Alternator/Battery

Here's a possibly stupid question I'm a little unclear on. On most GA planes, does the electrical system run directly off the alternator in flight, or does it run off the battery and the alternator constantly keeps the battery charge at a certain level? I always understood it ran directly off the alternator, but I had an electrical failure in a 172 and the explanation the school gave me was that the connection on the battery was loose (it had happened before).

Also, on the Piper ammeters (that show electrical load rather than rate of charge), what's the indication that an electrical failure is likely? Lower than normal load, or higher? The kind Cessna uses always seem a little easier for me to understand at first glance.

Most alternators you find in GA airplanes are not self exciting type alternators- in other words you need battery power for the alternator to generate electrical power. Probably one of the reasons that in the post 1976 (or there abouts), 172 the battery is a -R item. In other words if the battery is not operational the airplane does not conform to its type certificate and is not airworthy. So technically if you get a jump start in those 172s you are flying a unairworthy airplane.

I'll give this a try

To explain whats happening in flight, imagine cutting off the alternator. Everything still working? Yep. The battery is still powering everything perfectly fine except now your time limited on your battery's charge.

Now cut off the battery and everything goes dark. The alternators direct electrical power is too much to feed directly into the electrical system, this is why the battery switch must be on to for the alternator to make any power. Also like blackhawk said above, the alternator needs power from a battery to get "excited" and make power.

The alternators primary job is to recharge the battery. This is why the alternator is usually a higher voltage than the battery (14v alternator with 12v battery OR 28v Alternator with 24v battery).

So to answer your first question, the electrical system is powered off of the battery.

I haven't flown a piper in several years but I remember those ammeters. I think they were referred to as load meters. Don't they display the alternators total load output? So a failed alternator would be a zero indication?


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Ok, that makes sense. So basically you shouldn't be able to turn off the BAT side of the master switch and just use the ALT side, meaning the power is always coming through the battery? It seems like I remember people telling me that it's always coming directly from the alternator but I could never really find a solid explanation in the books.

Try it sometime. The Master Switch is mechanically limited. You can either have both BATT - ON ALT - ON, BATT - ON ALT - OFF, or BATT - OFF ALT - OFF. Try switching the ALT side on with the BATT side off, it won't work. A mechanism inside the switch switches both sides on.

Unless you are flying newer piper, then you have 2 switches.

Pretty sure once the field is energized, the acu will take over the energizing part and continue to feed some of the power back into the alt. I seem to remember being able to turn puff the Batt one everything was running

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I would have to disagree. Per the Cirrus SR-22 POH:

Yes I realize y'all were discussing Pipers and Cessnas, but this is what I know and I don't believe it's much different, but in this case, BAT 1 and ALT 1 are both connected to the Main Distribution Bus. In flight, the alternator is supplying all the electrical power to that bus and is charging the battery (by virtue of having a higher voltage output).

This is probably as good a diagram as any:



(Taken from this site.)

Everything is connected to ground (the aircraft structure), which is the common reference voltage everything gets measured against.

Closing the BATT side of the Master Switch goes from ground into the Battery Contactor (aka a solenoid). The Battery Contactor is a remotely operated electromagnetic switch, which looks like a coil of wire surrounding a magnet connected to the large switch that actually supplies battery power from the (+) terminal. When you close the BATT side of the master switch, current flows through that coil of wire in the Battery Contactor and creates a magnetic field, which pulls the magnetic switch down and allows current to flow from the (+) terminal of the battery through the ammeter (if it's the +/- kind) and into the Primary Bus bar. It can also be shunted away through the Starter Solenoid prior to reaching the Primary Bus Bar, to turn the starter. The only reason the Battery Contactor & Starter Solenoids exist (rather than you just opening and closing the battery switch manually) is to keep your hands away from unpleasant high current electrical arcs. Thoughtful of them right? That's the entire battery half of the electrical system.

