alternators and the battery

[aslan1982]

On a piper warrior/seneca what exactly does the battery power

1. is it used to start the engine?
2. does it power the likes of the radio, gps, turn co-ordinator, lights...
3. does it just provide 12 volt power
4. Is the alternator just there to recharge the battery?
5. When you see a load on the ammeter dial, does this mean that, that amount of amps is being drained from the battery or is that amount of amps being fed into recharge the battery?
6. Can the alternator provide 24 volts and if so what on a piper would be 24 volt and 12 volt?

Many thanks for the help to clear up my understanding of this

[Mark1234]

1) Yep.
2) Kinda. Yes if the engine isn't running, and yes if the alternator has failed!
3) It depends on the battery - different batteries supply different voltages, depends on your aircraft systems. You might also have regulators to give different voltage feeds, and/or have an inverter on a more complex a/c that provides ac power from DC, but unlikely on warrior types.
4) It recharges the battery, and provides power for the systems when the engine is running. Think of the battery as a resevoir.
5) depends on the ammeter and how it is wired. Some are centre zero and show flow in/out of the battery - they will read positive when the battery is charging, and negative when it is draining. Some are showing load on the system and always deflect in a positive sense - it's worth knowing what your usual load is.
6) if it's a 24volt alternator, sure. Depends what it was designed for. That's rather aircraft specific.

Also note that on most aircraft there is some sort of arrangement with the master switch that allows you to switch the alternator / battery on separately.

[dont overfil]

The battery powers eveything and the alternator charges the battery.

If the aircraft has a 12 volt system the alternator will have to have an output of more than 12 volts to charge the battery, Usually about 14 volts under load. There is an electronic circuit to control the amount of charge.

There are different ways of displaying the load but I think the PA28s I've flown show the alternator output in amps. Others sometimes show charge and discharge. ie the "0" in the middle of the guage.

Some aircraft are 12 volt and others are 24 volt. They will need the correct alternator for the aircraft.

DO.

[aslan1982]

thanks Mark for your help

[aslan1982]

just so im clear

While Im flying and I see on my ammeter lets say 40 amps

1. This is the amount of amps being fed into my Battery by the alternator. If it is zero the alternator is broken
or
2. This is the amount of amps being used by my battery and the alternator will be working in the background toping up the battery


So the battery will power everything and the alternator will just keep recharging the battery??

[BackPacker]

aslan, you might want to take a good look at the description of the electric system of your particular aircraft. This should be in the POH. As others have said, there are several locations where an ammeter can be inserted in the electric system and the exact location differs from airplane to airplane. Furthermore, there are other indicators spread out through the system that you have to keep an eye on, and know what they're supposed to be indicating.

After all, you have three major components that can have current flowing between them: the alternator, the battery and the rest of the electric system (and this all could be split over several buses, to be precise). If you only have one ammeter, it could theoretically be placed in at least three locations. So with a single ammeter you will never be able to get a full picture of what your electric system is doing. You will always need to combine this with, for instance, the low voltage light or the alternator failure indicator to understand what's going on.

In the POH of the PA-28-161 Cadet (issued Sep 9, 1988), I found the following:


As installed, the ammeter does not show battery discharge; rather, it indicates the electrical load on the alternator in amperes. With all the electrical equipment off and the battery master and alternator switches on, the ammeter will indicate the charging rate of the battery. As each electrical unit is switched on, the ammeter will indicate the total ampere draw of all units including the battery.

So if everything is correct then in the cruise the 40 amps consumed by the electric system is totally supplied by the alternator since the battery is, and remains, full. (Although the PA28 POH suggests that keeping the battery topped off will consume about 2A anyway.)

But more likely what you will actually see is that right after starting the current is higher than 40 amps. The excess is used to top off the battery, which was drained during pre-engine-start actions and the engine start itself. After a few minutes the battery is topped off and the current drops to 40.

