Thread: trunk mounted battery short protection

Tweet

I've been through a lot of tech. lines at the drags, with bat. in the rear, and the best I can remember, no one never ask me if my alt. had a cut off on it, so I'm saying as long as you disconnect the bat. it don't make no dif. what your alt does. some people cut them off for more power, but for nothing else. what safety factor would it be anyway with the bat. cut off at the back of the car and the alt. cut off on the dash. safety people wouldn't know that.??????????

Mike, they always test my cutoff switch at tech inspection at least here at Bandimere speedway, usually right after testing the neutral safety switch. That's how I found out it originally was not wired correctly because the motor kept running, albeit pretty rough.

Perhaps some tracks are less stringent than others....

Denny, I was considering installing a relay to cut off the alternator to battery connection so as to cut the load to the engine when racing, this can be significant load with electric water pump and fan etc. Are there some specific NHRA rules that would make this more complicated than a simple relay?

Chris

The positive cable does not need protection, but a disconnect switch is a good idea, even if your are not drag racing. It should be run directly to starter positive post. Make sure cable run is protected from heat sources, sharp edges, etc.

Power distribution for fuse panel can be from disconnect switch. This wire should be atleast 8 AWG with a fusible link. The fusible link is always 2 sizes smaller (12AWG) and should be 5-3/4" long.

Run a second wire from disconnect switch (same size) to a remote terminal block in engine compartment. The ouput of alternator will also go to this terminal block. Use at least an 8 AWG wire for this with a fusible link. This second run is your major load side, headlights, electric fan, pump, etc. All loads should have relays to handle that function. The fuse panel side is only for control function. Use a 3 wire alternator (one wire alternators were designed by GM for tractors only, as they run at one constant speed. They also require atleast 1600-1800 RPM initially to cause the field to self-excite. But when your RPM drops back to idle, the field can fall off and alternator output stops. The 2 small wires on side plug are as follows; one is for system voltage sensing and the other provides field from ignition. This wire you install a diode to prevent backfeeding when you turn off ignition. For MSD ignitions this is a must.

Hope this helps.

Skids72, do you mind saying where you placed the cutoff switch? For a long time I thought a "kill switch" should be in the cockpit as DennyW shows but then Tech1 is trying to tell us the specs for a 300 mph dragster with an outside rear cutoff. I can at least understand your circuit as the easiest with just a longer 8 gauge wire from the alternator to the battery. I note your method only needs a single pole switch of high amp rating but for a few bucks more the Moroso switch Tech2 suggested would allow cutting off both the battery wire and the alternator wire and give 300 amp continuous flow. The '29 cockpit is pretty tight with radio speakers in the kick panels, so I'm not trying to keep this thread going uneccessarily but I'm still learning. I need to go back to Arcet and get some more 2/0 black ground cable to run from the battery forward to a bolt on the trans at the back of the engine because I don't see any good connection for a ground on the engine except the trans mount. The Painless relay DennyW shows is more like what I had expected for a kill switch but at this point I am just trying to figure out one good way to do it. The TPI kit gives a nice total circuit diagram but does not include a cutoff or kill switch which I would like to add.

Catsasswiring, I bought a 100 amp one-wire alternator that is supposed to start charging at about 1000 rpm but I have also heard stories about a lot of alternators not working so I will just have to keep trying till I get one that works and charges at low rpm.

Don Shillady
Retired Scientist/teen rodder

To Don Shillady

Your alternator should have a plastic plug on the side, near the cooling blade. Remove it and install a plug available at any parts house to make it a 3-wire alternator. The importance of the field wire when ignition is on, is that your alternator will always have a field on it regardless of RPM. GM alternators power curve starts at about 2600 RPM. Your pulley ratio of crank to alternator needs to be 2-1/2 - 3 to one. This ensures enough RPM on alternator.
More info. available at "madelectrical.com"

Don, my cutoff switch is mounted behind the rear license plate. It is hidden by the license plate and I remove the license plate at the strip. I've seen others that mount the switch just below the deck lid of the trunk or even through a taillight. One fellow I met last year drilled a hole through the center of his license plate and had the switch there...... pretty slick but I think I'll keep what I have because I didn't want the switch visible when on the road.

catsasswiring: sounds to me like you know what you're talking about when it comes to wiring. Can you comment on my previous idea of using a relay to cut alternator out to reduce load for racing? Would this idea be worth pursuing?

