NHRA Approved Battery Disconnect

[tmcolegr]

Latest project: install an NHRA approved battery disconnect

Disclaimer: yes I have already searched and read through all 9,281 threads that pertain to NHRA approved battery disconnect switches and I either haven't stumbled across one that answers this question or it's never been discussed.

Subject vehicle is a 2005 S197 GT. Battery has already been relocated to the trunk - center mounted, directly over the rear axle, behind the seats. I have 3 dedicated battery cables run from the front of the vehicle all the way back to the trunk mounted battery: dedicated 4 gage (+) battery cable for the alternator circuit, dedicated 1 gage (+) battery cable for the starter/BEC circuit, and a dedicated 1 gage (-) battery cable for vehicle ground.

Most everyone (not all) agrees that both the (+) battery cable for the BEC/starter circuit and the (+) battery cable for the alternator circuit must be interrupted to completely de-energize the electrical system and to shut the vehicle off. To accomplish this, 2 separate devices must be used: 1 to disconnect the starter/BEC circuit and 1 to disconnect the alternator circuit. A manual or electrically operated disconnect switch is the most popular method of disconnecting the starter/BEC circuit while a solenoid is usually the preferred method of disconnecting the alternator circuit.

However, if you read through all the threads that use a solenoid to interrupt the alternator circuit, almost everyone agrees that the solenoid should be energized by a power source that is supplied by the key switch. The solenoid, if not energized by the key switch and left on over a prolonged period of time, will eventually drain the battery. The preferred method is using a relay, that is key switch activated, to energize the solenoid. Additionally the solenoid when energized, even though it is rated for alternator amperage, gets warm or in some cases even hot. I spoke to Painless Performance Products and even their 50105 High Amp Alternator Shutdown Relay will get warm.

If I interpreted the NHRA rule correctly it simply states that a single knob (labeled "Push Off) protruding through the rear of the vehicle must be able to be pushed to completely de-energize the (+) side of the vehicles electrical system and shut down the vehicle. I am confused as to why no one chooses to use 2 separate battery disconnect switches that are manually operated by a single knob. Let me elaborate. In my case I plan on drilling a hole through the rear bumper cover directly behind the license plate. The license plate could be reinstalled to cover the hole when the rod/knob is not installed. The battery disconnect switches could be mounted horizontally (reducing overall height) next to each other in the spare tire compartment. Both disconnect switches would be mounted on the same plane, with a rod connecting the 2 disconnect switch levers together, and a single rod protruding through the back of the vehicle. The (+) battery cable for the starter/BEC circuit would be connected to one disconnect switch and the (+) battery cable for the alternator circuit connected to the other disconnect switch - keeping both electrical circuits completely separated, but both disconnect switches operated simultaneously by a single rod/knob. This manual method of disconnecting the alternator circuit completely eliminates the need for: a relay that is activated by the key switch thus preventing the battery from going dead over a prolonged period of time, an electrically operated solenoid which can get warm/hot when energized, and all associated wiring. This should result in a simpler, cleaner and more reliable installation.

I have given this considerable thought/research and (2) Flaming River FR1003-2 disconnect switches (each rated for 250 amps @ 12V continuous duty) will serve the purpose. I even spoke to Kevin @ Flaming River's Tech Support about this method and questioned the additional effort required to push the knob to operate both disconnect switches simultaneously. His thoughts were this should not be an issue as the force required to operate the 2 disconnect switches would not be excessive. Once installed the OEM trunk mat could be reinstalled and the entire installation would be hidden and out of sight.

Has anyone tried this method or can they offer valid reasons why this will not work? If not, Monday I plan on ordering the Flaming River disconnect switches and proceeding with this project. I will post pictures of the installation.

Everyone's opinions, pro or con are welcome.

 

[SteveP]

Hey man. Was nice talking to you about this yesterday as well. When I get back home, Ill take pics how mine is run without the solenoid.

 

[tmcolegr]

Was great talking to you. Thanks for your input. All comments pro or con are welcome.

 

[GB10]

I have a switch that you turn to the off position mounted behind the license plate like you are talking about. I can install my license plate to hide it and once I go to the track I can just remove the plate and install the handle on the switch.

I relocated my battery to the trunk and ran 2 wires for the + wires. The alternator + wire goes on the same terminal as the battery + wire and on the other side of the switch is the + wire for the fuse panel and starter. When I turn off the switch all power is de-energized. I don't have any solenoids since those can fail while it is much less likely for the manual switch to fail. Only thing is the switch needs to be rated for at least 175A continuous duty.

Here is the switch I used: http://www.summitracing.com/parts/LNG-45785/

I can draw up a quick wiring diagram if you need me to.

