Battery vs Generator Question

[chris]

Hey guys,

I'm getting a little confused here, as I have heard some conflicting theories.

I also believed that in normal powered flight, the generator or alternator provides all the electrical power, while the battery simple serves as a reservoir of energy, waiting to be used in case of an emergency.

Some people, however, have told me that the battery is in fact used along with the gen/alt in normal powered flight.

I am currently reading the Turbine Pilot's Manual, and in one of the electrical diagrams, it seems to imply that both provide power in normal flight. (the generator powers certain buses and keeps the battery charged, while the battery powers other buses).

For the cessna I fly, you can disconnect the alternator via the split master switch, but you cannot disconnect the battery side and have only the alternator running. Does anyone know why this is the case? This seems to imply that the battery is serving a very important role, and Cessna does not want you disconnecting it! ie. you can operate with the alternator disconnected in flight, but you CANT disconnect the battery.

Comments?

Thanks in advance,

Chris.

 

[derg]

We have a 'battery bus' on the MD-88, but it's not powered by the battery in flight under normal electrical conditions.

The "battery bus" is actually powered by the "battery charger" which comes off of the uh...shoot, lemme think, I think it's off of the left DC bus. Don't quote me cuz I always do a 'trivial bullcrap memory dump' on the flight home from recurrent annually.

But the battery bus is only powered by the battery when there's no AC power available -- like if you walk up to an 'electrically dead' airplane and turn the battery switch on with no other power (APU, Ground Power, etc) available.

 

[EatSleepFly]

[ QUOTE ]
I am currently reading the Turbine Pilot's Manual, and in one of the electrical diagrams, it seems to imply that both provide power in normal flight. (the generator powers certain buses and keeps the battery charged, while the battery powers other buses).

[/ QUOTE ]

In the ATR, there are two (well, three but two for the sake of this reply) hot battery busses- the Hot Main Bus, and the Hot Emergency Bus. Even when the battery switch is off, these busses are still powered by the battery. Once the DC generators come online, these hot busses are powered by their respective Main DC bus. The Main DC busses can never be powered by a battery. Therefore, in flight the batteries don't normally supply anything. If they are discharging when even one DC generator is operating, its an abnormality, and we'll get an indication on the overhead panel.

[ QUOTE ]
For the cessna I fly, you can disconnect the alternator via the split master switch, but you cannot disconnect the battery side and have only the alternator running. Does anyone know why this is the case?

[/ QUOTE ]

The battery acts as protection for the avionics from a spike in alternator output.

 

[USMCmech]

Your are right on both counts, but you have their relationship confused.

[ QUOTE ]
I believed that in normal powered flight, the generator or alternator provides all the electrical power, while the battery simple serves as a reservoir of energy, waiting to be used in case of an emergency.

[/ QUOTE ]

Completly true and accurate.

Think of it like your towns water pump and water tower. The pump (alt/gen) draws the water up and creats the pressure, while the tower (battery) holds any excess and uses it to maintain things durring times of high demand.


[ QUOTE ]
For the cessna I fly, you can disconnect the alternator via the split master switch, but you cannot disconnect the battery side and have only the alternator running. Does anyone know why this is the case? This seems to imply that the battery is serving a very important role, and Cessna does not want you disconnecting it! ie. you can operate with the alternator disconnected in flight, but you CANT disconnect the battery.

[/ QUOTE ]

The battery must be conected because it is part of the electrical circut. The alternator (either AC or DC) requires an electrical charge to work. It gets this from the battery initialy. The Rotor inside must be excited by DC from the battery. A standard alternator won't charge a completley dead battery.

Some newer "brushless" alternators can get around this by useing a permanent magnet in the Rotor. Also DC generators don't require any electricity to work either.


[ QUOTE ]
I am currently reading the Turbine Pilot's Manual, and in one of the electrical diagrams, it seems to imply that both provide power in normal flight. (the generator powers certain buses and keeps the battery charged, while the battery powers other buses).

[/ QUOTE ]

Larger aircraft use multple AC and DC busses. As well as multiple AC and DC alternators and generators. Some are conected to the battery and some are not. However the battery should not provide power under most circumsatnces.

Think back to the water tower, just because it's conected to the system dosen't mean it is being used.


I hope that helps.

 

[ananoman]

[ QUOTE ]
Some people, however, have told me that the battery is in fact used along with the gen/alt in normal powered flight.

[/ QUOTE ]
Some think this because if you look at the diagrams, the alternator generally feeds the electrical system through the battery. But, since the alternator puts out more voltage than the battery during normal operations, the battery is not really doing anything other than providing 'surge' protection.
[ QUOTE ]
I am currently reading the Turbine Pilot's Manual, and in one of the electrical diagrams, it seems to imply that both provide power in normal flight. (the generator powers certain buses and keeps the battery charged, while the battery powers other buses).

[/ QUOTE ]
Just because something is called the 'battery bus' does not mean that it is always powered by the battery. Like Doug says, if you walk out to a dead airplane and turn on the battery switch, the battery will power the battery bus. However, as soon as you start up an engine, then a generator or alternator will take over. The advantage of having a separate 'battery bus' in more complex aircraft is that if you suffer a total generator failure, everything that is not on the battery bus is automatically shed from the system. It also makes life easier for the battery when starting up, as it does not have to try to power the entire electrical system when the battery master is turned on.
[ QUOTE ]
For the cessna I fly, you can disconnect the alternator via the split master switch, but you cannot disconnect the battery side and have only the alternator running. Does anyone know why this is the case? This seems to imply that the battery is serving a very important role, and Cessna does not want you disconnecting it! ie. you can operate with the alternator disconnected in flight, but you CANT disconnect the battery.

[/ QUOTE ]
Not all light aircraft are like this. Most Pipers can be flown with the battery switch off in certain conditions. Usually this is only done if there is a short in the battery. Operating like this can cause damage to the radios and other electrical components, but it is preferrable to a total electrical failure.