D113

call me crazy, but did I not iterate everything between my post above and this one what pretty much everyone else just said? (save a few details?)

Slimsta.. I'm afraid there are some problems with your father's math.

I1+I2= is really not applicable in this scenario.. and please don't take any offense to this statement, but an electrical engineer should no have made any of the above statements for fear they coudl lose all credibility.

Here are some facts:

Electricity will take the shortest path.

If there was no battery in the system, the pull of electricity would come right off the alternator. Since there's a battery in the system, there's effectively a reservoir of available power at your disposal. When the engine is off, there's no charging system, so the power comes right off the battery. If the engine is on, the power gets pulled from the system.. and since there's a battery on it, the system's essentially an endless supply of power, if your power out does not exceed your power in.

MR2NR made an interesting point that I didn't even think about:

I said: If you get in a head-on with an unfused alt->batt grounding out in the process you'll likely have an engine fire/fuel system fire. MR2NR made an interesting point about having the opposing vehicles Emblem/or yours on the steering wheel stamped in your forehead too. This is very possible as a shorted battery could render any airbag system inoperable... so any chance you had getting out before you were engulfed in flames is drastically reduced due to your airbags not deploying!.


If your system fuse of 100A in the fusebox hasn't blown, your regular car items arent drawing 100A. If your amp to battery connection has a 100A fuse and it hasn't blown yet, then you aren't drawing a full 100A (or at least for a long enough time). The only issue that could exist is that if you have a 100-120A ALT and a direct connection, you may pop this fuse due to combined audio system/car system demands. That's why we recommend going a bit higher in the fuse size than your Alt's output rating. A 150A fuse will blow just as quickly as a 100A fuse in a grounding out scenario (vehicle collision) so you shouln't worry about having a 150A here, and a 100A there.

The only issue that could arise is that once the system has a direct alt to battery connection, is that the whole system is in parallel at this point and this could blow your system fuse rendering nearly your entire vehicle useless. Thankfully though, there's a number of different point of resistance in the ALT->fusebox-battery path on some vehicles (mine namely), that would likely mean that the current draw of the audio system would likely be a direct path down the "yellow-brick-road" of a 4Awg run direct to the alt. Thankfully the battery in the system will take the brunt of the current drawing hits.

Fuse ratings do not get added together to give an overall system draw (your example of 150 + 100. If your system was for some strange reason fused twice on the way to the amp, if you exceeded 100A you'd blow the 100 A fuse regardless of where it was inline (before or after the 150A)

This is why I'm concerned the advice you've been given from the parental unit is GROSSLY innacurate or misleading.

Another factor too is voltage drop. Lets say your amp(s) have a 50A fuse built in. It's very likely that this 50A is to compensate for voltage drops in the 10-11.5V range. That being said, at 12v the same current is less due to higher voltage and even less at 14.4v when your car is running at optimal (highway driving, no headlights, no AC, no defrosters)

This being said, you are probably not drawing anywhere close to 100A full-tilt. For example.. if an amp draws 1150W at 11.5v (100A), then at 12v you are drawing 95A and at 14.4 you are drawing 80A.

In closing. Fuse your alt->batt at a level just slightly larger than you Alt's output.

Shorty420

If your not sure 100% on the size of the factory fuse between the Alternator and the Battery then you should check the nameplate on the alternator and determine the rated output. Once you know that then oversize the fuse by about 20-25%. This will protect the alternator from pushing out over its rated load and any shorts that happen on the alternator side of the fuse.

Most likely the Alternator to battery wire is 8 AWG going thorough a stock fuse, this should be disconnected from both ends and removed from the car. Then run your new 4AWG wire between the (+) alternator and the (+) battery and place your new inline fuse about 1ft from the battery.

Next determine the load that your stereo will draw 1000W RMS , check your voltage when running normally to determine your voltage 11.5-14.5 Volts then calculate your current. again oversize the fuse by about 25% Place the fuse as close to the battery again running back to your amp.

Dukk

IMO you should not add this wire. If you DO choose to add it, you should do it properly and fuse it at both ends.

D113

Hey Dukk,

Like your idea of a fused at both ends wire, however I can't figure out why?

Head-on collision + still spinning alt?

FusionMadsen

iTrader: (4)

What do you mean why? The wire could be severed anywhere, therefore you need to fuse each side, otherwise one side will short out while the other is protected by the fuse.

D113

Simple question honestly.. the only way I can see there being power on the Alt side of the fuse near the battery is like I said above.. if the alternator is still spinning then maybe. The original battery to alt connection is fused.. so where is the power coming from if the alternator isn't moving?

If this is this case.. then I answered my own question above.

Any questions?

Shorty420

I would have to say that fusing both ends is overkill as the alternator is only going to be generating current when the engine is running and at a high RPM. By fusing close to the battery you protect the battery from a short on the positive if the wire gets severed.

I would fuse close to the alternator if i ran the wire across the very front of my car and it would be impacted first by an accident.

The chances that you would be in a big enough accident to sever the 4AWG wire in your engine compartment and still have the belt turning at a high enough RPM to generate high current from the alternator is IMO low.

D113

Agreed.. but I guess the moral of the story is better safe than sorry at $35 ish a fuse holder.