How long can I fly on battery alone?

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jconway2002

Pre-takeoff checklist
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jconway2002
I am trying to get an estimate on how long I could fly on the battery alone in the event of an electrical system failure. The plane is a Cessna 152 with 24 volt battery.

Is there some sort of math I can do to figure this out?
 
You can fly on a battery alone as long as you have fuel for.

How long the battery will last depends on what you leave running. Turn off the master and it will last the length of the flight.
 
jconway2002 said:
I am trying to get an estimate on how long I could fly on the battery alone in the event of an electrical system failure. The plane is a Cessna 152 with 24 volt battery.

Is there some sort of math I can do to figure this out?
Click to expand...

Sure there is! Linky :D
 
The engine doesn't run on battery, it uses magnetos. They avionics and stuff do. The starter, of course uses battery, but once it is started, you will run. No math needed.
 
thanks everyone. I phrased my question wrong however. I meant how long will the battery last without the alternator charging it. I dont know why I wrote what I wrote originally.
 
Not sure of the exact answer, but FAR 23.1353 is interesting in this regard:

(1) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing electrical power to those loads that are essential to continued safe flight and landing for:

(i) At least 30 minutes for airplanes that are certificated with a maximum altitude of 25,000 feet or less; and

(ii) At least 60 minutes for airplanes that are certificated with a maximum altitude over 25,000 feet.

(2) The time period includes the time to recognize the loss of generated power and to take appropriate load shedding action.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/B68CC9734E940D1F86257996006CB148
 
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jconway2002 said:
thanks everyone. I phrased my question wrong however. I meant how long will the battery last without the alternator charging it. I dont know why I wrote what I wrote originally.
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That's sort of like asking, "How long will a gallon of millk last?" More information is needed. But the link provided can get you started. It's a function of the amp hours of the battery and the individual electrical draw of whatever you have turned on inside the airplane. Once you have all those loads you can computer your battery runtime.
 
The one time it happened to me, I didn't realize it, as that particular 172 had an alternator light that was itself suspect ("just reset the breaker", was what I was told to do). On that flight, the battery lasted about an hour, including a landing and shutdown/restart, with minimal radio work... might have lasted longer if I had not used flaps for the second landing. :D
Realized what had happened when I went to start it later and the battery was dead as a doornail. Why I never checked the ammeter after "clearing" the warning light I don't know, but it was the last time I failed to keep an eye on that. :yes: Having to take a 3-hour Greyhound ride will really drive that sort of thing home, LOL.

...time estimated above is based on the assumption that the alternator actually failed the first time the light came on- it may have been after the first landing, which was only about 15 minutes before the second.

Short answer: it won't last as long as you'd like, especially if you use anything that requires electricity.
 
Ok, coming from an un-experienced never really flown future pilot who has been reading his textbook and knows a little about electricity and engines...

As said above, your engine doesn't need the battery/alternator to keep going. So what you're really worried about is your instruments, radio, and lights.

I wouldn't trust my life to any battery if I could avoid it. So I'd point the airplane towards the airport I need to go to, then kill everything I didn't absolutely need until it was time to land- probably even hit the master switch if there was nowhere nearby to land. Then bring it all back up in time to approach the airport.

If you only turned on the instruments when you really needed to... say periodically to get a navigational fix and right before your approach/landing you could probably milk a couple hours of everything.
 
Bottom line is there is no way to no just how much time you have, so all you can do is reduce the electrical load to the minimum needed for the conditions of flight and get yourself on the ground as soon as you can do so safely.
 
You first have to decide what's "essential," and then determine the total draw, before you can begin to calculate how much time you have.

You can estimate the total draw mathematically by adding up the rated draw on all the "essential" equipment, or you can add up all the fuse values for that equipment. But some fuses may protect both "essential" and "non-essential" components, which may complicate things.

You also can directly measure the draw by turning all that stuff on (on the ground) and measuring the actual draw using an inductive ammeter, or by measuring voltage drop and doing the math.

