Alternator Voltage

[Great Ozzie]

I have not been persnickety when it comes to FSX aircraft and system replication of RW aircraft; A2A is helping to change all that.

So today when watching the voltage / amp drop when cranking on the starter... I happened to notice that after engine start, volts were at just 25.5v.

I decided to fly for a half hour or so tonite and see what the voltage did. Shortly after takeoff, it rose to approx. 26.3v and stayed there for the duration of the flight.

The ALT system is a 28 volt system (24v battery)... so I was a little surprised to see ALT volts remain at 26.3v. The system overvolts @ around 31.5v and undervolts @ around 24.5v (± a few tenths). I expected the ALT to output something around 28 'and a half' volts when at operating near cruise rpms.

If this has already been reported / discussed... well... in the words of Miss Emily Litella let me just say, "Never mind!".

-Rob O.

[Oracle427]

You're right, it should be reading 28V, I never noticed it remained low even after a few minutes of running at cruise power.

[Ron Attwood]

Well spotted Ozzie. I saw that myself 5 minutes after I installed the 172. Tsk tsk I thought, sloppy work!

[lonewulf47]

I do not agree completely ! For lead-acid batteries which are constantly charged (standby operation) the nomial voltage should be 2.2 V/Cell, thus 26.4 V for the 12-cell accumulator. Only batteries which are used in cyclic operation (charge - discharge) should be pushed up to 2.4 V/cell. Automotive accumulators as well as the types used in aviation are normally considered as used in standby operation. Keeping a cell-voltage constantly over 2.2 V will lead to "overcooking" and thus will reduce battery life.

[Mr.Mugel]

So, a typical 24V battery system contains two 12V Pb Batteries. That´s the classic way, still used in modern truck systems as I´ve learnt with. Each 12V Battery consists of 6 2V Pb cells.

When you start the engine, you drain a lot from the engine with a high current. With high discharge currents, the cuircit may go down to 10-10.5V/Battery. So about 20-22 Volts while engaging starter is quite okay. With automotive design, the engine will start with a voltage of 9V out of 12V original, in winter, and still manage to produce sparks in the ignition.
So we got a desired level of 24V, a minimum of about 20V. Than you need to charge the battery.
To charge it, you need to supply a higher voltage from the alternator than what the circuit incluiding the batterie has right now. So when the battery is empty like 21V, you can charge it with 24V, but not when it quite full, like 26V, than you need to charge it with at least 28V, or it will take hours till it´s loaded back to a desired voltage, so it can power the starter to start a cold engine.

The maximum amount of Voltage a 12V Pb battery can take before cooking out the water in the battery acid mixture is 14.7V. So 29.4 volts for a 24V battery pack. To stay on the safe side, a 24V pack is usually loaded at 26-27V, the voltage that has to be on the whole circuit.

I don´t exactly know if I´ve answered your question, as I am quite drunk right now, but it was my best shot, the facts mentioned should be right at least.

Edit: this agrees to Lonewulf. 30V would already cook the batteries, so it should be avoided. 26-28 is a desired value.

[pilotgod]

Hmm...the 1976 C-172N with a 24V system that I fly seems to be quite content in the 27.4V range. With avionics off she will charge at 27.8V. Not the full 28V of the alternator, but higher than 26.3 I see on the A2A aircraft. I would consult the other aircraft I fly, but the ones that aren't 12V do not have a voltmeter.

[AKar]

According to 172R/S AMM, the Alternator Control Unit "... controls the alternator field circuit to supply a main bus voltage of approximately 28.5 volts." The low voltage annunciation is provided whenever the bus voltage is "less than 24.5 +0.35 or -0.35 volts", and over-voltage protection "...disengages the aircraft ALT FIELD circuit breaker. This removes the power from the alternator system if there is an over-voltage condition greater than 31.75 +0.5 or -0.5 volts." An approximate reading of 28 volts, and always less than 29 volts, should be there by 1300 RPM with functional ACU.

[Ron Attwood]

I love it when you guys talk dirty.

[Great Ozzie]

I do not agree completely

Wow Ron... look what the lonewulf says about ur comment... you going to take that?

I'd fight back if it were me.

For lead-acid batteries which are constantly charged (standby operation) the nomial voltage should be 2.2 V/Cell, thus 26.4 V for the 12-cell accumulator. Only batteries which are used in cyclic operation (charge - discharge) should be pushed up to 2.4 V/cell. Automotive accumulators as well as the types used in aviation are normally considered as used in standby operation. Keeping a cell-voltage constantly over 2.2 V will lead to "overcooking" and thus will reduce battery life.

Ok...

I found a chart for Charge & Float Voltages, based on Air Temp, out of a BatteryMINDer Manual (model 24041-AA-S5). Please see the pic below.

We (BatteryMINDer) are the only approved manufacturer of maintenance chargers for use on all Concorde Aviation batteries.

According to 172R/S AMM, the Alternator Control Unit "... controls the alternator field circuit to supply a main bus voltage of approximately 28.5 volts."

If this is indeed what the AMM says ( "Aircraft Maintenance Manual" -- for Ron) and I have absolutely no reason to doubt that setting, then I would expect to see around 28v.

30V would already cook the batteries,

I am going to have to disagree with this as far as the C172R is concerned. As has been stated (a couple times) an overvolt condition does not exist until approx. 31.5v -- as can be verified in the C172R POH. If I saw 30v (for any length of time) it would be the same if I saw 26v... I'd want the VR (Voltage Regulator -- for Ron) checked and adjusted back to 28.5v.

Authentic battery. The battery capacity is based on temperature.

Very cool... very

-Rob O.

[AKar]

I might add that it is implied in the ACU maintenance practices chapter that anything above 29 V is considered too much at least when testing it after work done on it (even if the voltage is not enough to trip the CB):

"If the voltmeter shows more than 29 volts, immediately set the ALT MASTER switch to the OFF position and stop the engine.

NOTE: The ACU regulation circuit is non operational. The ALT FLD circuit breaker should open if the voltage is more than 32 volts."



(Note the less accurate "more than 32 volts" than the one I referred to earlier - even these actual manuals are not that precise on every line!)

[Al FR-153]

I love it when you guys talk dirty.

Ron, they are not only talking dirty, they are dropping acid!

The bigger problem is that I understand what they are saying, and it is getting DEEP