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.