The Alternator works by rotating an electromagnet called a Rotor inside fixed coils of wire called a Stator. Several alternating currents are induced in the various Stator coils (usually 3) by the changing magnetic field, and these go through a bunch of diodes (electrical one-way check valves using semiconductors) called a Rectifier which converts the various out of phase AC currents into a continuous DC current, which then goes out the B (Battery) lead of the alternator, and is hooked up directly to the Primary Bus Bar through the ALT circuit breaker. If the alternator just used regular magnets in the spinning internal rotor, no outside electricity would be required for it to run, but it would also only be able to output a fixed current. Since the load on the airplane's electrical system changes depending on what's switched on or off and how much the battery has been discharged, we need the alternator output current to be able to vary up and down - to provide a lot of current to recharge the battery after a start or only a little bit of current to keep the electrical system running if most of the loads are switched off. To accomplish this, the rotor inside the alternator has a Field Coil and a variable Field Current is supplied into the alternator (through the F terminal) that makes the rotor's magnetic field stronger or weaker and changes the alternator output.

In the diagram above, the Field Current is supplied by shunting away some current from the alternator B terminal output at the ALT circuit breaker, and sending it down through another ALT FIELD circuit breaker and into the ALT side of the Master Switch. When you close the ALT half of the Master Switch, that current continues - sometimes through an overvoltage protection unit - and into the power input terminal on the Voltage Regulator (aka the Alternator Control Unit in this diagram). The Voltage Regulator's job is to compare a reference voltage in the system against the B-output from the alternator (+/- sense terminals on the ACU) and decide whether to increase or decrease the alternator output voltage. It then takes the power input current (from the ALT half of the Master Switch) and uses it to supply the Field Current to F-terminal on the alternator (which goes to the field coil in the rotor). And that's basically it... in excruciating detail.

In simple terms: The battery and alternator are connected independently to the Primary Bus Bar. When the alternator fails (or is switched off) current flows out of the (+) battery terminal into the bus bar to power the various electrical loads. When the battery is charging, the alternator supplies an excess voltage (commanded by the voltage regulator) to send current through the Primary Bus Bar and into the (+) battery terminal.

The Cessna-style (+)(-) and (0) ammeter works because it is literally wired in series with the (+) battery lead wire, which is routed behind the panel between the Battery Contactor (operated by the Master Switch) and the Primary Bus Bar. When current is flowing from the bus bar into the battery, the ammeter shows a (+) deflection. If the alternator fails or is switched off with the ALT half of the Master Switch, current flows through the ammeter in the opposite direction - from the (+) terminal of the battery into the Primary Bus Bar - thus the needle is deflected the opposite direction towards (-) (also the ACU detects a low voltage condition and illuminates the "Low Voltage" warning light). If the battery charge is topped off and the electrical system is in happy equilibrium then the needle is vertical at (0), because no current is flowing either into or out of the battery.

The Piper-style Loadmeter reads % of rated alternator output (in amps from zero to the max rated output). Loadmeters are usually used in larger electrical systems where running the main battery hot wire through a panel-mounted ammeter is impractical. Instead, a Loadmeter is wired in parallel with the (B) alternator output terminal, between the Alternator and the ALT circuit breaker (and Primary Bus Bar). In this case, you are measuring the amount of current flowing out of the alternator, not the amount of current flowing into or out of the battery. Thinking about it in that context, if your battery were charging would you need more or less than the usual current supplied by the alternator while the system is in equilibrium? The answer is more, because now the alternator is working harder to supply current to all the electrical loads on the panel PLUS charging the battery. What about if the alternator fails, or is in the process of failing? The supplied output current from the alternator will be less, or zero! To convince yourself, take a look at your loadmeter during flight after you've been in the air for a while (and the battery has had time to recharge). Compare that equilibrium amperage to what you see immediately after start with all your electrical loads turned on (avionics, lights, pitot heat, etc). It should be less during flight than after start, assuming I'm not crazy and don't have the concept backwards in my head.

As always, hope that helped!

(Source: Dale Crane, Aviation Maintenance Technician Series, Airframe Volume 2: Systems)

Also, in the case of a C-172R (I dug out my old POH's), the low voltage annunciator comes on when voltage drops to 24.5 volts. If the 24 volt battery was the only thing powering the entire electrical system, wouldn't the low voltage annunciator be on all the time?

Good call. I've never flown a newer Piper and was restricting my comment to the usual older training fleet aircraft (C172 vs. PA-28), which share the standard mechanically interconnected red split-master switch. But you're absolutely right, the ACU shunts off some of the alternator output current (through the ALT switch) to power the alternator field coil, so it's a self-sustaining process. I think the mechanical interconnect in the switch is just the engineers covering their collective asses by ensuring that the battery will automatically take over supplying the bus bar with power rather than relying on the pilot to remember to manually switch it on.

