New Voltage

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Orionsax7

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I did the headlight upgrade to my 71 Demon. Powered a pair of relays with the original headlight power lines, and used the relays to power the headlight via the alternator. Lights are nice and bright now!

Only thing is that now the car runs with over 14volts at anything over idle. If the headlights are on it runs 12-13 volts but if no extra electrical load is on it, it sees 14+ volts at the fusebox.

Is this normal?

-Alex
 
Yes. With a fully charged battery, your alternator should keep the voltage right at 14.2V.
 
No it's not normal!

As Greg stated you should see 13.8 to 14.2 volts across the battery whether you have the lights on or not.

I think you issue is you are powering the lights from the alternator. You should pick up the power for your relays from the battery or near the battery. I would take the power from the same point that power is fed to the rest of the car.
 
It's fine. Don't worry about it unless you're seeing over 16 or less than 12 at ANY time, except while cranking.
 
Ok. I've heard both "always power new electrical loads at the battery" and "Always power new electrivcal loads from the alternator." Which is true?

Thanks for the input everyone.
 
Orionsax7 said:
Ok. I've heard both "always power new electrical loads at the battery" and "Always power new electrivcal loads from the alternator." Which is true?
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The alternator is the power source. The battery is for cranking the motor, and to provide reserve power when the motor is not running.
 
Two reasons you don't want to power accessories directly rom the alternator;

1. The output of an alternator is AC (hence the name alternator) and diodes are used to rectifiy the output. But, it's still a pulsing DC signal which can cause problems with some accessories like stereos. The battery acts like a big capacitor so you get clean DC power when you take your power from the battery.

2. You are shunting off a portion of the alternator's output to run the lights. This is confusing the voltage regulator and is the reason you get 12-13 volts (which is not enough to charge the battery consistently) with the lights on and 14 with the lights off.

FWIW, I have never heard anyone recomend taking power directly from the alternator output terminal to power an accessory.
 
The alternator is by far the best location to power your headlights when adding the relays. One of the largest benefits is the reduction of amp draw through the firewall plug and the associated small wiring. The lights literally have their own circuit now instead of using the undersized and 35+ year old wiring. Numerous websites recommend this approach including a lighting consultant friend of mine who deals with high end HID lighting and such.

When you do this be sure to use fusible links or the appropriate fuses between the Alt and the relays. I installed a 78 Amp alt and used 2 seperate 12 ga wire and 16 ga fusible links to power the relays.
 
DOHCRT said:
The alternator is by far the best location to power your headlights when adding the relays. One of the largest benefits is the reduction of amp draw through the firewall plug and the associated small wiring. The lights literally have their own circuit now instead of using the undersized and 35+ year old wiring. Numerous websites recommend this approach including a lighting consultant friend of mine who deals with high end HID lighting and such.

When you do this be sure to use fusible links or the appropriate fuses between the Alt and the relays. I installed a 78 Amp alt and used 2 seperate 12 ga wire and 16 ga fusible links to power the relays.
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This is exactly what I did...
 
Alternators do in fact produce rectified dc voltage but Dave is correct about it not being a clean signal. If you look at it on an ocilliscope you will see this. I know, I know, who uses an ocilliscope on a car? Whether you do or not the sine wave (voltage) is not clean coming right off the alternator. The battery does in fact clean up the voltage signal like a capacitor.

I do agree that it is best not to run alot of current through old wiring and old bulkhead connections so that is the only reason I see to wire high current devices to the alternator directly. If your going to do this it would probably be best to get a large capacitor (like the kind used to accent stereos) to add to the circuit (in parallel with the alternator output) to smooth out the ripple.
 
I'm just curious, what effect does the rectified DC current have on the lighting? I don't notice any flickering or change in lighting intensity. Will the bulbs wear out faster? What are the down falls of this if you are only powering headlights? I will also be adding electric fans and plan to use the alt as a power source also, will this current cause negative effects for the fan motors? How would one wire in the stereo type capacitor to act as a buffer?
 
