Alternators vs Battery

[uwochris]

Hey guys,

I was speaking to some people today and I got some conflicting opinions. I'm hoping you guys can clarify!

1. Some people argued that the aircraft's electrical systems are powered directly by the battery, while the alternator is simply there to reenergize the battery and to prevent it from dying.

2. Others argued that the battery is only used during starting and as a back-up power source. The alternator(s) are driven by the engines and they produce and directly supply the electrical systems with their energy.

Comments?

 

[CA1900]

#2 is correct. Note that the voltage of the alternator (14 or 28 volt) is higher than that of the battery (12 or 24 volt). The battery simply doesn't have the voltage to overpower the alternator -- quite the opposite, actually, which is how the battery keeps its charge. Think of voltage like water pressure: the source with higher pressure will win.

Once the alternator's running, it's powering the entire electrical system.

 

[Speedtree]

Number 2 is more correct. If you look at your a/c systems book (for most a/c) you will note that the alternators charge at 28v and you have a 24v battery. This is to prevent them from competing and possibly draining your battery. You are correct that the battery is for starting and once the alternator control kicks online, the alternator is supplying the system.

 

[Speedtree]

damnn he beat me to it. But I'm better looking

 

[mtrv]

And the battery must be online, acting as a reservoir, to absorb spikes, and excess current from the alternator. Hopefully, the alternators regulator won't fail and supply a runaway current condition.

This is why a split alt/battery switch as turns the battery side on, anytime the alt switch is on.

 

[pgcfii2002]

The battery is primarily used for startup, while the alternator provides power to all of the aircraft electrical systems. If the battery were in fact used during flight, it would probably go dead a few hours into the trip. Also, the alternator usually puts out a higher voltage than the battery, so it can charge it.

 

[JohnnyP]

....

Your not saying just the battery by itself is gonna give you a few hours are ya? Thats wishful thinking.

 

[erj-145mech]

Your not saying just the battery by itself is gonna give you a few hours are ya? Thats wishful thinking.

That would depend on the load, wouldn't it. An older Cessna 150's running load is pretty low for VFR daytime. The two gas gauges and maybe a radio is all there is if the beacon isn't on. Older 150's have venturi turn and bank's, the oil temp is a bourdon tube, the clock is wind up, so not much draw from the 25 amp battery. A properly tuned engine would fire up after 4 or 5 blades, so not much draw on the starter.

 

[Gorilla]

A lead-acid or NiCD battery also can provide huge currents for short periods of time. Pure theory now - Lets say you have a twin equipped with 2 alternators and 1 battery. The left engine dies and you attempt a restart. With the left starter motor turned on, that motor will immediately demand huge amounts of current. The motor doesn't care what feeds it, it just needs a LOT of DC current. When there is a large demand on a system, you get a phenomenon called voltage drop. As the electrical system voltage drops, it reaches a point where the battery can begin to drain, and it fills in the slack left by the operating alternator, which can't handle the current demand of the left starter motor. In reality, the output of the operating alternator is probably current-limited; the system won't allow it to provide more than "X" amperes, and/or there may be electrical switching functions which isolate only the battery to provide cranking power, but the theory remains - the battery exists as a reservoir and as a source of very high amperage current when needed.

Think of a battery like a big water tank, which under low demand, fills up with water. During high demand periods, the water tank can provide all the flow needed for a period of time, until it's drained.

 

[JohnnyP]

....

erj-mech,

Thats true, i wasnt thinking of the demands on an older, less-advanced system.

 

[avbug]

The orginal question can't be answered without knowing the specifics of the particular system in question. Because of the wide variety of electrical operating system configurations in use among individual aircraft designs, there isn't a one-size-fits-all answer.

An alternator is an electrical generator. It produces electricity. In almost all aircraft systems, it is capable of powering most of the various electrical busses at any given time, with or without the availability of the battery.

The battery is usually capable of power some or all of the electrical systems on board. On some aircraft, generators/alternators must be in operation in order to power some systems. This is done to prevent an excessive load on the battery. The battery will support a limited load for a limited period of time.

The electrical system is designed to operate within a very narrow voltage band. The highest voltage attached to the system at any given time may be said to be operating the system. If that voltage is higher than the battery voltage, then the battery will be receiving support from that higher voltage, if the battery is connected to the system.

The battery may be separated from the electrical system using the battery master relay, which, when opened, serves to prevent electrical flow between the aircraft battery bus and the battery.

NiCad Batteries and Lead Acid batteries are very different in characteristics, limitations, and capabilities. Both have the same basic property in that they serve to provide a limted source of electrical voltage, and both receive electricity continuously when a higher voltage source such as a generator or alternator is operating, and the battery master is closed to attach the battery switch to the electrical system.

The electrical system is comprised of a number of "busses," or sites that are served by various electrical sources. Each buss serves specific items in the aircraft, such as hydraulic pumps, avionics inverters, etc. Some busses may be served by the battery when everything is turned off, and these are called "hot battery busses" because they get electricity all the time. Typically these are critical items and small things such entry or compartment lights that may be activated without supplying power to the entire aircraft electrical system.

In flight, the aircraft electrical system is being powered by whatever is providing the highest voltage at the time, to the aircraft electrical system. This may be the battery, or it may be the alternator or generator. A generator that experiences a low voltage situation will be taken off line and the aircraft battery may be drained, if it's the only other source of electricity, it's attached to the aircraft bus system, and it's the highest source of voltage. A generator which experiences an over voltage system will generally get taken off line by a regulator, generator control unit, or other system protection, and other sources will continue to provide the electricity.

