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Fusing alternator to battery cable

Scottie

Well-known member
Joined
Mar 25, 2023
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How many people fuse the alternator to battery cable? Just finished up wiring my cj6 and on the fence about adding fuse. Seems like it’s 50/50 on this.

I know Mechman alternators recommends against it, at least for their high amp alts.
 
Did it once on a drag car with the battery in the trunk,the car had to have a functional battery disconnect that would kill the engine
So the alt wire had to go back to the battery
Other then that never
 
I'm also in the "no" camp.

be cognizant of how and were you run the wire, err on the side of too much wire, let it do it's thing. Alternator isn't doing anything unless the engine is running. You're likely to notice an underhood fire while driving and if there is ANY chance that the alternator cutting out would shut off the engine, it would make it difficult and less safe to drive the extra 10-90 seconds it would take to get to a safe spot to address the fire.
 
well, guess i've got a fusible link in some stuff. That's a big difference compared to a fuse though, much more tolerant of short duration high amp and takes longer to pop in the event of an event.
 
Most OEs put a fusible link in the alternator wire. Since they're spending profit on it, must be somewhat important.

A battery distribution point like most 2000s GM trucks have:

1702506774095.png


With the smaller fusible link (small black wire here, alternator on these is generally 145A) to the alternator would be easy to install on most rigs. Nearly free at the Gank-A-Part to boot.
 
Only place I've seen it done is in camper builds with high output alternators that are topping off a LiFePO4 house battery bank. At modest discharge levels the alternators should be able to keep up with the especially low impedance/high current draw of those batteries, but if the batteries are deeply discharged, they can draw an insane amount of current that'll burn out the alternator quickly. The proper way to charge LiFePO4 is with a BIM or DC-DC, but this method is simple and cheap I guess.
 
Rockwood said:
The black one. That's a stolen random Internet image. Here's the one on mine (currently swapping an LM7 into an XJ):

Fusible Link.jpg


8mm^2 wire, smaller than 8AWG.
Click to expand...
Gotchu.
Makes me want to test some wire gauge to failure and see what are the results.
 
Bebop said:
Gotchu.
Makes me want to test some wire gauge to failure and see what are the results.
Click to expand...
Yeah. Had me a bit confused when I saw it on disassembly, but if GM says its good enough for this alternator, fuck it. I'll be testing the shit out of this unit with my cooling setup (600W GM brushless fan, plus the 40A fused stock XJ fan), so I guess I'll find out if its sufficient.

Stock XJ alternator has a slightly smaller fusible link built into the stock alternator wire. It's hidden under split-loom and a bunch of tape, but its there.
 
Both of my older trucks with upgraded alternators have breakers in the charge cable for cheap insurance. Neither puts enough load on that wire to damage anything it's 100% for accidental damage causing a short.
 
Without a fuse or fuseable link in the alternator power wire one can smoke the alternator if the cables are hooked up incorrectly when giving another car a jump.
 
TrikeKid said:
Both of my older trucks with upgraded alternators have breakers in the charge cable for cheap insurance. Neither puts enough load on that wire to damage anything it's 100% for accidental damage causing a short.
Click to expand...
Yeah, I doubt it'll smoke the wire, more curious on voltage drop.
 
If the battery wire is fused at the source (battery) then a big wire (within fuse rating) will blow the fuse if shorted and shutting the engine off will stop charging, seems sufficient.

No need to add a shunt or thin wire to the mix IMO.
 
I decided not to add a fuse. Cable is sized at 1/0 for future alternator upgrade. This is what mechman recommends for their 320a alternator. They also recommend no fuse.
 
PAE said:
Without a fuse or fuseable link in the alternator power wire one can smoke the alternator if the cables are hooked up incorrectly when giving another car a jump.
Click to expand...
So don't be a retard?
 