 

[tmcolegr]

I relocated my battery to the trunk and ran 2 wires for the + wires. The alternator + wire goes on the same terminal as the battery + wire and on the other side of the switch is the + wire for the fuse panel and starter. When I turn off the switch all power is de-energized. I don't have any solenoids since those can fail while it is much less likely for the manual switch to fail. Only thing is the switch needs to be rated for at least 175A continuous duty.

First off I appreciate you posting in my thread. But with the alternator wire connected to the battery side of the disconnect switch all power has not been de-energized as the alternator circuit is still connected to the battery.

 

[GB10]

The purpose of the kill switch is to kill the motor and all electronics. With the alternator wire connected to the battery + wire the motor and all electronics die since they do not have power to remain powered on. The alternator can only create electricity if the pulley is spinning and once I hit the switch the motor stops and no more power is present. I do still have power from the battery connected to the alternator output terminal, but it will not keep the motor running since there are no electronics with power feeding them.

 

[tmcolegr]

so let me check for understanding...

With the alternator (+) cable attached to the battery (+) side of the battery disconnect switch, the vehicle will shut off as the power source has been removed from the starter and BEC circuit?

If that same alternator (+) cable was connected to the load side of the battery disconnect switch, the vehicle would keep running as the alternator would continue to feed current to the starter and BEC circuit anytime the engine was running.

Please confirm

 

[GB10]

Yes, that is correct. I am drawing up a diagram right now for both ways; one being the way I did it and the other the way you originally planned. Both work, but I prefer simplicity and reliability over ease of install.

 

[tmcolegr]

Yes, that is correct. I am drawing up a diagram right now for both ways; one being the way I did it and the other the way you originally planned. Both work, but I prefer simplicity and reliability over ease of install.

I clearly understand what you're stating. I was under the impression the (+) battery current had to be terminated to the entire vehicle (including the alternator circuit) to be in compliance with the NHRA rule? If not your method eliminates the need for the second battery disconnect switch which becomes redundant and unnecessary.

Your signature states you have a 2010 Mustang. I assume this vehicle is the one you're using for the test mule? Have you ever found this method to have an adverse affect on any of the electronics: alternator, PCM, etc.?

 

[GB10]

No problems whatsoever. I ran #2 wire from the front of the car to the rear for the battery relocation and cutoff switch. Everything operates normally since I used wire bigger than I needed. Technically #3 wire was sufficient, but voltage drop on DC is much greater than AC, so I decided to use #2 just in case I decide to power something new from the fuse panel rather than directly from the battery.

Here is a wiring diagram for the method I used for future reference by others if needed

 

[tmcolegr]

The Longacre switch you posted the link to is a DPST switch. The main power circuit is rated for 175 amps continuous while the alternator circuit is only rated for 20 amps. I can only assume you're not using the smaller terminals for the alternator circuit on that disconnect switch?

I had planned on using a SPST switch. I should be able to use a SPST switch and connect the alternator (+) battery terminal to the (+) post of the disconnect switch. Power to the starter/BEC would be interrupted, shutting off the vehicle, while power would remain continuous to the alternator.

 

[GB10]

No, I am not using those terminals. It has been a while since I did it and I had to look again to see what I did. I did it just like you are saying. I'll revise the diagram in my previous post. I originally planned on using those terminals until I got the switch and noticed the alternator terminals were only rated for 20A.

 

[tmcolegr]

Thanks for the diagram. If you don't mind me asking, what software program did you use to create that diagram? Where did you get the symbols from? Lastly how did you save it and attach it to your post?

Back to the original topic. You mentioned you used 2 gage cable for the (+) starter/BEC circuit. Based on your posts it sounds like you also ran a separate (+) battery cable for the alternator all the way back to the battery disconnect switch. What size cable did you use for the alternator? I'm using 1 gage welding cable with a 200 amp fuse for the starter/BEC circuit and 4 gage welding cable with a 150 amp fuse for the alternator circuit. Also as I mentioned previously, I ran a 1 gage (-) negative cable from the battery all the way up to the engine compartment. I did not terminate the ground cable in the trunk and simply ground the engine to the chassis. I intentionally used welding cable as it has a higher strand count, carries more amperage when compared with the same size battery cable, and is more flexible.





