Voltage Fluctuations - why?

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rhyary

rhyary

Joined
Apr 5, 2006
Threads
167
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1,481
Location
Albany, NY
Calling on all electrical experts.

I have an instant volt meter in the cab and been watching my voltage regularly. I can logically explain any voltage fluctuation with the exception of one thing.

I don't understand why some times my voltage drops to 13.8, and then go back to 14.1 or 14.2.

Before you jump into conclusions, I wanted to say that everything is working great on my truck. I am not looking for troubleshooting techniques. I am looking for logical and knowledgeable explanation because I like to know minute details about how things work.

BTW every voltage range is due to ambient temperatures.
Low number during the last two months in -13f to 25f.
High number during the fall, summer.

Alternator is a Toyota 150amp from cdan using photoman bracket. There is no voltage fluctuation as it relate to RPM nor due to electrical load. Yes it is that good!

Cables were all beefed up, there is no tiny corroded OEM cable in the system :-)

Here is the sequence I observe every morning:
Morning batteries at rest 12.55 or 12.6 (as expected)
Start: drop to 8 or 10V (as expected)
Charging: 14.2 to 14.4 (as expected)
Maintaining 13.8 (as expected but not always observed)
Charging: 14.2 (not expected)

Sometimes the charging 14.2 will go on for a long time on a long trip. I would expect it to drop to 13.8V

Sometimes after the drop to 13.8V, it will pickup again to 14.2V and will not go down to 13.8V again.

This does not change if I am connecting the secondary battery or disconnect it. I can do that from the cabin.



I would expect that once the 13.8V shows up on a long trip, it will stay there to the end of the trip.

Rami
 
Here is how I control the secondary battery

Switch
left-switches.webp
 
Are you charging the batteries while paralleled or individually? I am not an electrical guru, but if I understand correctly charging two different types of batteries while paralleled is hard on the alternator? Maybe this is why you are seeing the fluctuations. What happens with only one battery connected?

BTW: your battery combiner switch install is classy!
 
Thanks for the complement.

Yes I can disconnect the second battery using the "classy" switch :-)

No the behavior does not change if only the main starting battery is active.
 
This falls in the realm of things not to worry about IMO. However, I would love to know where you got that battery switch.
 
This is a the blue sea switch that comes with the blue sea isolator. I got it from West Marine. My biggest surprise was how neatly it fits the OEM blank spot. Trust me, I lost sleep over this issue!

Unfortunately I do tend to worry about stuff like this and in the process I get to learn and know a lot of cool information.

Incidentally, I can't recall even once where I pursued information not worth worrying about and being disappointed worrying about it once I learned the reason. :-)
 
This change in voltage is less than 10%, sound ok to me... but here is a smarty pants explanation..

It could be hysteresis in the voltage regulator, which is really not too common but does occur. There are several other reasons why you could be seeing what you're seeing. I'll explain a little how this stuff works, it may make more sense.

An alternator is an exciter controlled (no permanent magnets) generator and voltage regulator combo. An alternator has an exciter winding in the spinning armature that induces a current in the stator windings, usually 3 phase AC into a bridge rectifier that converts it to pulsed DC output. The more current into the exciter the more current is induced on the output.

Almost all generators are current devices, meaning for a given current in the exciter coil plus work, there will be a current induced on the output coils. For a fixed current in the exciter at a fixed RPM there will be a fixed current induced on the output. If you increase the load and there is not enough current to to supply the demand, the voltage drops. When the load is decreased the voltage increases until the point where all the current is consumed. As rpms increase, the induced current on the stator increases, conversely as rpms lower it reduces the induced current. Without a regulator and with no load and you can get easily 100s of thousands of volts on the output. That is until the insulation starts breaking down and shorting out the stator windings.

