Voltage regulation in the FJ is the same as any other vehicle.
Goes kinda like this:
1) The alternator provides AC power.
2) The AC is rectified to DC via a bridge rectifier.
3) The DC is fed through circuitry that regulates the charge around +13.8v maximum at a high current.
Now, with regards to sometimes seeing 13.2 and sometimes seeing 13.8 is because of the varying load. You're seeing 13.8 at idle because it's outputting the maximum in a no-load condition (generally -- there are some exceptions). If you start to increase/decrease (or vary) the load, you will notice fluctuations in the DC output voltage.
While the load can causes the change in voltage, it is often the state of charge of the battery.
The voltage regulator will command the alternator put out a 13.8 volts, up to about140 amps. But if the state of charge of the battery is such that it draws high charging current, the alternator will back off the voltage until the battery is charged to a point where 13.8 volts can be applied to it without huge charging currents.
SO it is a combination of the load and the state of charge of the battery.
You could have almost no accessories running and have the state of charge of the battery be low enough that it would eat up almost all the alternator current, causing a say 13.2 volts to occur. On the other hand, you could have the battery at a higher state of charge and have tons of accessories running and that would reduce the available voltage also, again due to the current limit.
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BCMT6, Pkg2, KOs, rack, rails, hitch, Abrams, I-Way, Baja 480C, 150W maplight, aux backuplights, aux battery, gerrycan rack, dead buzzers/DRL
While the load can causes the change in voltage, it is often the state of charge of the battery.
The voltage regulator will command the alternator put out a 13.8 volts, up to about140 amps. But if the state of charge of the battery is such that it draws high charging current, the alternator will back off the voltage until the battery is charged to a point where 13.8 volts can be applied to it without huge charging currents.
SO it is a combination of the load and the state of charge of the battery.
You could have almost no accessories running and have the state of charge of the battery be low enough that it would eat up almost all the alternator current, causing a say 13.2 volts to occur. On the other hand, you could have the battery at a higher state of charge and have tons of accessories running and that would reduce the available voltage also, again due to the current limit.
White Rim, this is true and if you think about it, we're actually saying the same thing. The battery would in effect be the increased (still varied) load in that instance.
If the state of the charge on the battery is low, the battery is at a lower potential than the alternator is putting out and thus, the current is going to flow from the higher potential to the lower potential.
By "varied" load, I don't mean the load you're necessarily putting on the system. I'm referring to the system as a whole and the battery is inclusive in this system as well. The battery in a charging state is considered an electrical load.
So that's what I would expect, but here's what I have observed...
Almost no load and fully charged battery gives a 13.2 even at 2K RPMS. If the car hasn't been started for a day, then it goes to 13.8.
So what I'm wondering is if you have almost no load and a very good battery, will the alternator back off the voltage because there isn't any demand? It certainly appears to be doing that.
Oddly enough, a partially discharged lead acid battery, when placed on constant current charge at about 50 amps, will rapidly rise to 13.8 volts or so (14.2 for a deep discharge) and then, as the charge level increases, the voltage will actually DROP (shockingly enough haha). THen the voltage will gradually increase to about 13.5 volts again.
I cannot remember the chemistry behind this.
This assumes about a 75 degree operating temperature.
Another factor is that the open circuit voltage of a lead acid battery decreases with temperature. So at say 0 degrees F, a lead acid battery will have a higher voltage than at 75 degrees F, (but it will provide less starting current, CCAs. THe CCAs is the currrent that can be drawn from the battery at 0F without the voltage going below 9 volts, and is between 600 and 1200 amps typically)
And so when the battery heats up in the engine compartment its voltage will go down a little, even at full charge.
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BCMT6, Pkg2, KOs, rack, rails, hitch, Abrams, I-Way, Baja 480C, 150W maplight, aux backuplights, aux battery, gerrycan rack, dead buzzers/DRL
Last edited by White Rim : 05-01-2008 at 06:29 AM.
Reason: clarification
Oddly enough, a partially discharged lead acid battery, when placed on constant current charge at about 50 amps, will rapidly rise to 13.8 volts or so (14.2 for a deep discharge) and then, as the charge level increases, the voltage will actually DROP (shockingly enough haha). THen the voltage will gradually increase to about 13.5 volts again.
I cannot remember the chemistry behind this.
This assumes about a 75 degree operating temperature.
Another factor is that the open circuit voltage of a lead acid battery decreases with temperature. So at say 0 degrees F, a lead acid battery will have a higher voltage than at 75 degrees F, (but it will provide less starting current, CCAs. THe CCAs is the currrent that can be drawn from the battery at 0F without the voltage going below 9 volts, and is between 600 and 1200 amps typically)
And so when the battery heats up in the engine compartment its voltage will go down a little, even at full charge.
All very true, all very good information. Someone knows their lead acid stats!
About the temperature - that stuff is all modeled at 25C, or 77F. You just about hit that nail right square on the head.
While we're discussing temperature effects, let's consider another thing that can contribute to strange or erratic battery behavior. Don't forget about age as well. The internal resistance of the battery will increase with age. The internal resistance may also increase substantially if an attempt is made to pull too much from the battery too quickly with a large amount of current draw. Over time, this increase will present itself as a lower overall voltage (and seemingly reduced current capacity) retained by the battery.
So what I'm wondering is if you have almost no load and a very good battery, will the alternator back off the voltage because there isn't any demand? It certainly appears to be doing that.
Hmm... It may have to do with this, which I had in an old link under "automotive" in my browser:
Quote:
The sensor for the regulator is usually connected to the positive post of the battery and when it senses a low voltage condition, allows more energy to flow in the rotor coil, creating more energy in the stator coils. It does this through a series of points, raising the points off of the contacts to allow less current to flow, or dropping points onto the contacts to increase current flow.
If you think about it, it could be preset to something like 13V and the 13.2V is still not registering as a "low voltage condition."
When the car starts, if it's at 12.8 or something, the charging circuit is going to kick into high gear and maybe charge it back up to a preset 13.8V and kick back off again.
Here's an idea: Before you start your FJ next time, see what the battery reading is. Report back and we'll see if this theory has any merit.
J+J+FJ - '08 Titanium MT6 - Not stock, pinstriped paintjob. 
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