I still have a couple concerns with either disconnect switch setup:

• The single battery disconnect switch is a simpler and cleaner setup. However the alternator circuit is still energized by the battery at all times as it is not interrupted by the disconnect switch. I'm confident your setup works as you described, but I'm not sure I feel comfortable with the alternator (+) battery circuit not being able to be interrupted in the event of an emergency. Kind of defeats some of the purpose of the whole disconnect switch.
• The dual disconnect switch also has drawbacks above and beyond the additional switch and associated cables. Even though the alternator circuit has been interrupted there are concerns associated with that process. I've been warned by more than one shop that abruptly disconnecting (either with a solenoid or manually operated disconnect switch) the (+) battery cable from an alternator that is already charging (with the engine running) is extremely hard on the alternator and can lead to premature alternator failures.

FWIW the only time this (+) battery disconnect switch would even come into play, would be during a tech inspection or in the event of an actual emergency. I already have a manually operated disconnect switch in the (-) cable that I use when performing routine maintenance/repairs on the electrical system that require the battery to be disconnected. Unfortunately it doesn't meet NHRA rules.


PS: Longacre makes a DPST switch that's bad ass. Battery circuit is rated at 175 amps continuous and the alternator circuit is rated at 125 amps.

I really expected more to chime in on this thread. It may be that they read the title of the thread and figured this topic has already been beaten to death. Thanks for your input.

 

[05stroker]

Thanks for the diagram. If you don't mind me asking, what software program did you use to create that diagram? Where did you get the symbols from? Lastly how did you save it and attach it to your post?

Back to the original topic. You mentioned you used 2 gage cable for the (+) starter/BEC circuit. Based on your posts it sounds like you also ran a separate (+) battery cable for the alternator all the way back to the battery disconnect switch. What size cable did you use for the alternator? I'm using 1 gage welding cable with a 200 amp fuse for the starter/BEC circuit and 4 gage welding cable with a 150 amp fuse for the alternator circuit. Also as I mentioned previously, I ran a 1 gage (-) negative cable from the battery all the way up to the engine compartment. I did not terminate the ground cable in the trunk and simply ground the engine to the chassis. I intentionally used welding cable as it has a higher strand count, carries more amperage when compared with the same size battery cable, and is more flexible.





I still have a couple concerns with either disconnect switch setup:

• The single battery disconnect switch is a simpler and cleaner setup. However the alternator circuit is still energized by the battery at all times as it is not interrupted by the disconnect switch. I'm confident your setup works as you described, but I'm not sure I feel comfortable with the alternator (+) battery circuit not being able to be interrupted in the event of an emergency. Kind of defeats some of the purpose of the whole disconnect switch.
• The dual disconnect switch also has drawbacks above and beyond the additional switch and associated cables. Even though the alternator circuit has been interrupted there are concerns associated with that process. I've been warned by more than one shop that abruptly disconnecting (either with a solenoid or manually operated disconnect switch) the (+) battery cable from an alternator that is already charging (with the engine running) is extremely hard on the alternator and can lead to premature alternator failures.

FWIW the only time this (+) battery disconnect switch would even come into play, would be during a tech inspection or in the event of an actual emergency. I already have a manually operated disconnect switch in the (-) cable that I use when performing routine maintenance/repairs on the electrical system that require the battery to be disconnected. Unfortunately it doesn't meet NHRA rules.


PS: Longacre makes a DPST switch that's bad ass. Battery circuit is rated at 175 amps continuous and the alternator circuit is rated at 125 amps.

I really expected more to chime in on this thread. It may be that they read the title of the thread and figured this topic has already been beaten to death. Thanks for your input.

When the car comes from the fac. the alt. cable goes strait to the battery.

I have a Painless solenoid on mine but after reading this it has got me thinking about removing it. Mine is set up like the above diagram but has the solenoid also. If I dont need that hot ass thing it is gone.

 

[tmcolegr]

When the car comes from the fac. the alt. cable goes strait to the battery.

I have a Painless solenoid on mine but after reading this it has got me thinking about removing it. Mine is set up like the above diagram but has the solenoid also. If I don't need that hot ass thing it is gone.

Exactly!!

I am seriously considering going with single battery disconnect switch that only interrupts the (+) battery cable that goes up front to the starter/BEC circuit. By powering the alternator (+) cable off of the battery side of the disconnect switch, the vehicle still should shut off just as GB10 states!! Once all current going to the starter/BEC has been interrupted the vehicle should shut off. This prevents having to have a disconnect switch or solenoid in the alternator circuit. Only down side is the alternator circuit will be powered up directly from the battery at all times. However it came that way from the factory and only had an 8 gage fusible link in the alternator (+) cable. By using a 150 amp fuse in the alternator circuit I have accomplished the exact same scenario as when the vehicle was built.

Yours thoughts??

 

[05stroker]

Exactly!!

