Alternator Question!

I installed an new Plane Power AL12-EI60 alternator today. The reason for doing so was because at startup my old alternator was only putting out 13.5v at about 30 amps. ( I have an Earthx battery with draws about 30-40 amps after startup until it tops off) As the current draw drops the voltage will increase but on to about 14.1 volts on the buss. The paperwork that came with this alternator says that it should put out 14.5v at all RPM's and current draw all the way up to it's rated output of 60 amps.

Why is the voltage so low at first? As the current drops the voltage comes up. At approx. 12 amp current draw my voltage is still only reading about 14.1 volts. I have checked all grounds and they have been cleaned and are making a great contact. I also checked the output voltage at the alternator and it is 14.4v with a meter and my bus voltage on the G3X was reading 13.6V. I did confirm with a volt meter that the bus voltage was reading correctly. 8 gauge wire, 4 foot run to the buss from the alternator.

Any idea what might be happening?

A 8 gauge wire can tolerate a certain amount of amps at specific conditions.
The current goes from the alternator to the bus and then back to the alternator via the battery or load and the engine GND-wire. A low current like 1 amp in this 8 gauge wire will result in a almost zero voltage drop in the wire itself. Assuming a normal current of 15 A will result in a voltage drop around 0.3 V in the wire under normal flying. Between the battery and the bus you have a contactor with it´s own voltage drop of about 0.3 V.
This alternator AL12-EI60 has a BUILT IN voltage regulator.
It therefore sees the voltage on the B-lead out from the alternator and it tries to regulate this voltage to 14.5 V. Therefore the bus voltage will be lower, in the range of 13.9 or so.
Loading an alternator to 30 amps or more will result in a larger voltage drop
from the alternator to the bus, battery and the G3X, typically 0.8V.
Why is it so ? The wire area is fixed, more amps will produce an increased
drop in the voltage. Plane Power also sells alternators with EXTERNAL regulator that senses bus voltage. Then you have the possibility to set the bus voltage as you like. This way you can compensate for losses in the wire and load conditions.

Good luck

The Wizzard said:

The paperwork that came with this alternator says that it should put out 14.5v at all RPM's and current draw all the way up to it's rated output of 60 amps.

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This is flat out false, unless they have invented some new physics. It simply can't happen at ALL rpms. It takes power to make power, and if the alternator is turning slowly, like idle rpm, it simply won't make full output current - none of them will.

It's marketing, looks great in glossy print, and is worthless.

thanks for the excellent explanation

Thanks for the excellent explanation gentleman! Makes perfect sense.

Avanza said:

Plane Power also sells alternators with EXTERNAL regulator that senses bus voltage.

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The PP version with internal regular also senses voltage. It does so via the field wire that has a 5 amp fuse. That generally is attached to the buss with a breaker on the panel, and has caused some builders/owners to have experienced voltage higher than 14.5vdc as a result of voltage drop (poor connection resistance or switches) back to the alternator.

One does have to be careful in selection of the source for the field supply and source point for the panel voltage. As reference, I chose the panel side of the master for the voltage source and although there is some electrical separation from the buss supply, my voltage reads 14.4vdc (Internal regulated PP - 60A).

The Wizzard said:

I
Why is the voltage so low at first? As the current drops the voltage comes up.

Any idea what might be happening?

Click to expand...

That is how the system works. Inverse relationship between current delivered and voltage. The higher the current draw, the lower the output voltage will be, with many factors influencing the curve. Most set the voltage at low draw, as that is where it needs governing. You can search google for a more scientific and detailed explanation of why. Unwilling to type that much.

Take a look at the voltage drop during your engine start, the current its releasing to start the engine drops the power bank down, not sure thats correct on how to say it. I look at it as a sealed box loaded full with 14 volts, you open the amp power valve to start the engine or power a radio and that voltage will go down as the power is released and once the alternator sees this it slowly apply charge to it at the rate it is able to.
Anyway I toasted a new AGM J16 battery by leaving the master on for a few hours.. brought the volts down to around 12.2 volts and instead of putting slow charger on it to bring it back, I just started the engine and flipped my alternator on and that alternator sensed the low batt volt and ATTACKED the battery with 18 amps for about 15 minutes than slowly decreased the attack back to a normal 1 or 2 amp charge rate. During the 18 amp charge the volts the Alternator was applying to the bus and battery were 13.5ish and as the charge rate went down to normal the volts slowly came back to normal 14.2, any more than 14.2 is not good for a 12 volt battery

If I would have had a BIGGER alternator say 100 amp instead of 40 amp it may have been powerful enough to keep the voltage at 14.2 as it was destroying my battery with all those amps. If you ever leave the battery on and run down an AGM battery, put a slow charger on it.. at least with those little orange sbs j16 batteries. Mine was only six months old when the alternator destroyed it, it never held a good charge again after that

Charing a battery

If the alternator in the aircraft is used for charging a AGM J16 battery, the voltage is regulated to 14,5 V. The battery is capable of a fairly high charging current. The internal resistance in the battery determines the amps while charging. If the battery has been drained for several hours and has a low voltage ie 12 V, it also has got a low internal resistance resulting in a high charging current (amps) provided that an alternator is used.
When during charging the voltage of the alternator and the battery comes
closer to one another, this results in a decreasing current (amps).
What kills a battery is leaving the master switch on.

Good luck

What kills a battery is not how it was discharged, it's how it is treated after the discharge.

Example..... PC680 about three years old. Left the master switch on.... battery fully discharged. Nothing, no reading on the test meter.

Connected a 12v dc adapter 500ma to the battery and waited (maybe days needed) for it to top out. A slow discharge needs a slow recovery.... 500ma.

Ran the battery in the RV for another 3 years, then started to worry about it. Replaced with a new 680 and the old one went into the riding mower and has been there for 9 years now. Placed on a 500ma charger twice a year for 8 hours.

Don't trust what I say? try it yourself the next time you slowly kill your battery.

If you don't have a 500ma adapter laying around..... https://www.youtube.com/watch?v=V2f-MZ2HRHQ&list=TLPQMDQwMzIwMjPx5wsX31uLEw&index=1

Don't be in a hurry, it will take days.