At any given time, an alternator, generator, or the battery may be providing the operating power for your aircraft electrical systems, depending on the configuration and system status at the time.

 

[GravityHater]

Hey Avbug, how about this burning question on electrical systems... assume it is a C172:

Anyone have a link to basic electrical systems for light a/c?
The thing that is stumping me is the alt-vr-bus wiring......
I see the Alt has two large gauge wires naturally output and ground.
It also has the field wire between the alternator and the voltage regulator.
I suspect the VR senses bus voltage and feeds the info to the alternator which adjusts the output accordingly.
What about the "I" wire? It goes from the alternator to the voltage regulator, and also into the cabin..... is this how the vr senses the bus voltage?

 

[erj-145mech]

The "I" wire doesn't go to or from an alternator, it's from the voltage regulator and is for the Indicator, or or alternator out/inop circuit.

And JohnnyP, the batterys on the ERJ is goood for about 13-14 minutes without any of the 5 generators on line. If they were fully charged in the first place.

 

[Big Dog]

I think it depends highly on the airplane. In a C172M, I once had the negative terminal come loose from the battery. With the alternator running fine before, I lost the entire electrical system. No-nothing, completely black. So I would say in that case it's number 1. In that a/c, you can switch off just the ALT but not just the BAT.
In the Baron, you can shut off the BAT and the electrical system seems to do fine. In that a/c, it seems to be number 2.
Discuss....

 

[pgcfii2002]

Battery on a small GA airplane will provide some power for a few hours, I think it depends on the ampre hour rating (?).

 

[USMCmech]

Battery on a small GA airplane will provide some power for a few hours, I think it depends on the ampre hour rating (?).

It depends on the size of the battery and the load placed on it.

You probably could run your nav lights for several hours on a full battery. However nav lights + strobes + landing lights + Xponder + radios can drain it in less than 30 minutes. The number on the circut breaker will tell you how many amps that device will consume over an hours time.

Also transmiting on any radio consumes about 10 times the power that just listening does.

 

[JohnnyP]

..

erj-mech,

13-15 min's isnt a lot of time when youve got nothing left and need to get down, at least to me it isnt. Not terribly reassuring.

Maybe someone else more knowledgable knows this, i dont have a copy of the transport certification req's in front of me, but youd think they would require the aircraft to have more time than that in an essential pwr. situation? Is there anything of that sort thats req'rd for transport aircraft as far as time on batt's alone? In the dash we are supposed to get 30 min's out of them.

 

[Flyerjosh]

erj-mech,

13-15 min's isnt a lot of time when youve got nothing left and need to get down, at least to me it isnt. Not terribly reassuring.

Maybe someone else more knowledgable knows this, i dont have a copy of the transport certification req's in front of me, but youd think they would require the aircraft to have more time than that in an essential pwr. situation? Is there anything of that sort thats req'rd for transport aircraft as far as time on batt's alone? In the dash we are supposed to get 30 min's out of them.

For part 25 aircraft (transport category certification), the batteries have to power emergency standby instruments and radios for 30 minutes after a total primary electrical failure.

Here's some applicable regs:

§ 25.1351(6)(d)- Operation without normal electrical power.

It must be shown by analysis, tests, or both, that the airplane can be operated safely in VFR conditions, for a period of not less than five minutes, with the normal electrical power (electrical power sources excluding the battery) inoperative, with critical type fuel (from the standpoint of flameout and restart capability), and with the airplane initially at the maximum certificated altitude.

§ 29.1303 Flight and navigation instruments.

The following are required flight and navigational instruments:

(3) Continues reliable operation for a minimum of 30 minutes after total failure of the electrical generating system;


§ 23.1353 Storage battery design and installation. (applies to NORMAL, UTILITY, ACROBATIC, and COMMUTER category airplanes)

(h) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing. The 30 minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load shedding action.

 

[avbug]

Shoulda, woulda, coulda...doesn't mean the battery WILL last. Never count on it, because as often as not...it won't. That battery may be long in service and due for a deep cycle...at the next inspection...tomorrow. You might not have noticed the failure right away. It may be partially gone. You may have a cold soaked battery that's not going to give your full figure...Remember that battery requirements are certification requirements...not necessarily what the aircraft will do in the real world. Just what it's been demonstrated to do, for type certification.

The duration of the battery is very much dependant on multiple factors, not the least of which is the load under which it's operating. A ten ampre-hour battery, for example, will provide one amp for ten hours, or ten amps for one hour. In theory.

Same for backup power supplies.

If you're seeing an electrical problem, you may be seeing something you don't yet know about. Loss of electrical power or fluctuation might be a short that's starting a fire...it might be a control failure that will cost you all your electrical, including battery power. It may be a number of things...don't count on being able to fall back to battery. Often with electrical things, you're seeing the tip of the ice berg, and the problem that manifests isn't the real problem. This can be especially true of a shorted item on a bus affecting other items.

Most of the time a problem can be traced to a grounding issue. That sounds simple enough, but it can be a bear to troubleshoot...something you don't have the luxury of doing (and shouldn't do) in flight, even with test equipment, wiring diagrams, and maintenance publications in a well lighted shop. Additionally, a ground can present as any number of problems...and a short is a ground. You may have bigger fish to fry than loss of a generator, a popped circuit breaker, or the duration of your battery power.