[Pilot DAR]

Lots of great information here, it's covered the subject pretty well.

One of the simplest descriptions of the two different ways an ammeter can be wired, can be found part way into the Electronics International instructions , which are here:

http://www.buy-ei.com/Information/OI%20&%20II%20VA-1A.pdf

[Pull what]

The battery powers eveything and the alternator charges the battery.The battery dosn't power anything when the alternator and rectifier are online.

The battery is purely for emergency power and starting the engine.

Its the TYPE of ammeter that is important not its position

There are two types, the charge/discharge type and the load type.

The charge/discharge type will remain centered so long as the system output can meet the system demand. Beyond this point the needle will indicate a discharge and use of battery power.

The load type of ammeter will begin near zero and rise as more electrical load is put online. The voltage warning light will indicate if the load requirement is beyond the alternator's ability to produce. The load meter reflects the actual electrical load as it is turned on. A load ammeter at zero or discharge is saying that you are using battery power.

[Islander2]

Pull what said:Its the TYPE of ammeter that is important not its positionNonsense! True one would normally use the charge/discharge type of ammeter when locating it between the master solenoid and the busbar (typically Cessna), and the loadmeter type of ammeter if locating it between the alternator and the busbar (typically Piper), but it is the POSITION of the ammeter that determines the nature of the information that is being provided to the pilot, NOT the TYPE of ammeter.

To understand the purpose of the ammeter instrument and interpret it accordingly, the pilot HAS to understand where in the electrical system it is located. Agreed, the type of instrument is a clue, but I've flown older aeroplanes where the instrument has been replaced with the wrong type and it is by no means the definitive answer!

[BackPacker]

In fact, what you will see if you look at the innards of both types of ammeters, is that they are constructed similarly. A coil through which runs a current, which creates a magnetic field, and a little magnet connected to the spring-loaded needle reacting to that field. (Or the magnet is fixed and the coil connected to the spring-loaded needle - the effect is the same.) Plus a shunt to handle the majority of the amps. The difference between both types is simply the position of the needle when there is zero current.

Ammeter - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ammeter)

In a pinch you could probably swap one type for the other and still get more-or-less meaningful results. If you know how they work that is, and where they're place in the electric system. And if the needle doesn't deflect beyond the scale.

And that would definitely not be an approved solution of course.

[Pull what]

OK then put it another way its important to know which type of ammeter you have-so its position isnt important!

In the Charge/Discharge Meter configuration, the ammeter is directly in series with the battery.

In the Load type configuration, the ammeter is in series with the alternator.

The 'innards' of both instruments are the same, it's the scale and the information thats different!

[172driver]

The battery dosn't power anything when the alternator and rectifier are online.

The battery is purely for emergency power and starting the engine.

Not necessarily according to Mr. Cessna. From the POH of the 172RG I mostly fly:

The airplane is equipped with a 28-volt, direct-current electrical system. The system uses a battery, located aft of the rear cabin wall as the source of electrical energy and a belt-driven 60 amp alternator to maintain the battery's state of charge

[Pull what]

Put it another way, the alternator is the main source of power to the aircraft systems after engine start, not the battery.

Basic PPL aircraft general and principles of flight exam question!

[Pilot DAR]

Well, trying to deliniate whether it is the alternator or battery which provides the electrical power to the aircraft systems is kinda' like trying to determine if the propeller pulls into a low pressure area, or pushes against a higher pressure area, to propel the aircraft. The right answer is it's either and both.

Without an alternator/generator, a battery is a wonderful source of electricity for a finite period. Without a battery, an alternator is a troublesome source of electricity for a much longer period of time. They work in harmony, each doing their own rather different job. Ammeters are basically the same as to how they work, be it center zero, or end of scale zero, the means of operation is the same. If you wire an end of scale zero ammeter differently, it will indicate backward off the scale.

