NHRA Approved Battery Disconnect

[05stroker]

So what you're saying is that it is NHRA-acceptable to allow the high current line between the battery and the alternator to remain hot even after the required switch is thrown "off"? If so, is there any requirement that it at least be fused?


Norm

Good question !

I do have mine fused anyhow. No way there is a hot battery cable running the length of the cabin with no fuse .

So,is it legal ?

 

[GB10]

I have never failed tech with this wiring configuration. If it gives you piece of mind then you could install an inline fast-acting fuse on the alternator wire by the switch. This would allow the fuse to blow if the alternator wire was pinched for some reason. However, this is unneccesary for NHRA tech.

 

[tmcolegr]

Good question !

I do have mine fused anyhow. No way there is a hot battery cable running the length of the cabin with no fuse .

So,is it legal ?

+1 same here

I have a 150 amp fuse at the battery for the (+) battery cable that runs all the way up to the alternator. Technically no the alternator is not being disabled, but the fuse should blow in the event of a short in the alternator or cable - serves the same purpose as the 8 gage fuse link in the OEM alernator (+) battery cable.

 

[Norm Peterson]

I have never failed tech with this wiring configuration. If it gives you piece of mind then you could install an inline fast-acting fuse on the alternator wire by the switch. This would allow the fuse to blow if the alternator wire was pinched for some reason. However, this is unneccesary for NHRA tech.

SCCA Solo requires Modified category cars on race rubber to have a master switch that cuts power to everything except an onboard fire system.

Not a reflection on your wiring installation, but the NHRA requirement really does seem to come up short when cars doing up to maybe 80 mph running eseentially one at a time on a large open parking lot have a stricter requirement than cars running side by side and between walls at 140+.

SCCA's requirement probably followed over from the club racing side, but it shouldn't be any big secret that crashes do tend to pinch things, including wires and fuel lines . . .


Norm

 

[TexasKyle]

"This cutoff switch must be connected to the positive side of the electrical system and must stop all electrical functions, including magneto ignition"

To me that says you have to kill the link between the Alt/battery and the fuse panel. Purpose being that in the event of a crash, fire etc, the car engine can be shutdown from outside the vehicle by safety personnel.

That means putting the cable coming from the Alt on the battery side of the disconnect switch. The other side of the switch is the power lead to the fuse box. You wouldnt have to open the cable coming from the Alt, because when the engine dies, so does the Alt. Killing power to the main lead kills the ignition, which shuts the car down.

The car will run without the alt, or without the battery (if it was running before killing the battery) but it can't run with both of them disconnected from the ignition. That is the logic I am using here anyhow.

 

[CPRsm]

I'm sure it's just the way you worded that which has me confused.

You connected the cable from the battery and the cable to the alternator to the (+) battery side of the disconnect switch and the battery cable to the starter/BEC to the load side of the disconnect switch correct??

Not quite, that would keep the alt hot.

2 cables come from the front of the car to the trunk. One alt cable, one from the starter post. These two will go to one side of the cut off switch. The other side of the switch will go to the battery. Once you have these hooked up you run a wire from the starter post to the fuse box. This will re-connect power to the car.

 

[tmcolegr]

Not quite, that would keep the alt hot.

2 cables come from the front of the car to the trunk. One alt cable, one from the starter post. These two will go to one side of the cut off switch. The other side of the switch will go to the battery. Once you have these hooked up you run a wire from the starter post to the fuse box. This will re-connect power to the car.

Your correct the alternator will be hot all the time.

Your scenario incorrect

In your scenario the cable you just ran from the starter post to the fuse panel is also connected to the alternator cable which will keep the engine running as long as the alternator is producing current.

The alternator cable must be connected to the (+) battery side of the disconnect switch. The starter and fuse box cable needs to be connected to the load side of the disconnect switch. Pushing the disconnect switch interrupts the power supply to the starter and fuse panel. This in itself will shut down the vehicle as the alternator is not supplying current to either of those circuits.

GB10s previously posted schematic is correct.

 

[GB10]

SCCA Solo requires Modified category cars on race rubber to have a master switch that cuts power to everything except an onboard fire system.

Not a reflection on your wiring installation, but the NHRA requirement really does seem to come up short when cars doing up to maybe 80 mph running eseentially one at a time on a large open parking lot have a stricter requirement than cars running side by side and between walls at 140+.

SCCA's requirement probably followed over from the club racing side, but it shouldn't be any big secret that crashes do tend to pinch things, including wires and fuel lines . . .


Norm

I agree with what you are saying and I also agree that cutting power to everything is a good idea. NHRA requirements seem to be much less stringent than SCCA rules, but unless there was an incident which involved a pinched alternator wire where a death or serious injury occured, then NHRA will unlikely change the rule. I don't agree with it, but that seems to be the case with all of their rules.

Not quite, that would keep the alt hot.

2 cables come from the front of the car to the trunk. One alt cable, one from the starter post. These two will go to one side of the cut off switch. The other side of the switch will go to the battery. Once you have these hooked up you run a wire from the starter post to the fuse box. This will re-connect power to the car.