Without knowing the draw, your question is not answerable. Whatever method you choose to determine the draw, add on a safety margin. Also, when calculating battery drain, remember that rated battery capacities are for new, perfectly-functioning, fully-charged batteries, which is not a realistic scenario; so work in a safety factor for that, as well.

-Rich
 
jconway2002 said:
thanks everyone. I phrased my question wrong however. I meant how long will the battery last without the alternator charging it. I dont know why I wrote what I wrote originally.
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At night you need to leave the lights on, so it will run down faster.

During the day, if you are VFR you can turn off everything electrical except the trasponder and anti-collision light and it may outlast your fuel (assuming a strobe not a rotating beacon). And, if you turned those two things off as well, who would ever do anything about it?
 
Most 150s have neglected batteries that have never seen or known desulfation. After one year I surmise that it's rare for one to pass ICAW instructions. Most mechs just don't have the stuff for the 90% discharge test, and really dislike things getting realy really hot inside the shop......
 
bbchien said:
Most 150s have neglected batteries that have never seen or known desulfation. After one year I surmise that it's rare for one to pass ICAW instructions. Most mechs just don't have the stuff for the 90% discharge test, and really dislike things getting realy really hot inside the shop......
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Bruce has a very valid point, many owners wait until their battery's capacity is so depleted it can no longer be counted on for starting the engine and unfortunately for the pilot who loses their alternator in the air, a battery with significantly less than half it's rated capacity will often still start the engine, especially if it's not real cold. In addition, unless there's an obvious indication of alternator failure (i.e. an idiot light), most pilots aren't likely to notice the problem until after the battery becomes so discharged that the voltage falls off so far that radios etc. quit working. Typically at that point you have less than 5 minutes before all electrical power is lost.
 
DavidWhite said:
Depends on how many amp hours the battery is and how many amp hours are getting drawn from it
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This is the answer...however many batteries aren't brand new so the math probably differs from reality.
 
jconway2002 said:
I am trying to get an estimate on how long I could fly on the battery alone in the event of an electrical system failure. The plane is a Cessna 152 with 24 volt battery.

Is there some sort of math I can do to figure this out?
Click to expand...

As was mentioned in another post, a 152 will keep flying until you run out of fuel, regardless of whether power is available from the alternator and/or battery. The reason is that the magnetos generate their own power from the turning of the engine.

The fuel gauges are electrical, so if you fly with the master switch turned off, you will have to turn it on if you want to check them.

How long the battery will retain its charge will depend on what lights and other electrical equipment you need to have turned on, how much current it is drawing, what the ampere-hour capacity of the battery is, whether the battery is fully charged at the time you notice the failure, etc. Unfortunately, it's not practical to get that information in flight.
 
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taters said:
This is the answer...however many batteries aren't brand new so the math probably differs from reality.
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...and how long it has been since the battery has passed a 90% discharge test....(as in usually never)....

How many 150's do you ever see, out at the tiedown, on a desulfator? Really.

They have solar powered ones that are perfectly adequate for the G-25.
I've only seen one in 40 years of aviation (on a 150).
 
bbchien said:
...and how long it has been since the battery has passed a 90% discharge test....(as in usually never)....

How many 150's do you ever see, out at the tiedown, on a desulfator? Really.

They have solar powered ones that are perfectly adequate for the G-25.
I've only seen one in 40 years of aviation (on a 150).
Click to expand...

I've seen none..

the IA replaces my G-25 every other annual, so far thats worked well.
 
At 13.6 amp hours, it will support a load of 1 amp continuously for 13.6 hours. Of course, that was when the battery was brand new, and in perfect condition. Let's say your battery in the plane is about 80% of it's useful life, that would be 10.88 amp/hours. If the T&B uses 1.2 amps, and the nav uses .4, com uses .25(non xmit), txp uses 1.5 amp. Add it up and divide by the amp/hours.
 
