sdl2112

I am trying to identify the type fuse used on a 2006 A3 alternator. There are several of this style fuse in a row connected to a bus bar at the front of the fuse box. There is a spare slot I would like to use to power amplifiers from but for the life of me I've searched but haven't found any that look like them. The alternator fuse is marked 200A on the body with tabs on either side with mounting holes. I would rather not contact the service department because I don't want them to know what I'm up to and will probably charge way too much. Thanks for any info.

MacSub

on the power cable. You don't use the 75X busbar connection as this is not rated for additional heavy current items.

sdl2112

It would be difficult to connect directly to the battery without modifications to the battery box. You would also introduce additional resistance. All current (except starting current) already flows through the bus bar so it should be sized adequately. Since the majority of the time you listen with the engine running all power comes from the alternator. If I connect to the battery the current would flow from the alternator to the bus bar, from the bus bar to the battery then from the battery to the amps, much better to connect to the bus bar.

HIS4

It's probably got an 8 AWG feed which is where I get the 50A rating from. Definitely not enough to power both the accessories and a high powered stereo.

It is true that while the engine is on most of the power comes from the alternator but the battery is not bypassed so your assumption about the way current flows through the system is wrong. Typically you have feeders from the alternator to the battery and from the battery to the fused distribution block. While the engine is on, the battery is still in series with the alternator. Typically the alternator's voltage is slightly higher (around 14V) than the battery (12.6 fully charged). In the event of a large current draw, the alternator's voltage will want to drop but the battery will somewhat regulate it because it will resist instantaneous voltage fluctuations.

Hooking up the amp to the battery is the correct way to do it.

MacSub

You really don't know much about car electrics do you?

HIS4

Well I wasn't going to put it that way but yeah, I was thinking the same thing.

sdl2112

Funny how the question originally asked was not addressed and turned into how I should wire my system. Btw, the fuse type is a MIDI, it just looks different than the type typically offered by Monster or the like.

Now it is pretty clear who knows their basic electronics. This is not limited to car audio. This is simple power distribution. My car electrical distribution system has a 140A alternator connected with 25 mm^2 (3-4 awg) wire, no not 8 awg. This connects to a bus bar through a 200A fuse. The physical size of the bus bar suggests it is good for 100-150A with 30C rise. From the bus bar there are several feeds, one is to the fuse panel with blade style fuses, others are the electrical power steering, cooling fans ect, and another is to the battery. Connecting to the battery in my case makes no sense and only introduces an additional series resistance plus it is not a convenient connection point for me.

To the point of where power is coming from. All power comes from the alternator with the engine running. If not you won't be listening very long. If your lights dim you my have an inadequate alternator, short current demands can come from the battery if the alternator regulation is not fast enough. When sizing conductors you look at the average or rms power flow. Again in my case whether I connect the amps to the battery or the bus bar all current still runs through the bus bar, again with engine running. A lot of what determines the sizing of car audio conductors is the peak ratings and to limit voltage drop. If I have a 1000W amp it will virtually never supply a long term average of 1000W, look for yourself with a scope. If you limit your music listening so that the peaks are not clipped, than your average is considerably lower. Thus you size your conductors for voltage drop not necessarily current rating. I bring this up to show the bus bar rating is plenty adequate.

One point I'm trying to make is think things through yourself. Just because someone wired their system a certain way doesn't mean it is correct for all.

HIS4

So if your description of the system is correct and that bar is protected only by an upstream fuse of 200A it has to be rated for at least 200A. 3 parallel 4 AWG feeds would handle just about 200A. As far as sizing of conductors, yes you can size off estimated demand but if you're dealing with a single amp system you're basically stuck with matching the overcurrent protection of the amp at a minimum. If the amp has a 100A fuse, you wouldn't want to size your distribution for less because you could possibly trip your main breaker or blow your main fuse if the current draw from the amp exceeds your overcurrent protection. You could oversize it and assume that the fuse at the amp will protect the amp from from drawing any more current than it can handle.

I don't think the resistance of connecting to the battery will be significant enough to make a difference in this case since you're dealing with relatively short runs of large cable. Since I don't know the most specific details of the A3 electrical system the only advice I can offer is use your common sense. As long as you can protect the conductors and not overload the upstream feeder or associated equipment, you should be fine. As far as the fuse, take a picture of it and I may be able to give you a better idea. Does it look almost like a large relay but with a clear window on top that you can see the fuse element through. I know I've seen these in some newer cars like Corvettes.

MacSub

That busbar is powering the whole car, if you want to treble the amperage being drawn from it, go ahead, I'm sure Audi designed it for just such aftermarket applications (NOT). Do you honestly think that busbar is connected directly to the alternator?
Dimming lights here you come.
You also forgot to factor in the incredible inefficiencies in car audio amps, so while your calculations maybe correct in theory, you need to double the result to account for losses/inefficiencies in the way the amp delivers 1kw (if it ever gets near that).
"Additional series resistance" my ****, sounds more like you just can't be bothered to do the job properly. Have fun......

sdl2112

Why don't you back your theories up with some real engineering numbers. My guess is you can't. What is connecting directly to the battery going improve? Nothing, it makes it worse in my case. How can you not understand that? You must not understand the underlying principles of electrical engineering. Connecting to the bus bar with the engine running I actually have parallel cable runs from the alternator and battery lowering my source impedance in the event of a high current demand. Connected to the battery these cable runs are now in series from the alternator. What do you mean "my ****" it is real. If I were upgrading to a HO alternator things would be different, I would have to make changes. It looks as though Audi sized things correctly, I would hope so. It has a 140A alternator, everything is sized for that. No more no less. If I try to draw more than this with accessories on the lights will dim. Do I think the bus bar is directly connected to the alternator, dugh...no. Have I calculated the voltage drop at worst case? Yes of coarse. At 140A the voltage drop from the alternator to the bus bar is 100mV. I can live with that. Where did you get the idea I wasn't taking into account amplifier inefficiencies? I was merely using 1kw as an example of output rating. Of coarse you need to look at the efficiency of the amp for the input current. Have fun too...