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I did a search on recent posts and found that 13.5-14.5 is ideal for the cars charging system. I just put in a 100amp one wire alt. It seems to work fine (so far) but when I have all of my electrical devices on the voltage on my meter reads in the high 12v range. I plan to hook a digital meter directly to the alternator to get a true reading but I have not done so yet. My question is: if the voltage is below 13.5, should I get a alternator with an even higher amp rating? I have an electric fan, fuel pump, and 1200 watt stereo system. The voltage drops into the 12v range only when Everything is on (lights, fan, pump, stereo, etc.) I'd rather shell out the extra cash for higher amp alternator than have a problem later with the electrical system. On the other hand, I don't want to get a new one if I don't have to.
Thanks
 

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Where are you measuring the voltage? If it's inside the car, like on the switched side of the ignition switch then 12.5+ with everything on is fine. You can expect .5 to 1-Volt drop across the power feed wire.

Something else to consider- The alternator terminal voltage is set to say 13.8 volts with very little load on it. For the load to increase the system voltage must decrease. The lower the system voltage the higher the current generated by the alternator.
 

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It's fine. You have nothing to worry about.



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I disagree that everything is alright. Maybe, maybe not.

The test is to test the voltage at the battery when everything is on. This is a measurement at the BATTERY, not the dash. As one poster commented, there are voltage drops on the way to the dash, so the real test will be at the battery.

A battery is at 12.58-12.65 VDC when fully charged. If you can turn on everything in your car, and still have voltage above 105% of full battery voltage..meaning at least 13volts at the battery, which means there is charging current for the battery in reserve, then your alternator is okay. If not, go for an upgrade and don't forget the wiring and fusible links.

My guess, is that it's close, and depending on the feed wire, relays (you do run relays...right...for fan, stereo etc) will determine the outcome.

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Maybe you should fall back to plan A. Measure what is happening on the back of the alternator. If it's a voltage drop problem you can work from there. A lot of great answers so far. I think that everyone is correct on how they look at the problem and try and save you some bucks. All for that.
My take is that you are running marginal. You added 2 large power hogs to the existing GM designed circuit. One being the fans and the other the radio. The fans are a constant drain, 50 amps or better. With your 100 amp alternator you increased its size only about 40 amps over the original alternator size. Close enough.
Adding the mega-stereo put you in a new league. Can you get by? Yes, keep the volume down. The stereo takes less at lower volume. Want to crank it up at idle? The GM engineers didn't figure you doing that. Need to up the alternator size.
 

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You can also add an additional battery. It will act like a capacitor and give you reserve for the LOUD stereo. You can hide it in the trunk (usually close to all the amps, etc.)
 

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What RPM were you testing at? Try testing at about 1500-2000RPM, or your typical driving RPM. You won't get full output from the alternator at idle or 600rpm.

Peter
 
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