So why does the referenced Cessna POH say that the alternator maintains the battery's state of charge? because the aircraft load is constantly discharging it otherwise! If you flick on the landing light, with the nav lights on, and operate electric flaps or an electric hydraulic system, you will momentarily exceed the capacity of the alternator. Do you pop the breaker? No, 'cause the battery takes the load for that time. When you turn final, and pull the power to idle, with the landing light on, you can be assured that the battery is powering everything, and will continue to do so, until you add power, because at idle, the alternator is doing very little. Indeed, this condition is very hard on alternator trives, for reasons a little beyond the scope of this post. My habit is to turn off the alternator side of the master switch when landing at night, to ease the load on the alternator drive. My battery is very useful at that time. Of course, a bit of ammeter monitored ground high idle afterword, brinngs the battery back up to where it should be.

So, the subject can be argued either way, but the bottom line, is the battery should be treated and respected as the source of electricity for the aircraft, with the alternator charging it. If I thought that the battery were no longer providing, or able to provide power in flight, I would most likely turn off everything electrical - certainly all avionics.

For the discussion about the position of the ammeter in the circuit, I again refer posters to the EI document I linked earlier. If you wanted the perfect indication of the function of the electrical system, you would actually use two ammeters, each connected in the circuit in a differnt place. The EI dcoument makes it very clear as to why.

The battery dosn't power anything when the alternator and rectifier are online.

The battery is purely for emergency power and starting the engine.

Disagreed.

The battery, in addition to the foregoing, acts as a giant capacitor in a way, absorbing and returning power to the system as needed. Without it, you have problems.

Pilot DAR

[IO540]

Pilot DAR has it right.

There is a lot of "terminology" splitting hairs here, the sort of thing which the exam writers love :)

An alternator, with its voltage regulator, is a constant voltage power supply.

Unfortunately, while (with a sophisticated voltage regulator) it could work by itself, most current implementations do rely on the presence of a battery for stability and a smooth output voltage. The alternator generates a 3-phase output which even when full-wave rectified still has a lot of ripple on it and would play havoc with a lot of avionics, if the battery was disconnected.

If the alternator, or its rectifier, or its voltage regulator, fails then you are running from the battery until it goes flat.

Unless the voltage regulator fails into a short in which case the thing will run until the battery overcharges :)

The only alternator system I know of which can run with a smooth output without a battery is the GAMI Supplenator but that does't "exist" yet.

[Fuji Abound]

Yep IO has got it in one line:

If the alternator, or its rectifier, or its voltage regulator, fails then you are running from the battery until it goes flat.

Practically that is the reality, and dont forget it. I have had two alternators fail in flight and landed before the battery went flat but I am in no doubt it would have gone flat. I had an alternator fail in my Caterham a little while back and sure enough the battery went flat and the car stopped - of course at least the aircraft engine will keep running unless it is a Thielert. :}

[jxk]

That's why in the UK there is/was a requirement to fit a low volt light to warn of an alternator failure. I know of someone flying an N reg with no such system, finding out that his alternator had failed only because his handheld GPS (connected to cigar lighter) went dead. The flight manual should also contain an estimate as to how long the battery would last with only the essentials running. Again a UK requirement.

[IO540]

It is IMHO completely unacceptable for an aircraft engine to simply stop as a result of an electrical failure elsewhere.

Alternators do pack up, and batteries definitely go flat.

I don't know why Thielert did not fit a second little alternator just for the engine. That's why GAMI developed their "supplenator" - to power the FADEC system. These little alternators have been around for years, and some are STCs for stuff like Bonanzas, IIRC.

[Crash one]

Has anyone mentioned yet that the magnetos provide an independent electrical supply to the spark plugs? The engine will not stop as a result of alternator/rectifier failure or battery flatness.
Electric flaps, undercart, avionics, coffee machine etc, will not work but the engine will continue to run.
Yes, newer designs are doing away with the old fashioned mags & they will fail when electrical systems go pop. Good thing, bad thing?

