C152 Battery - How long does it last?

[Private_flyer]

A friend of mine got asked on his skills test how long the battery will last for on a C172 if not charged from alternator....

I looked up the POH for C152 but can't find it anywhere... I know it's a 24V battery but could someone tell me how long I would have in the battery and how I work it out?

Thank you in advance!

[jxc]

Really would depend on what you have running radios lights etc

[Islander2]

Assuming G-reg, you will find it in the aeroplane's AFM/POH under the 'Emergency Procedures' section. It's an addition to the manual required by CAP 747 GR No. 6 (previously Airworthiness Notice 88) in respect of Low Bus Volts Warning.

[Captain Smithy]

Depends what the battery's capacity is, given in Amp-hours (Ah). This is how much current the battery will be able to supply without any recharge for 1 hour, e.g. 30Ah means the battery will supply 30 Amps of current for 1 hour. Or 15A for 2 hours, 10A for 3 etc.

If the alternator was to fail you would switch off all unnecissary electrical equipment in order to preserve the battery's charge so you can still work the radio, flaps or anything else vital.

Usually the Ah value is labelled on the battery though, is it not?

Smithy

[172driver]

As stated, it depends a lot on the load (i.e. what you have switched on) and of course on the state of the battery. For the 172 the rule of thumb - assuming non-essential stuff switched off - is approx 30 mins.

[Captain Smithy]

Forgot to add that if the battery has been sitting in the back at -5C for an hour or so in the cruise at 5000 feet or whatever then due to being cold soaked at low temperature the charge will be somewhat depleted.

Best answer to your friend's examiner would probably be "not very long"; but certainly long enough for a return to terra firma pronto to get it fixed. Either that or tell ATC what has happened, switch everything off, get out the map, compass and stopwatch and find an airfield that accepts no radio.

Smithy

[privateer01]

As captain Smitty said.....

It depends on the amp/hr rating of the battery.

Also the state of charge and the temp.

Its actually a bit more complex then that...however I doubt the examiner wanted to know about the specific gravity of the electrolyte, or the sulfation level of the plates, or power loss in the aircraft electrical system etc.

[Islander2]

Best answer to your friend's examiner would probably be "not very long"I wonder. Probably better on a skill test to demonstrate that you've read and understood the emergency procedures section of the aeroplane's Flight Manual, don't you think?

Oh, and the battery duration has to be at least 30 minutes when only 75% of its rated capacity is available and where the 'action to be followed following illumination of the Low Bus Warning Volts warning light' is promptly completed ... drills that are set out in the same supplement to the AFM.

[Alan_D]

I got asked the same thing on my skills test, but it was phrased along the lines of 'if you're about 1/2 hour from home and the alternator fails, what should you do'.
I bored him a bit about battery capacity and current draw, but the answer he was looking for was 'enough to get home by turning off non-essentials'.

After all the engine will keep going on a C152, you may loose a few instruments, the radio and the flaps eventually, but nothing to stop a safe landing at a suitable airfield.

[Pilot DAR]

I think the answer you are looking for can be found in the text of FAR 23.1353, which reads as follows:

[(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.]

In real life, it might be longer, but don't count on it. If you follow the flight manual procedure for electrical system failure, and your battery is in good condition to begin with, it should not be less.

I hope that helps....

[Final 3 Greens]

'if you're about 1/2 hour from home and the alternator fails, what should you do'.

Strikes me that the only reasonable response to that question is "consider all the pertinent facts and then make an informed decision."

I had a similar situation in the US, halfway across a stretch of water, at dusk.

Should I return to the airport I left, 10 mins ago or continue to destination, 20 mins on?

The thought of potentially losing my landing lights and radio (for the pilot operated runway lights) made it a no brainer. The battery did it's job for those 10 mins.

Another day, under different circumstances, may have required a different decision.

I agree with Pilot DAR's citation of the FAR and his conlcusion, but the dilemma for me was that in a rental aircraft, you don't really know what condition the battery is in.

[Miken100]

What a great thread... made me reflect on what I would do and useful info and learning in it.

[Mike744]

Agreed, what a great thread.
As a student I'd like to know a bit more.