Ideally, cutting power to the alternator is a good idea. However, your wiring method will not kill the motor and all electronics if the alternator and starter/fuse box wires are on the same post of the load side of the switch. The alternator generates electricity which is still connected to the fuse box even when the kill switch is depressed. The only function of the kill switch in your scenario would be to stop the alternator from charging the battery.

 

[CPRsm]

The only function of the kill switch in your scenario would be to stop the alternator from charging the battery.

Yup ! Kinda anyway.

Your scenario incorrect

In your scenario the cable you just ran from the starter post to the fuse panel is also connected to the alternator cable which will keep the engine running as long as the alternator is producing current.

Not really incorrect. Different maybe. Both of yours will work. But in your methods the wire going from the battery to Alt will remain hot after the kill. Mine will not. From the switch forward, the car is completely dead, alt wire included. Only live feed is the 1.5 foot section or so from the batt to switch. I could switch the alt wire to the other side of the terminal I suppose. A little anal retentive on my end I guess, but better I think. The Alt needs the battery to charge.

 

[05stroker]

Not quite, that would keep the alt hot.

2 cables come from the front of the car to the trunk. One alt cable, one from the starter post. These two will go to one side of the cut off switch. The other side of the switch will go to the battery. Once you have these hooked up you run a wire from the starter post to the fuse box. This will re-connect power to the car.

Your correct the alternator will be hot all the time.

Your scenario incorrect

In your scenario the cable you just ran from the starter post to the fuse panel is also connected to the alternator cable which will keep the engine running as long as the alternator is producing current.

The alternator cable must be connected to the (+) battery side of the disconnect switch. The starter and fuse box cable needs to be connected to the load side of the disconnect switch. Pushing the disconnect switch interrupts the power supply to the starter and fuse panel. This in itself will shut down the vehicle as the alternator is not supplying current to either of those circuits.

GB10s previously posted schematic is correct.

Sounds like you guys are talking about the same thing ,just wording it differently.


Here is how I see what ya'll are saying, the batt 12v + and the alt. go to one side of the disconnect, the other side goes to the starter and fuse box ect. With it setup like this the car will in fact be shut off when the disconnect is switched but , cable from the alt to the batt. will still be connected and rely on a fuse to terminate that connection. This is how I have mine setup at the moment. Along with a fuse between the battery and starter ect.

 

[CPRsm]

Sounds like you guys are talking about the same thing ,just wording it differently.


Here is how I see what ya'll are saying, the batt 12v + and the alt. go to one side of the disconnect,

Yes, that's how they're doing it. Like the pic. But on mine, the alt wire in this diagram would go on the other side of the switch.

 

[05stroker]

Yes, that's how they're doing it. Like the pic. But on mine, the alt wire in this diagram would go on the other side of the switch.

Then why wouldn't the alt keep the car running ? The way you are saying is just like removing the positive terminal on the battery and we all know that still allows the car to run if the alt is working correctly. Am I missing some thing ?

 

[TexasKyle]

Then why wouldn't the alt keep the car running ? The way you are saying is just like removing the positive terminal on the battery and we all know that still allows the car to run if the alt is working correctly. Am I missing some thing ?

You are correct. It would need to be wired per the drawing CPR posted. Moving the Alt cable to the other side of the switch would not kill the car by pushing the button.

 

[tmcolegr]

Then why wouldn't the alt keep the car running ? The way you are saying is just like removing the positive terminal on the battery and we all know that still allows the car to run if the alt is working correctly. Am I missing some thing ?

That's exactly my point!! If the alternator is connected to the other post on the battery disconnect switch, the vehicle is not going to shutoff when the disconnect switch is pushed, as the alternator is still supplying current to the fuse panel.

The alternator cable must be connected to the battery side of the disconnect switch and should also be fused for circuit protection.

 

[TexasKyle]

That's exactly my point!! If the alternator is connected to the other post on the battery disconnect switch, the vehicle is not going to shutoff when the disconnect switch is pushed, as the alternator is still supplying current to the fuse panel.

The alternator cable must be connected to the battery side of the disconnect switch and should also be fused for circuit protection.

Correct, on both accounts

 

[tmcolegr]

Correct, on both accounts

I think you and I were both posting the same info at the same time.

 

[CPRsm]

Wagers ?

 

[TexasKyle]

Wagers ?

On what? That I am right? Sure, we can wager. If you use the drawing you posted but move the alt cable to the opposite side of the switch than is shown, I guarantee that the car will not shutoff while running when you hit the kill switch.

If it's not running, it won't start with the kill switch engaged, but that's it.

Having said that, I will wager whatever you like.

 

[CPRsm]

Name it. That way when I take it from you there are no hard feelings, it was your idea.

Anyone else wanna get in on this ?
Man, that's an awesome smilie LOL

 

[TexasKyle]

One bazillion dollars. It's not like I can collect from you anyhow. Actually, all I want is a video of your face when you hit the kill switch and nothing happens.