The life expectancy of the filiments in the lights won't be appreciably shortened by the dirty DC signal. If you don't notice any ill effects or flickering in the bulb output I wouldn't worry about it.

Alex's original post was that he had an issue that he was seeing 12-13 volts at the fuse panel with the lights on and 14 volts with them off. I am assuming that he had 14 volts when the lights were on or off before adding the relays. That being the case then by moving the light load to the alternator output terminal is some how (bad ground, bad connections, etc. somewhere in the system) causing the voltage regulator to not be driving the alternator enough to ensure there is adequate voltage in the system.

Good engineering practice tells you to provide common points for providing power and grounds (that's one reason why you don't see accessories powered from the alternator from the factory). It eliminates all kinds of weird issues that come up from ground loops and multiple connections. If you have a concern that the charging circuit (the issue with inadequate wiring throught the light switch has altready been addressed by the relays) through the fire wall is not adequate to deal with the 9 amp load the head lights add to the system then you would be better served to fix that then put what I would consider a bandaid on it by taking power from the alternator.

IMHO, if you have dim lights due to old wiring and you are not add lights that draw additional current then the correct thing to do is fix the wiring. The factory wiring is quite adequate for standard sealed beam bulbs.
 
dgc333 said:
Alex's original post was that he had an issue that he was seeing 12-13 volts at the fuse panel with the lights on and 14 volts with them off. I am assuming that he had 14 volts when the lights were on or off before adding the relays. That being the case then by moving the light load to the alternator output terminal is some how (bad ground, bad connections, etc. somewhere in the system) causing the voltage regulator to not be driving the alternator enough to ensure there is adequate voltage in the system.
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Respectfully Dave, I do not agree with this theory. I believe that what Alex experienced is still a voltage drop in the factory wiring between the alternator and the fuse box. When the headlights are turned on, you also get parking lights, tail lights, side marker lights and instrument panel lights. All these together can add up to several Amps, and these are still being powered through the original wiring.

I made the statement that with a fully charged battery, and no load, the alternator (voltage regulator) should try to maintain approximately 14.2V. However, with a large load such as headlights, I would expect the alternator output to drop to somewhere between 13.4V and 13.8V with the engine idling. Now couple that with a 1V drop in the wire harness to the fuse box, and it is not surprising Alex measured 12-13V there. It would be interesting to see exactly what the voltage was at the alternator under the same conditions.

dgc333 said:
Good engineering practice tells you to provide common points for providing power and grounds (that's one reason why you don't see accessories powered from the alternator from the factory). It eliminates all kinds of weird issues that come up from ground loops and multiple connections. If you have a concern that the charging circuit (the issue with inadequate wiring throught the light switch has altready been addressed by the relays) through the fire wall is not adequate to deal with the 9 amp load the head lights add to the system then you would be better served to fix that then put what I would consider a bandaid on it by taking power from the alternator.
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In a perfect world, I would agree 100%. But these 30 year old Mopars are not perfect. So yes, taking the power directly from the alternator is a bandaid for the poor factory wiring.

dgc333 said:
IMHO, if you have dim lights due to old wiring and you are not add lights that draw additional current then the correct thing to do is fix the wiring. The factory wiring is quite adequate for standard sealed beam bulbs.
Click to expand...
I think this depends on your definition of adequate. I have personally measured over 1V drop between the fuse box and the headlamps on my Dart. This does not include any voltage drop in the wiring from the alternator to the fuse box. To me, a >1V drop in a 14V circuit is unacceptable. Maybe my standards are too high.
 
Thanks for the great input. Tomorrow being Veteren's Day, I have the day off from school and will have time to take many many voltage drop readings and record them so I can figure out exactly where I'm losing power. I'll post the findings up as well.

-Alex
 
Excellent discussion guys. It covers a lot of areas that most of us non-electrical types never even think of when we are attempting to upgrade our old Mopars. Food for thought!!
 
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