Running power wire direct off alt?

[winkychevelle 10+ year member]

how about we find out what the car in question is?

toyota mr2 mkii

 

[TheUnderFighter 10+ year member]

Pontiac Fiero. haha. I dunno. I'd suggest just running a tiny battery next to the amps in the back. That way you just need a short run of wire from the alt to the new battery.

 

[jolly_26 10+ year member]

toyota mr2 mkii

Sure is mate //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

I missed the mid-engined part. my bad.

haha in that case I can see where you were coming from. I'll check out the size of the factory wiring tomorrow or something, been so cold lately it's hard to force myself outside, and if it is a large gauge then I will definitely just run off the alt.

 

[maylar 10+ year member]

Whenever the car is running, the car's charging system is going to be 13.6-14.4, your car doesn't just run at 12.6v if the alternator is not outputting the bulk of its current output. The battery itself acts as a load on the charging system until the alternator cannot adequately supply the current draw of the vehicle and its accessories. You want the battery there to be able to handle the flux in current demand not the alternator itself.

The alternator powers the vehicle once the engine is running. Battery acts as a load until it gets recharged after starting. In fact once the engine is running, you can (theoretically) disconnect the battery.

Running the power wire to the alternator itself will subject the alternator and voltage regulator to voltage spikes that will harm and drastically diminish the life of the alternator.

That's rediculous. The battery and alt are THE SAME POINT, connected by a fat wire.

Factory alternators rarely, if ever, supply meaningful current at idle. Factory alternators are 1. designed to run at about 80% of its charging capacity (i.e. if you have a 100a alternator, it was designed to have 20 amps of buffer after running ALL accessories in the car), and 2. don't produce meaningful current til => 2kRPM.

If that was true your lights would dim at idle.

Yes, the voltage will be ~14v but voltage is only part of the equation and not necessarily the important part.

Riiight... And where do you suppose that 14V comes from?

Please read how alternators, batteries, and the overall charging system in your car works first.

I suggest you follow your own advice.

 

[NFrazier 10+ year member]

The alternator powers the vehicle once the engine is running. Battery acts as a load until it gets recharged after starting. In fact once the engine is running, you can (theoretically) disconnect the battery.

.....You're right, that's what I said. The battery acts as a load on the charging system. If there is a drastic spike and the voltage regulator doesn't react quick enough, what's picking up the slack? The battery.

If that was true your lights would dim at idle.

You neglect to actually read things, or you simply ignore words. I did not say the alternator didn't produce ANY current, I said MEANINGFUL current. As in the 100A alternator is not going to be able to produce 100A at 800 engine RPM. The alternator still has the ability to supply current at idle, but it's not going to supply all of the current required to run all accessories in the vehicle, and any additional that the car wasn't designed for.

Riiight... And where do you suppose that 14V comes from?

I don't get the point of this comment. The alternator is running and supply the charging system when the vehicle is running, I never said it wasn't. When the vehicle is running the alternator pulling is rotating, thus it's on and supplying 13.6-14.4v.

I've seen this done on cars and have adverse effects on both the car itself and the aftermarket products that were installed. Yes it could have been that the alternator was malfunctioning, the voltage regulator was malfunctioning, etc.

The vehicle in question is midengined, it ***** to work on and it ***** to have to go through two firewalls.

CAN you run it off the alternator, theoretically yes. Would I ever do it, no.

 

[maylar 10+ year member]

.....You're right, that's what I said. The battery acts as a load on the charging system. If there is a drastic spike and the voltage regulator doesn't react quick enough, what's picking up the slack? The battery.

We agree on something then. And that's why I said you can "theoretically" remove the battery once the engine is running. In practice that's a bad idea because the battery absorbs transients.

You neglect to actually read things, or you simply ignore words. I did not say the alternator didn't produce ANY current, I said MEANINGFUL current. As in the 100A alternator is not going to be able to produce 100A at 800 engine RPM. The alternator still has the ability to supply current at idle, but it's not going to supply all of the current required to run all accessories in the vehicle, and any additional that the car wasn't designed for.

That was true in the old days. Under heavy load at idle the battery would supply some current because the alternator couldn't keep up. Voltage would drop to 12.5V. But modern charging systems do indeed have the ability to supply almost full rated current at idle speeds. Put an amp clamp on your alt wire sometime and see for yourself. Look at it this way - if the voltage is above 12.6V then the alt must be supplying all of the current.

CAN you run it off the alternator, theoretically yes. Would I ever do it, no.

There's where we disagree. My point is that the alternator and battery are electrically the same, separated by a length of wire. In the OP's case the wire is just longer than usual. But the voltage source is the alternator as long as the engine is running. It makes no difference which end of the wire you connect to. With engine off, that long wire would be significant but no different than running a wire from battery to the trunk in a "normal" install.

 

[bubbagumper6 5,000+ posts]

alts are to float your batteries not provide a constant charge, you want to burn it up go a head and run it off the alt.

You sir, are retarded. You need a better understanding of fundamental electronics.

 

[bubbagumper6 5,000+ posts]

if the voltage is above 12.6V then the alt must be supplying all of the current.

Just wanted to quote this so maybe some of you will soak it in.

 

[bubbagumper6 5,000+ posts]

lulz double postin up in this bitch

 

[madcad95 10+ year member]

You sir, are retarded. You need a better understanding of fundamental electronics.

This

 

[jolly_26 10+ year member]

Well after all of this great banter I am sorry to report that the car won't be getting a sub after all. I realised whilst cutting up parts of the car to remove weight that having a sub was just stupid in this car. To clarify, my MR2 is quite different to the crappy USDM ones that you guys get. I have a sub on the way but I'm just going to set it up in my room haha. This means that my system will be less than 200 watts rms, and the amp for the mids and highs is small enough to just go under the seat. I will run from the battery, as with the amps inside it'll actually be easier.

To update on the car, the doors have had a 25% coverage of AT 60mil, and mids and highs wired and positioned into them. Tweets are aimed, and mids are factory located other than spacers. Waiting on one more RCA cable and I'll be able to get it all done. Will be running active crossed off a Soundstream LW4.500 amp, and I may do a sealed 8 too if I can find a LW1.400 for a reasonable price.

Cheers for all the help. Also, to contribute to the thread, after more research I am of the opinion that @madcad95 @maylar @bubbagumper6 and @winkychevelle are correct. Not to say that everyone else is completely wrong, for some situations, systems and cars you are possibly right, but I'd say just not this one.