Denny, I like your drawing just fine but I think you were targeting a different application. I know HOW I would install a relay to do it.... what I'm looking for is whether it's a good thing to do and whether it's legal. I guess I can check with NHRA on that latter part. Intuitively, it seems like it would be a good idea to run the motor/fan/water pump off the battery for a 1/4 mile (save a few HP?) then switch the alternator back in after the traps.

Agreed, except for one point: total load is on the whole system not just the battery. With lots of goodies running on electrical, the alternator needs to work harder making it more difficult to spin mechanically. Take the electrical load off the alt and it spins more freely and robs less from the crank at least that's my understanding of how it all works

Okay... perhaps I could have chosen better wording Point is there is an electrical load on the alternator which becomes a mechanical load on the crank if the alt is electrically hooked in to the system. Cutting this electrical load should result in reduced mechanical load on the engine(?)

Cables routed through appropriately sized grouments should provide the needed protection.

jc

To Skids72 and DennyW

You need to run to separate systems (Load) & (Control). Reasons are for your fuse panel does not need the load, most aftermarkets are not sufficiently rated (Painless). Enos Components does make a 300A solid state mother board for his fuse panel. Second, drag race applications, tech will have you run engine, then open kill switch. On a system not separated, your voltage regulator will see volt drop when engine dies, this can cause a voltage spike and fry your MSD electronics box. TRUST ME! I have been wiring street rods, drag racing of all classes for 10 years. Larry Dixon Sr. (1970 Winter Nationals Winner) sends me his customers. I have been working in power plant for 32 years in operations as a senior control operator and shift supervisor (we make electricity).

Hope this helps. catsasswiring@yahoo.com

Forgot to mention, alternator cut out not needed with my set up.
As far as horse power loss for drag racing, the pulley ratio should be a 2 to 1. Alternator speed should not exceed 26,000 RPM. HP loss for drag racing is minimal. We run a '72 all pontiac, alcohol injected, run 8.60's. I have an GM 10SI series on it.

catasswiring

Thanks, catsasswiring. I never doubted your credentials... I could tell you know your stuff. As for voltage spikes killing MSD box... how are you saying this will be prevented? Separate control and load systems.... does this mean run everything with relays? I understand the benefit but I don't understand where the voltage spike risk is from and how specifically to prevent it. I have not installed MSD (...yet.... I do have all the parts) The only precaution I was aware of was to install the diode in the exciter wire to the alt.... what am I missing? I certainly don't want to fry this new toy.

Also, it sounds to me the HP gain cutting alternator while running is not worth the hassle?

Many thanks,

Chris

Let's assume you are generating 100 amps at 14.5 V dc and 95% efficiency. Then you are drawing about 2 hp from your engine.

Only on paper do I believe will this factor alone cost you a race.

Unless you are running a self-generating ignition (magneto) then the waning 12V may not be a great idea in general, albeit drag races are very quick, no?! Your MSD-coil will like 14.5 V much better!

One wire alternators are self-excited. You have to kick them up initially (at start-up or whenever) to kick them on. Once on I am not aware that they do not perfrom as well as 3 wire alternators. I would certainly like to see some performance curves ...............

The diode alone will not smooth voltage spikes, but then again I am not aware of MSD killing voltage spikes in a 12 V dc automotive charging system.

Kitz

How do you get to 7 HP from 14.5 V and 100 A?

Alternator load = V I = 14.5 * 100 = 1450 watts.

1 HP = 746 watts. ~ 2 HP


Kitz