I am seriously considering going with single battery disconnect switch that only interrupts the (+) battery cable that goes up front to the starter/BEC circuit. By powering the alternator (+) cable off of the battery side of the disconnect switch, the vehicle still should shut off just as GB10 states!! Once all current going to the starter/BEC has been interrupted the vehicle should shut off. This prevents having to have a disconnect switch or solenoid in the alternator circuit. Only down side is the alternator circuit will be powered up directly from the battery at all times. However it came that way from the factory and only had an 8 gage fusible link in the alternator (+) cable. By using a 150 amp fuse in the alternator circuit I have accomplished the exact same scenario as when the vehicle was built.

Yours thoughts??

I already have 175a fused terminals on both the battery cable and the 6 ga I have going from the alt. to the trunk. All I would have to do is remove the solenoid. I originally had the solenoid because I only had the batt. cable going to the trunk . I added the alt. cable when I moved the buss fuse box and every thing from the engine bay. The way it is now I see no reason this would be any issue. I am working on the car today , I think this will be added to my list.

Thanks for the idea guys!

 

[GB10]

Thanks for the diagram. If you don't mind me asking, what software program did you use to create that diagram? Where did you get the symbols from? Lastly how did you save it and attach it to your post?

Back to the original topic. You mentioned you used 2 gage cable for the (+) starter/BEC circuit. Based on your posts it sounds like you also ran a separate (+) battery cable for the alternator all the way back to the battery disconnect switch. What size cable did you use for the alternator? I'm using 1 gage welding cable with a 200 amp fuse for the starter/BEC circuit and 4 gage welding cable with a 150 amp fuse for the alternator circuit. Also as I mentioned previously, I ran a 1 gage (-) negative cable from the battery all the way up to the engine compartment. I did not terminate the ground cable in the trunk and simply ground the engine to the chassis. I intentionally used welding cable as it has a higher strand count, carries more amperage when compared with the same size battery cable, and is more flexible.






I still have a couple concerns with either disconnect switch setup:

• The single battery disconnect switch is a simpler and cleaner setup. However the alternator circuit is still energized by the battery at all times as it is not interrupted by the disconnect switch. I'm confident your setup works as you described, but I'm not sure I feel comfortable with the alternator (+) battery circuit not being able to be interrupted in the event of an emergency. Kind of defeats some of the purpose of the whole disconnect switch.
• The dual disconnect switch also has drawbacks above and beyond the additional switch and associated cables. Even though the alternator circuit has been interrupted there are concerns associated with that process. I've been warned by more than one shop that abruptly disconnecting (either with a solenoid or manually operated disconnect switch) the (+) battery cable from an alternator that is already charging (with the engine running) is extremely hard on the alternator and can lead to premature alternator failures.

FWIW the only time this (+) battery disconnect switch would even come into play, would be during a tech inspection or in the event of an actual emergency. I already have a manually operated disconnect switch in the (-) cable that I use when performing routine maintenance/repairs on the electrical system that require the battery to be disconnected. Unfortunately it doesn't meet NHRA rules.


PS: Longacre makes a DPST switch that's bad ass. Battery circuit is rated at 175 amps continuous and the alternator circuit is rated at 125 amps.

I really expected more to chime in on this thread. It may be that they read the title of the thread and figured this topic has already been beaten to death. Thanks for your input.

I just made the diagram in paint and saved the diagram as a jpeg then inserted the image from my photobucket account. I use those symbols everyday since I am an industrial controls electrician, but for the sake of this diagram I just drew everything in paint. I have software at work to make line diagrams, but I am rarely in the office to use it.

I understand what you are saying regarding the removal of the alternator and battery from the same termination point as it is not truly isolating the power sources, but it does meet tech requirements since all electronics are killed this way.

Regarding the wire sizes, I used a #2 welding cable for the alternator and a #2 welding cable for the fuse panel/starter. I combined the starter with the fuse panel since only one will operate at a time. The starter engaging momentarily will not exceed the current carrying capacity of #2 wire since it is a momentary load rather than continuous like the circuits from the fuse panel. I also ran a #2 - battery cable for the starter along with a #2 chassis ground.

 

[CPRsm]

I used the same switch on my car. I ran the positive to one side. The other side I ran one wire to the alt, and to the power supply for the car. Switch it off and it kills EVERYTHING, including a running car.

 

[tmcolegr]

I used the same switch on my car. I ran the positive to one side. The other side I ran one wire to the alt, and to the power supply for the car. Switch it off and it kills EVERYTHING, including a running car.

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??

 

[Norm Peterson]

I understand what you are saying regarding the removal of the alternator and battery from the same termination point as it is not truly isolating the power sources, but it does meet tech requirements since all electronics are killed this way.

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