[Nipper2]

Indeed, that's why serious engines have a permanent magnet alternator (PMA) fitted. A PMA is just about the simplest electrical machine you can design (and thus one of the most reliable). It has but one moving part and a very crude, inefficient but reliable control system. Why Thielert did not fit one is beyond me too.

[Pull what]

Put it another way, the alternator is the main source of power to the aircraft systems after engine start, not the battery.

Just to re-cap!

[BackPacker]

I just checked the DA-40 POH (for the 1.7 Thielert-engined version). Indeed it's only got one (90A) alternator, but it's got no less than three batteries in the VFR edition:

- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)

The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.

The IFR models also have a non-rechargeable dry battery to power the cabin floodlight and (electric) AH for 1 hour, completely independent of the electric system.

Now I'm not an electrical engineer but, although complex for such a small aircraft, the electric system of the DA40 does seem to be well thought out. My main worry would be if a failure elsewhere plus a failure of a critical relay to open/close a circuit somewhere would occur. For instance a short in the main bus, and a failure for the relays to isolate the ECU bus. Or something along those lines.

[IO540]

These (http://www.bandc.biz/standby-alternator-systems.aspx) look interesting.

In fact, they look awfully similar to the GAMI Supplenator (http://www.gami.com/supplenator/supplenator.php) which is not yet certified for anything.

I reckon GAMI are using the same generator assembly as B&C. They claim theirs works without a battery, and while GAMI did not answer a direct question on this in one forum a while ago, it seems obvious that they are modulating the field current sufficiently to generate a smooth output voltage without a battery. With a 3 phase AC generator and full wave rectification, and a decent closed loop regulator, this is completely possible.

Certifying this type of product cannot be hard, so IMHO GAMI are obviously holding back till the market is ready for their FADEC system.

What I don't understand is why aircraft makers don't simply fit 2 alternators, 2 batteries, two buses, and then have diode-fed batches of avionics. That's what the Cessna 400 does, IIRC, and it is very simple. Chance of a total electrical failure is practically zero. The extra weight is maybe 20kg.

[Crash one]

Quote:
Put it another way, the alternator is the main source of power to the aircraft systems after engine start, not the battery.
Just to re-cap!


Yes. EXCEPT the engine ignition system (Cessna/Piper, magneto driven outdated Lycon engines)

[Pull what]

There are a multitude of references to this on the net.

It seems Mr Cessna doesnt know as much about alternators as Mr BMW, but there again Cessna dont build engines!


BMW Alternator

Your BMW needs a source of energy for it to run and that energy is being supplied by the fuel stored on the vehicle’s fuel tank. But aside from fuel, the vehicle also needs an electrical energy source for the essential electric components like the distributor, electronic fuel injector and the engine management computer to work. The battery is often the primary source of this energy. The battery, however, would only function well if it is properly charged. To keep the battery charged, your BMW is equipped with another electrical energy source—the BMW alternator.

An alternator is an electrical machine or generator that converts mechanical energy to electrical energy in the form of alternating current. Alternators have a wide area of applications, although they are more popularly used for vehicle electrical systems. The main function of the alternator in a vehicle is to charge the battery while the engine is running. As a secondary function, the alternator also serves as the primary power source for the various electrical and electronic components installed in the vehicle while it is running.

[Pull what]

What I don't understand is why aircraft makers don't simply fit 2 alternators, 2 batteries, two buses, and then have diode-fed batches of avionics. That's what the Cessna 400 does, IIRC, and it is very simple. Chance of a total electrical failure is practically zero. The extra weight is maybe 20kg.

They do, its called a multi engined aircraft!

[IO540]

Sure, but carrying the extra engine works out awfully expensive, which is why almost nobody has been buying piston twins in the last couple of decades...

The DA42TDI came with a chance to change this, by having a reasonable fuel flow rate...