The battery is rated for 24 volts but may (and probably will), produce more when the alternator is initially shut off. Observe the voltage put out by the system after you shut off the alternator- it will initially be in the 24 volt range then will quickly drop.

Does a C-172 (newer model) have a 28-volt system or a 24-volt system?

According the the C-172R POH: "The airplane is equipped with a 28-volt, direct current electrical system."

Why would they say it's a 28 volt system, if the most voltage the system would receive through the battery only is, at best, slightly over 24 volts?

The POH also says: "The system is powered by a belt-driven, 60-amp alternator and a 24-volt battery."

It doesn't say the system is powered by a 24-volt battery, which is constantly being recharged by a 28-volt alternator, as I believe some are suggesting.

After looking at the diagram for my 210, I agree with you. It looks like the alternator and battery are both connected to the primary bus bar and are both capable of powering it.

The Shorts SD3 was similar in principal to this. The main bus was powered by the generators and the batteries were charged off of the bus, not the generator directly.

The original question still seems to have a vague answer. Which is running the system while in normal operation?

To the original poster, this was not a dumb question at all. Thanks for bringing it up. Im always


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The CASA is pretty much the same. The generators power the secondary+essential bus, or just the essential bus. From there the power goes to charge the batteries.

The alternator.

If you stick a 1.5-volt battery on your tongue, and a 9-volt battery at the same time, your body will feel 9 volts (the higher voltage). Same thing happens to the electrical bus. It feels the higher voltage (that of the alternator) and that's what gets sent down the line. The battery is standing by to power the bus when the alternator goes away. Same as taking the 9-volt battery off your tongue. Only then will you feel 1.5 volts.

I think you just answered your previous question about the C172R then. I would guess that the voltage potential between the alternator output and ground is about 28v. Yet after all the power is diverted out of the primary bus to the various system loads the voltage is probably closer to (but higher than) 24v (to keep the battery charged). Should the alternator fail, that 28v electrical system would become a 24v electrical system, because 24v is the maximum potential the electromotive force from the battery can induce between ground and the (+) battery terminal. Thoughts?

You can read the voltage directly from the digital clock in the R/SP. In normal operation, the Alternator should produce 28v to the bus, and then the components use 28v. When you shut the Alt off, it drops to 24v and continues to power the system until the battery runs out.

Excellent post, Inigo!

auw2fly, you are right. If the system is only running off of the battery the "VOLT" light will be on all the time. This is on purpose. It has exactly on the same purpose as the idiot light in your car. A 172R has a 24 volt direct current electrical system powered by a belt driven 28 volt, 60 amp alternator and a 24 volt *battery. If the alternator fails, the system runs off of the battery which is less than 24.5 volts and the idiot light will tell you that you need to get on the ground quickly.

In response to the whole 28/24 volt thing. I find it easier to understand electricity if you think of it like hydraulics. Volts = Pressure (i.e. 24 volts = 2400psi) Amps = fluid (i.e. 5 amps = 3 quarts of fluid) The alternator (like a hydraulic pump) keeps the system at 28 volts. The amount of amps (also read current or load) varies. Like Inigo explained above, the ACU controls the field current to match the required load. Think of the battery like a 2400psi accumulator. It stores fluid (amps) at 2400psi. To force more fluid into the accumulator (charge the battery) you have to have more than 2400 psi.


*If you look in the Equipment List in you POH you can find the Amp-hour rating for the battery. This is important to know if you have an alternator failure.

Sorry, but this couldn't be more wrong. First, saying that a dead battery is not operational is like saying an empty fuel tank in INOP. Just like going out to the ramp and finding a plane with low fuel, you would want to know why the fuel is low. If the battery is dead because the numbskull that flew it before you left the master on, charge it and go.

Second, as Inigo explained above a self-exciting alternator is also self-sustaining. It just needs initial battery voltage to begin the creation of current. Once the alternator is online, then it supplies it's own field current.

This is an awesome explination of electricity. I could never keep volts/amps/current straight. Thinking of it in these terms helps clarify though.

Thanks!