The regulator adjusts the current to the exciter so the output voltage go too high and doesn't fry the electronics and cook the battery. It also increases current to the exciter when the load is turned up like the headlights or the rmps drop to idle. Some regulators designs will charge up to a high water mark then shut off. It will not reengage until the voltage hits a low water mark. That switching gap is called hysteresis. Most other designs operate linearly, always providing current to the exciter. Sometimes the exciter armature gets magnetized causing an induced output without an exciter current. This can screw up linear regulators operations and cause the effect your seeing. The good thing is the magnetism wears off and the regulator/charging system returns to normal operation. If it doesn't wear off, the exciter needs to be flashed with reverse voltage which depolarizes the armature.

The regulator uses an internal voltage reference like a constant current or a constant breakdown voltage (zener) diode. The reference is used to measure against the alternator's output voltage. When the alternator voltage becomes too high, the current to the exciter is reduced, reducing the output current and resulting voltage. Depending on the design of the regulator there may be a hysteresis or a gap between when the exciter current is adjusted.

Electronic components operating characteristics changes over temperature and they all don't change the same. So there could be non-linear variances resulting in slight changes in operation. The same it true for soldering and other mechanical electrical connections. You may have a poor solder joint or dirty terminal that performs marginally only over a certain temperature range affecting the resulting voltage. Move out of the range, all works great.

All this electronics stuff described is also applicable to your volt meter. There could be nothing wrong with your vehicle and the meter you're using could be hosed, giving you false readings...
 
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It isn't something I would worry about. There are simple explanations, such as the VR fluctuation above. It could also be the current load on the system as you turn things on or off. The wires have intrinsic resistance and there is a voltage drop across a resistor network that depends on the current. V=IR
 
Thanks.
So what I take from the above explanation is that what cause the 10% fluctuation can not be traced to a functional operation and/or components but the variable components reacting differently to temp changes and or other environmental.

There is no functional rhyme and reason. Meaning, I can't find a relationship between operating the car and when the voltage display is 13.8 and 14.2. Trying to find some correlation to time after start, length of driving, time of the day, charge level. But nothing seems to be predictable and correlate.
 
Sun spots...
 
The reason, I believe we need to get to the bottom of this is that we can't, at least I can't, afford killing a $230 battery every year or a year and a half.

Under 2 other different threads we have relevant discussions

The aux was on battery tender for 10 hours. I'll report if I see different numbers
 
I think rhyary is referring to sun spots comment.

"Sunspots" is a term used by old radio engineers to explain stuff that is too unexplainable, defies reason or is out of scope of conventional engineering assessments and falls into the area of knowledge called " the stuff we don't know that we don't know".

The fact is you are making a determination that your charging system is not performing properly because one sensor of ambiguous pedigree is telling you so. Do you know for a fact that voltage display is correct ?

And talking to the fact of randomness your experiencing, maybe there is mechanical issues with something in your vehicle, your voltmeter or the installation of the meter that the road conditions, bumps vibrations, effect ?

And just as an FYI 13.8 volts is just about the voltage on a lead acid battery with no load. Auto batteries are usually 6 cell devices producing 2.1 to 2.13vdc per cell or between 12.6 and 12.84vdc with an open load.

What I find interesting, is....

The 13.8vdc you are seeing looks like the "float voltage", used to charge over long term i.e. "trickle charging". The 14.2vdc is the normal charging voltage....

Does the ecu in your 450 connect to the alternator charging system or the regulator ?
 
This is off West Marines web site.

BLUE SEA SYSTEMS
Starting Isolation (SI) Dual Sensing Automatic Charging Relay

Supports high-output alternators up to 120A.

You stated you have a 150A output alternator. Could this be part of your issues besides the isolator alternating from Batt to Batt through the regulator. You said it is working fine. Don't go OCD over it and lose any sleep. After all its doing its job.

JM2C
 
I was under the impression that the initial 14.2v at startup was the alternator topping off the battery after starting. Lowering to the 13.8v range when running. I know when I use my power windows the voltage raises to 14+ and goes back down when the window motor stops. I use a Datel volt meter in the cup holder spot to continuously monitor the voltage. The head lights don't have quite the same affect, being that its a constant draw. The blinkers on the other hand make the voltage fluctuate about .5 volt.
 
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