Battery required?

S

SierraPilot123

Well-Known Member
Is the battery required in a C-172 inorder to operate the avionics and other components of the electrical system while in-flight? In other words, if the battery failed, can we simply isolate it from the power grid (battery master off) and operate the electrics in the aircraft soley by the alternator output?

Thanks
 
Consult your POH, but I think the electronics get their power from the voltage regulator, which is taking voltage from the alternator and battery, and charges the battery, as needed. I don't think the voltage regulator will come on if there is no battery present. Also, I don't think you can just turn the BAT part of the master switch off.
 
The POH doesn't address this issue that I am aware of, but yes there is a rocker split switch for BAT and ALT. Looks like it should be able to run off just the alternator (or ACU) if need be, since the system diagram shows no dependence or power being routed through the battery. But I still don't really know for sure......hmmm???
 
In order for the alternators to operate without the battery, you'd need self-excitation. The Duchess's alternators have that feature, but I don't think the 172's do. Mainly because you cannot turn off the battery without turing off the alternator, but you can turn off the alternator without turning off the battery. That type of switch is typical of a system without self-exciting alternaotors, but I've never really studied the 172 electrical system closely enough to be 100% sure. But my best Sierra Academy educated guess would be that in the 172, the battery is required in order for the alternators to generate power.
 
Thanks Flyguy. That sounds convincing. I knew of the "self-excitation" feature in the BE-76 but wasn't sure if that was a feature or not in the Cessnas.
 
Even if you could run it, one of the main functions of a battery is to absorb electrical spikes so you could damage the avionics.
 
averyrm said:
Even if you could run it, one of the main functions of a battery is to absorb electrical spikes so you could damage the avionics.
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Well that brings up an interresting question I haven't thought of. If the battery is installed, but is completely dead, would it still absorb electrical spikes?
 
mojo6911 said:
That is what the voltage regulator does.
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The voltage regulator will protect the system from an alternator that is continually producing too much voltage, but can't really do much for a sudden power spike. The battery will absorb some of that shock though, and protect the electrical system.
 
The vlotage regulator is designed to absorbe spikes in vlotage (hence it's name).

The battery absorbs changes in Amps.

Both have to do with electricty, but are different things.
 
USMCmech said:
The vlotage regulator is designed to absorbe spikes in vlotage (hence it's name).

The battery absorbs changes in Amps.

Both have to do with electricty, but are different things.
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Okay, that makes sense, but then without the battery you still risk damage to the avionics due to an amperage spike, yes? And would a dead battery still be able to absorb a power spike?
 
The Circut breakers (or fuses) protect from excessive amps.



Electricty 101,

Ohms x amps = Vlotage (ohms law)

Amps are the amount of electricity.

Voltage is the force of eletricity

Ohms is resistance.


The amount of force it takes to push X number of amps throught a wire of Y resistance, is the vlotage.

Your alternator has two ratings, voltage, and Amps per hour. That means that it will make 40 amps in and hour at 14 volts.

Think of it like a water pump

To push X number of gallons up a hill Y feet tall, will require Z amount of pressure.

Therfore you need a pump that can push X GPH at Z pressure.
 
That's probably the clearest explanation of basic electrics ever. :)
 
I was taught that one of the purposes of the battery is to act as a shock absorber in case of a power spike. Apparenly it wasn't just a case of my instructor being misinformed either becasue averym seems to believe this also. Am I to understand that this is a myth?
 
Once the engine is running, the battery serves the same role as your towns water tower. It is a resivor of energy, for times when demand exceeds the rate that the alternator can supply.

Turn on every electrical device in your plane, radios transmiting, lights, pitot heat, ect. You are now drawing 55 amps per hour out of your 40 amp per hour altrnator. Where are the extra 15 amps coming from? Answer, your battery. If your battery has 65 amps, it can supply this load for 4 hours.

Once you turn off the landing light, and stop transmiting over the radio, you are only useing 35 amps, and 5 amps per hour are returned to the battery.


Same principle with a water tower, at half time, everybody goes to the bathroom, creating a short period of high demand. The water stored in the tower under pressure, drains and meets this demand. Durring the second half, the pumps refill the tower.


Power spikes useually refer to voltage spikes, which is the voltage regulators job. Think of a pressure wave inside a hose, because somebody parked a truck on it. If there is a weak spot in the hose, then it can burst.

The battery absorbs "spikes" in the draw of Amps, like the water tower.
 
No, if the battery has no extra electricity stored in it, then if you exceed the alternators ccapacity you will suffer a "brownout".

Go back to my example, Everything on, but not trasmiting over the radio your airplane is drawing 40 amps out of a 40 Aph alternator. Keying the microphone draws another 15 amps, but from where? The battery is completely dead and has none to give.

When this happens in your house it's called a "brownout". Electricity is still making it to your plug, but demand is exceeding supply, and therefore the "pressure" (voltage) drops. It happens in CA ocasionally.

This can be bad news for stuff like your GPS reciver.

Go back to the example of the water tower. If your towns water tower was empty and everybody flushed their toits at once, the pressure would suddenly drop, and none of the toilets would flush properly.
 
I don't think damage will occur, but malfunction will. Brownouts aren't harmful, but it may cause the avionics to appear like it's functioning, when it really isn't. ala lights are on, nobody's home
 
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