don’t run a bijillion Amps through the switch, instead you just burn the cables off at the starter.

fwiw cat makes a awesome keyed shutoff that holds up forever. I got like 40 of them in service and the only reason I had them fail is the keys rusting in them from the lack of use.

For my current project, I picked up an old porsche cutoff switch, where the handle to the switch comes off when it's shut off. I like it because it looks different than the standard Moroso style switches. The only problem though, is that when you turn it off, the handle/key literally just falls out. So you can't leave it in there if you want to.

Does the CAT key stay retained somehow when it's turned off? or do you have to remove it every time like this one?


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SLOWPOKE693

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Mine are diesel, so they run without a battery regardless :lmao:

That makes sense with the alternator, I guess the distributor doesn't need ground to make spark then? I know magneto distributors will spark on their own.

Think of the alternator as the positive and the engine as the negative. Everything stays grounded because the positive has a path back via the engine, frame ect.
 

ThePanzerFuhrer

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For my current project, I picked up an old porsche cutoff switch, where the handle to the switch comes off when it's shut off. I like it because it looks different than the standard Moroso style switches. The only problem though, is that when you turn it off, the handle/key literally just falls out. So you can't leave it in there if you want to.

Does the CAT key stay retained somehow when it's turned off? or do you have to remove it every time like this one?



Cat key will not come out unless off. Part number 7n-0719. I get them through my cat dealer not sure if the Amazon ones are oem or just Chinese knockoffs.
 
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ridenby

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So I should use the cutoff on the Positive cable , run the output of the alternator back to the switch. I guess the switch should be in some type of container to prevent being shorted by random crap?
 

WaterH

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In most forms of competitive motorsports the battery switch MUST be tied in to the positive side of the battery and alternator charging wire to meet the rules. As someone stated above, if its tied into the negative side, once the engine is running the battery switch won't do shit to turn off the electrical system. It is installed on the positive side so if there is ever a crash ect and fuel lines get ripped open the pumps and engine will not stay running when the switch it turned off. It's for saftey.

If all your looking to do is add one on to keep a battery from discharging, go on with your bad self and install it on the negative terminal.

I don't get this. Are you saying the rules require the positive battery disconnects and the charging wire disconnects from the positive side? (Meaning they must not be connected together) Otherwise, it will keep running. Is this some kind of double switch?

In aviation, they have two switches. One is the master that cuts the battery and the other is the alternator that cuts the field. A lot of times it will be two rocker switches right next to each other so you can switch as one or seperate. This works good as you can cut an overheating battery off and still run the aircraft. Or you can cut all electrical in a fire. The problem now a days is a lot of cars have alternators that don't have a field wire. (At least not an accessible one)
 

SLOWPOKE693

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I don't get this. Are you saying the rules require the positive battery disconnects and the charging wire disconnects from the positive side? (Meaning they must not be connected together) Otherwise, it will keep running. Is this some kind of double switch?


To answer your questions..... Yes, and No.

Nascar and NHRA allow you to run a high amp relay on the alternator charging wire. The battery switch breaks the connection to the positive battery wire and positive trigger wire on the relay and that shuts the engine and every other electrical component down with 1 switch flip.

Or you can run the battery positive wire through the switch and just have the alternator charging wire run to the battery side of the switch only.

Racecars almost exclusively run 1 wire alternators. Less=Lighter=Faster

I haven't heard of a battery overheating in like, never.... Must be an aviation thing. :flipoff2:
 
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SLOWPOKE693

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So I should use the cutoff on the Positive cable , run the output of the alternator back to the switch. I guess the switch should be in some type of container to prevent being shorted by random crap?

Read my post to WaterH, that should explain it a little better. You can use a high amp rated relay to accomplish the same thing. I always did it this way on my circle track and drag cars to prevent having to run a wire all the way to the switch. The furthur the switch is away from the alternator, the heavier gauge the wire has to be. I dont like adding unnecessary weight to stuff I'm trying to make go fast. :laughing:
 
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WaterH

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I haven't heard of a battery overheating in like, never.... Must be an aviation thing. :flipoff2:

I have to admit, I've never had a battery overheat on an aircraft. But it must be a thing because every year we would train the emergency procedure in the simulator.

Having said that, I did have a battery blow up on an old Jaguar XKE I restored. Can't remember if it had an alternator or a generator. But it had Lucas electronics. (Nuff said)
 

ridenby

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What a can of worms. Yes,some sanctioning groups require a pos. located cutoff. It seems a lot of folks say go with a neg. cutoff. I am going to do neg.cutoff at the battery for the simplicity. With luck I will be cutting power for non emergency reasons.
 

dnsfailure

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What a can of worms. Yes,some sanctioning groups require a pos. located cutoff. It seems a lot of folks say go with a neg. cutoff. I am going to do neg.cutoff at the battery for the simplicity. With luck I will be cutting power for non emergency reasons.

to be honest, that's the only real reason I run a cutoff switch (I don't race), to just turn off all the power when the vehicle is not in use.
 

WaterH

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to be honest, that's the only real reason I run a cutoff switch (I don't race), to just turn off all the power when the vehicle is not in use.

In that case, there is a good reason to cut the negative. If you're working on the truck and you drop a wrench, cable, metal part on the positive post and some other part of the truck, nothing happens. If you touch the negative to some part of the car, it's just a momentary power up. But if you cut the positive, doing the same can be exciting.
 
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