Painless and American auto wiring both have a 175A fuse between the alt and bat. But I've also ran a 6 or 8 straight from a GM one wire to bat many times, also seen it a million times. Usually it's a fairly short run through air. If using a fuseable link, it needs to be 3 or 4 gauges smaller. The fuseable link wire is standard wire, what makes it special is the jacket, it doesn't burn. I've tested 10g F.L. spliced to standard 10g wire, shorted them and the standard stuff went up in flames almost immediately, FL was fine.
 
Provience said:
I'm also in the "no" camp.

be cognizant of how and were you run the wire, err on the side of too much wire, let it do it's thing. Alternator isn't doing anything unless the engine is running. You're likely to notice an underhood fire while driving and if there is ANY chance that the alternator cutting out would shut off the engine, it would make it difficult and less safe to drive the extra 10-90 seconds it would take to get to a safe spot to address the fire.
Click to expand...
I had an alternator catch fire in a parked truck.
Would have burned the truck down but someone noticed smoke coming out of the hood.
Was so hot I pulled it out with welding gloves an hour later and it melted a big hole in the snow/ice.

No idea WTF happened. Truck had been parked for a couple hours.
 
AKnate said:
I had an alternator catch fire in a parked truck.
Would have burned the truck down but someone noticed smoke coming out of the hood.
Was so hot I pulled it out with welding gloves an hour later and it melted a big hole in the snow/ice.

No idea WTF happened. Truck had been parked for a couple hours.
Click to expand...
Good real world example, no problem and truck didn't burn down :beer:

That's a neat one though with a sweet internal short
 
This came from a rebuilder a few years back, he recommended no fuse.

possible causes of shorts in the alternator charge circuit.

Regulator Stays On: This causes the field to be continually on. Which puts about 6amps directly through the rotor to ground. This causes great heat, but the battery usually drains to zero before a fire. Regardless it is only 6amps, so neither a 130 or 200amp breaker will open.

Rotor Shorts Internally: This causes a directly positive to ground short. The 24-20gauge brush lead will melt instantly. Kind of acting like a fuse of about 10amps. Then the brush spring will try and carry the load, and melt as well. So neither the 130 or 200 breaker will open.

Diode Shorts: This causes a direct positive to ground short. The diode’s lead is only designed to carry 50amps (stock), 70amps (HD aftermarket). So it will melt almost instantly. So neither the 130 or 200 breaker will open.

Stator Shorts: This causes a heavy positive to negative short. This can cause a fire as all the amperage created by the alternator is going to ground. But the alternator is the source of the power, so the breakers will not be able to prevent this. Once the alternator has fried the stator, it then becomes possible for the power to come from the battery to the stator. But this is only if the diodes have failed by becoming shorted. If so see above.

Main Battery Post Becomes Loose: This can cause a fire, in fact 10,000 or 1000’s of fires happen on Ford products equipped with their 2G alternators, because of this. Since the fire is started by heat in the loose connection, it is the alternator that is producing the power that creates the fire. So breakers and fusible links will not and did not prevent this.

Main Battery Cable Shorts: This causes a direct to ground surge, usually large enough to burn/blow the wire in half instantly. If you are using a 4ga cable, it is possible several hundred
 
Most common reason for a fuse on the alternator I've seen isn't parts failure, it's brain failure: someone does something stupid and shorts shit. Had a mechanic work on a car of mine and didn't disconnect the battery, grounded the wrench. I found out when I went to drive it home and the battery light was on. Something I've done personally: dipstick rotated onto the positive pole and worked under the boot (unlucky award). Grounded to the windage tray inside the motor. Luckily, the fuse stopped it.

Sometimes shit happens. Maybe you have a truck LM7 and left a tool on it, then closed the hood. Maybe you hit a busload of nuns and grounded the alternator to something. Maybe you did like I did above. Maybe you're just an idiot. Either way, since most of us are running OE alternators of some sort, grabbing the matching fusible link isn't useless (though expensive if new: $74 new OEM, I'll grab a couple next trip to the JY). Yes, I'm aware the starter is never protected. Can't save em all. :flipoff2:

In short:

1707239341441.png


Fire bad.
 
In case you don’t believe me…

IMG_3391.jpeg


That’s why you fuse shit. :flipoff2:
 
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