Looking up replacement batteries for the C152 gives 24V anything from 10AH to 14AH but what I'd like to know what the relative loads placed on the battery are if the alternators fails, i.e What it unessential, what to shed first?
Nav lights I'd guess would be quite a high current draw?? Beacon??
R/T would be low consumption on receive, 500mA?? Higher on transmit 2A?? but only (hopefully) used on transmit for short periods.
Don't know what the average C152 transponder draws, any idea?
What current do the flaps draw? would they work on a lower voltage when the battery is dieing?
I suppose at the end of the day it depends on the battery condition - if it's an old battery it's capacity would be a lot lower - how often are the batteries replaced?
Thanks.

Mike

[mad_jock]

Day time:

Turn everything off bar the radio

I would tell which ever ATC service that I was talking to what my intentions were, an eta of where I was going and when I would next be in contact and on which frequency. Then the master switch would be turned off until I got to where I told them I would contact them again. Which would be about 15mins out from the field I intended to land at. And I wouldn't declare an emergency (that said it wouldn't be wrong for another pilot who felt that they wern't happy to declare pan or mayday as they see fit). It would be a flapless landing. It's happened a few times to me in 1300 hours in SEP.

Night time.:

Don't do singles at night far enough away from a runway so I would just get it as quickly as possible on the deck leaving the Nav lights on and radio. It would again be flapless with no landing light. And I would proberly declare a mayday cause I am a big girls blouse and don't want to be stuck holding for commercial traffic with a limited battery and potentially no radio which would be a pain for both me and ATC.

[RatherBeFlying]

In real life a lot will depend on the charge remaining in the battery at the time you discover the alternator is not charging it.

In gliders, we can fly all day on a single battery charge powering the radio, vario and computer, but there are those with a two or three battery setup.

By switching off the master, you also turn off the turn co-ordinator gyro which takes quite a bit of juice.

If you are on a flight plan at night, tower will make sure the lights are on at your expected arrival time. Of course with pilot controlled lighting you are on your own and a handheld would come in handy.

[BackPacker]

I would tell which ever ATC service that I was talking to what my intentions were, an eta of where I was going and when I would next be in contact and on which frequency. Then the master switch would be turned off until I got to where I told them I would contact them again. Which would be about 15mins out from the field I intended to land at.

What I would also try to do, if flying to a controlled field, is secure a clearance all the way into the circuit, preferably via a route or altitude that's otherwise unused, and agree with ATC that if by then my radio has indeed failed, I would hold on downwind waiting for light signals.

My home base is a controlled field so I would like to arrive there, if only because that's where the engineer and my car are going to be. Instead of an uncontrolled field 60 kilometers away that accepts non-radio aircraft.

[A and C]

This touches on a number of issues all of these have had some sort of attention above mut very little hard fact.

The GR 6 (AN 88) requirment is for at least 30 min of battery life after the failure check list had been done, some people calculate this, however I wanted to be a little more practical when I upgraded the avionics on my aircraft and so decided to meet the requirment by demonstrating the the time to failure following the loss of the alternator.

We assumed that following the low bus warning light 5 min was required to do the check list so the battery ran the following equipment for 5 min:- all aircraft indicators, electric turn & slip, pannel lighting, KCS 55 compass system & RMI, 2 x KX 165 nav /com, KN 62 DME, KR87 ADF, KN89B GPS, AT 150 ATC transponder, Collins audio & intercom.

After 5 min we switched off all the equipment except the aircraft indicators, turn & slip, the KCS55 compass system, one KX165, KN89B GPS & AT150 ATC transponder.
We occationaly transmitted on the KX165 to simulate communicating with the ground.

After 2 hours we turned on all the eqipment for 10 min to simulate an IFR approach, the whole test lasted 2 hours 15 min and we had no equipment failure due to lack of electrical power.

This test was conducted with an "average" 12v battery.

The Cessna 152 has a 24V battery and is likely to have at least as much power if it is reasonably charged and has been serviced properly......... and here comes the problem!

The battery fitted to your average club aircraft is unlikely to get much in the way of TLC, there is no requirment for a capacity check as this is not reccomended by the battery makers. Quite quickly after I got both my C152's the batterys failed and following this I replaced both the batterys and got a spare, I rotate the batterys around the fleet at the 50 hour checks and do the servicing on the spare battery when it is not in use, over the last 4 years I have not had a battey fail. This would seem to indicate that the average flying club battery gets a hard life and is not to be trusted.