[Fuji Abound]

I just checked the DA-40 POH (for the 1.7 Thielert-engined version). Indeed it's only got one (90A) alternator, but it's got no less than three batteries in the VFR edition:

- Main battery 12V, 35h Ah
- Alternator excitation battery 12V, 1.3 Ah
- ECU backup battery 12V, 12 Ah (powers ECU-B only)

The ECU backup battery should supply power to the ECU, if all else fails, for at least 30 minutes.

All true and you should get sufficient warning and have sufficient time to get the aircraft on the ground before the engine packs up.

However it is well worth having the backup battery under the back seat checked / replaced regularly. It should power the ECU for 30 minutes but, perhaps surprisingly, this assumes the battery is operating close to its maximium performance. In reality the batteries are often found to be way below their rated performance and 30 min is optomistic.

The sytem is OK but needs to be understood. I am not entirely sure you ever want to be worrying about the engine stopping on you while in IMC on partial panel and worrying whether you will get to your diversion quickly enough. Its fine for pottering around the local cabbage patch, maybe not really up to serious IFR touring.

[Pull what]

An alternator, with its voltage regulator, is a constant voltage power supply.

That's slightly misleading, the voltage regulator regulates voltage to around 1 volt on a 12 volt system and 2 on a 28 volt system. The alternator by itself would provide much higher voltages.

[IO540]

The alternator with its voltage regulator forms a constant voltage power supply.

The alternator itself, with a constant field current, and a constant resistance load, will generate a voltage roughly proportional to RPM. This is no good, which is why the voltage regulator is added, and this varies the field current so as to keep the output voltage relatively constant.

The regulator is usually rather primitive which is why the resulting voltage is not perfectly constant. However, in my TB20 I am getting 28.something and it varies over a fraction of a volt only.

The regulator doesn't drop the alternator output voltage. The output voltage still comes directly from the alternator output. The regulator controls that voltage by varying the field current, which is much lower than the normal load current.

[Pull what]

REGULATOR
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.

Not sure what you are saying-I am saying that the alternator output voltage isnt always constant.

[mikehallam]

Though an observed meter may show a 'steady 28 volts from your alternator/regulator, an oscilloscope may well show many high transient spikes.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.

[IO540]

the alternator output voltage isnt always constant.In a standard lead acid 12/24V motor vehicle battery system (identical to the GA aircraft version) the purpose of the voltage regulator is to maintain a constant 14/28V voltage across the battery.

The battery then takes care of its own charging, by drawing a current which is according to the difference between the charging voltage (14 or 28V) and its internal cell voltage (typically 12/24V).

I am an electronics hardware/software designer and used to design solid state regulators for motorbikes in the 1970s, back in the days when the regulator was essentially a relay with a vibrating contact and thus lasted somewhere south of no time at all.

It's a very simple system.

Doing it so that the system runs without a battery present (i.e. the alternator delivers a relatively smooth 14/28V) is more of a challenge and I don't think anybody bothers. It can be done by controlling the field current with a fast feedback loop, but may possibly require a customised alternator with a low inductance field winding. This is what GAMI claim to have done. It would be a neat thing to have because it would not matter if the battery became disconnected in flight, etc.

Though an observed meter may show a 'steady 28 volts from your alternator/regulator, an oscilloscope may well show many high transient spikes.
It's these the battery [and sometimes a capacitor] flatten.
Without them in the circuit the electronics may suffer.

Exactly. A scope would certainly show the ripple, at a frequency of 6 x alternator revs.

[dont overfil]

Aslan, see what you've started now. Imagine if your question had been controversial or had a spelling mistake!:ok:
DO.

[Pilot DAR]

I was thinking just the same thing!

I was wondering what would have become of this thread, if the question had been "How does the alternator work, while I momentarily push the aircraft over into near zero G, while electricity is being used?" Or, "What would happen if as a pilot, I used the battery to start the aircraft at LFAT?". Heaven help us, we'd be into our 6 th page, with some posts having been deleted by a vigilent moderator!