Two real world examples... First was a rented C172 flown from Dare County to Ocracoke. Dead on arrival. Second was a 300 mile triangle on a new alternator which died about an hour away from the end. New battery kept me squawking all the way home.
So the answer is... It depends. Have you ever tried to start the plane on a cold day and got nothing?
 
cowman said:
Ok, coming from an un-experienced never really flown future pilot who has been reading his textbook and knows a little about electricity and engines...

As said above, your engine doesn't need the battery/alternator to keep going. So what you're really worried about is your instruments, radio, and lights.

I wouldn't trust my life to any battery if I could avoid it. So I'd point the airplane towards the airport I need to go to, then kill everything I didn't absolutely need until it was time to land- probably even hit the master switch if there was nowhere nearby to land. Then bring it all back up in time to approach the airport.

If you only turned on the instruments when you really needed to... say periodically to get a navigational fix and right before your approach/landing you could probably milk a couple hours of everything.
Click to expand...

It has happened to us twice.

The first time our electrical system went away, we were brand-new pilots on our first outing away from known territory. We considered it an emergency and landed at the nearest airport despite the fact that they had no facilities and was unattended. Fortunately, a pilot landed shortly afterward who found us a mechanic and gave us a ride to and from a local hotel. Pilots are wonderful people.

By the time it happened the second time, we had flown across the country a few times and were much more comfortable with the capabilities of ourselves and the plane. So, the alternator failed while we were somewhere between New York and Oshkosh. I had the sectional open and immediately knew where we were and in a moment, knew where the nearest towered airport was, and how to get there using pilotage. We radioed the tower with, "We are 50 miles east and lost electrical power." He immediately gave us the weather and cleared us to land and cleared us to taxi and told us there was no need for further radio communication. We turned off the avionics master and kept flying until we landed there. Eventually, we hand-propped and flew to an airport where we could get a new alternator.

Notes on electrical failure:
1. You can fly without a battery until the fuel runs dry.

2. Do not hand-prop unless you have been trained to do it. Hand-propping is very dangerous.

3. Always know where you are and how to get to a good airport.

4. If safe, land at an airport that can service your plane.

5. Tower controllers are your friend. Pilots are, too.
 
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AuntPeggy said:
Notes on electrical failure:
1. You can fly without a battery until the fuel runs dry.

2. Do not hand-prop unless you have been trained to do it. Hand-propping is very dangerous.

3. Always know where you are and how to get to a good airport.

4. If safe, land at an airport that can service your plane.

5. Tower controllers are your friend. Pilots are, too.
Click to expand...


Great advice AP!
 
docmirror said:
At 13.6 amp hours, it will support a load of 1 amp continuously for 13.6 hours. Of course, that was when the battery was brand new, and in perfect condition. Let's say your battery in the plane is about 80% of it's useful life, that would be 10.88 amp/hours. If the T&B uses 1.2 amps, and the nav uses .4, com uses .25(non xmit), txp uses 1.5 amp. Add it up and divide by the amp/hours.
Click to expand...

Battery capacity isn't a fixed number, it varies with the discharge rate and temperature. Car batteries are (or used to be) rated at "C/20" (capacity when the discharge is spread over 20 hours) and the end of the discharge cycle occurs when the voltage drops to something like 1.9 volts per cell. For example a 6 cell (12v) battery rated at 100 AH will provide 5 amps for 20 hours at "standard" temp before the output voltage (with a 5 amp load) drops to 11.4v. That same battery would probably only handle a 20 amp load for 35-45 minutes because the higher current draw makes the battery less efficient and the internal resistance of the battery drops the output voltage to the cut off level at a higher state of charge with the higher current (Ohm's law and all).

Aircraft batteries (and most small rechargable batteries except car batteries) are rated at 1C which means that (when new) a 20 AH battery should be able to deliver 20A for 60 minutes at standard temp (usually 25°C).
 
Yeah something like that. Create a CFD model of cell capacity vs amperage draw for all internal equipment and you will....

Have wasted your time. If your alternator craps out shut everything down including your radios then turn them back on when you need them.
 
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