So the question on your lips now is how as a flying club member do I detect the imminent failure of a battery?

Shortly before both my batterys failed I had a number of reports of the pitot heat being very week, as this is one of the bigger loads that you can apply to the electrical system I think that it is probably the only "battery test" that a flying club member can use on an aircraft that he/she is renting.

I think that it is most important to not over react to an alternator failure, don't panic, do the GR 6 check list with care as with even a poor battery you are likely to have well over 30 min to get the aircraft on the ground with no loss of electrical services for the landing.

If it was a VFR day I would probably switch off all the electrics (turning them back on for a FREDA check at 20 min intervals) and continue to my destination, turning it all back on for landing, at night or IFR I would put the aircraft on the ground ASAP.

The last thing to note is that if the alternator fails don't forget to turn OFF the alternator part of the split master switch, powering the alternator field is of little use if the alternator has failed or is not turning due to a broken drive belt.

[BackPacker]

What you also need to remember is that the battery that provides you with electricity in the air, in case of an alternator failure, is the same battery that also runs the starter motor. Sometimes for at least five minutes or so, in case of a pilot who seriously gets his priming or startup sequence wrong. (I have witnessed a pilot attempting a start for 20 minutes with the fuel selector closed...:=)

The first sign of a weak battery will probably be that you can't start the aircraft anymore in cold conditions. How weak the battery needs to be to fail starting the engine, and how much juice can then still be extracted from it at low loads, probably differs from battery to battery and airframe to airframe. My gut feeling says that if you have no problem starting the engine, even if you mishandled the priming a bit, then the battery should have enough life left in it for a low load, for at least 30 minutes. But I'm no battery expert and I'm interested to hear opinions on this.

[A and C]

If you can't start the engine the alternator failure is hardly likely to be a problem! That is why I think that the pitot heat test is far more valid.

[Pilot DAR]

Good long story A and C.

Many years ago, I accidentally did a similar demonstration, all be it unknowingly, in my C150. After a series of summer day flights, only a few of which required the use of the radio, I found that the radio was not working well at all, and the received transmissions were quite garbled. I finished my landing, only to find that the flaps would not retract after landing.

After testing other electrical things, I realized I had a totally flat battery, and the alternator was not working either. Without shutting down, I flew home to correct the problem.

It turned out that the alternator had not been functioning for some unnown time, but probably many hours of flying over at least a day at least. The "self resetting" alternator field circuit breaker, hidden behind the panel, had opened, and remained that way, with a bit of lint stuck in the contacts. This failure does not illuminate the "low voltage" warning light, and if your current draw is very low (day VFR) is indistinguishable on the ammeter. This odd failure, is not even mentioned in the maintenance manual trouble shooting table, so it was a hard find, once I did realize I'd had a failure.

My plane is now modified to have the same 2 amp field breaker that all of the other Cessnas do.

I've had several alternator failures since, mostly at night. My experience is that the alternators are durable enough, it's the over voltage sensors which seem to quit with no warning, and take the alternator off line. There's a fix for this too, but that is beyond the scope of this thread...

My most useful tool for such night failures: You can buy very small self contained LED/battery bulbs for decorations. They are about .5" diameter, and just over an inch long. They can be jammed into, or taped to, those handy clear plastic suction cups, pointing toward the suction side. When you want to illuminate an instrument, turn on the LED, and stick the assembly to the center of the instrument glass. They provide just enough light to work perfectly, and will provide hours of light. A few of these in your flight bag are a great idea.

[Piper.Classique]

I like the idea of the little lights. Never seen any this side of the pond; where should I be looking for them?
Am I the only one here who has an on/off switch for the turn and slip? I once spent three months waiting for a new generator, charged the battery every other day or so, as all it had to run was the starter motor and radio (manual flaps, no transponder or horizon etc). That was tugging, so quite a lot of engine starts.

[Whopity]

how long the battery will last for on a C172 if not charged from alternator....If used wisely, it will last longer than the fuel in the tanks!