But, Aslan cunningly avoided using the words "cheap" or "spam can Cessna 150" in their post, and aside from capitalizaton errors, got the words spelled right as far as I noticed - so my duty is complete!

And, thanks everyone, I did learn a few things worth knowing - so there you go! I'm off now to lurk for the next transgression, then pounce, with editorial errors in my own critical rant about language perfection!

Pilot DAR

[172driver]

vigilent

:}


I know, I know.... hat, coat, door.... just couldn't resist!

[Fuji Abound]

capitalizaton


Think I will join you. :}

[IO540]

I think the famous "grocer's apostrophe (http://walkinthewords.********.com/2009/05/grocers-apostrophe.html)" is the worst by far.

[cockney steve]

Just to be pedantic. :- In automotive applications, Alternators come in "battery-sensed" and "machine-sensed" versions.....the net result is the same, the BATTERY should be maintained at a nominal 14.2 volts,for a "12V" lead-acid battery.
The posher alty's also have temperature-compensation built in,
The machine is set to the nominal voltage as a compromise,- to maintain a charge and replace a "surface-charge" (discharged by starting), yet not to overcharge, which would "boil" off the distilled-water part of the sulphuric-acid solution which forms the L/A battery's electrolyte.

Often, people are baffled,when , after starting with a flat /depleted battery, a short run restores it to full vigour,However, after a few hours' non-use, the surface-charge has dissipated and the battery is again depleted.

That is why an external charger is advised ,to deep-charge a battery and maintain it with a "float-charge"....cheap chargers will poke in a full charge,their output being reduced by the battery's rising voltage opposing the charger,-but an unregulated charger will gaily carry on "stoking" a battery and electrolyse the water (hence the warnings about not smoking and switching-off before disconnecting ) the liberated hydrogen and oxygen can give a hell of a bang and casing and acid get everywhere :\

Work it out, folks....a 36 AH battery will, at peak,deliver that amount...BUT USUALLY AT THE 12-HOUR RATE.....that means that THEORETICALLY a mint system would give a continuous 3 amps for 12 hours....we already established the load is typically~40 amps....so you'd be very lucky to get 30 minutes from alty -failure to loss of power due to voltage-drop.

As others stated, the battery acts as a small "buffer-tank" to provide starting energy and even-out the load on the alty.


Petrol V diesel !....the magneto is completely self-contained. engine will run without any external power.

Diesels commonly use a fuel-shutoff solenoid to switch off.....when the volts are inadequate,the valve closes and the donk starves. Modern Diesels also use electronics for injection, as opposed to the old mechanical injector-pumps with the pull-wire to stop.---same problems with lack of volts....the brain dies and kills the donk.

[brownbox]

All of you guys are technically correct here so let's not get into a confusing tizzy about details...

The battery is used primarily to start the engine and also in most cases to provide field excitation for the alternator / (generator if it's older). Some voltage regs are that good (prestlite etc) that the output is good enough alone without the battery in circuit - provided the system will let you do it!!


The alternator in the a/c is really no different from that at Sizewell! It doesn't need a battery!!:ok:

[Lurcherman]

Maybe the more important thing to know is what to do if you have a problem in flight.
I am stuck the wrong side of the pond with a brand new 161 cos alternator kept dropping out. Alt failure light first, 0 reading on amp metre then low voltage light. recycling CB brough it back for a bit then off again. Divert cos I'm not going oceanic with that! Main consideration is landing somewhere where I can get it fixed and idealy VFR. That worked as wasn't going to drop out of sky. Lots of help from ATC.
Detail facinating, but it's the practical side that maybe is more important.

[Pilot DAR]

wrong side of the pond

There's a wrong side?

[BackPacker]

It's just as with driving on British roads. There's a right side and a wrong side, also known as the left side.:}

[Lurcherman]

Apologies, nothing wrong with the hospitality!