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After cranking with external power - will the battery charge?


NicoN

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Coming from another discussion:

Supposed, my battery is completely drained because one of the convenience lights stayed on over 2 days:

My engine then is cranked via external power.

What happens if the external power is removed? Battery charges? Or Battery stops charging? And I have the same problem again after landing.

 

From the Cessna perspective I was told, with a complete drained battery it can happen that the battery relay again opens because of low battery voltage (even with the alternator producing power?).

The battery is detached from power and therefore stays uncharged. Your electric gear still works because the alternator keeps it alive.

Do Mooneys behave that way? I am not sure if I can read the schematic correctly

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Not sure if hypothetical or not, but this was my weekend 700nm from home. Convenience light bumped and left on.

Battery showed 7.6V and wouldn't even flip the master solenoid. We hooked up external power, it fired up and I flew home.

It cranked right up back at home. I'll see how it looks over the next couple weeks, but I suspect I'll have to replace it since it had a full drain.

 

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5 minutes ago, smwash02 said:

Not sure if hypothetical or not, but this was my weekend 700nm from home. Convenience light bumped and left on.

Battery showed 7.6V and wouldn't even flip the master solenoid. We hooked up external power, it fired up and I flew home.

It cranked right up back at home. I'll see how it looks over the next couple weeks, but I suspect I'll have to replace it since it had a full drain.

 

There are lots of anecdotal stories like this.   My hangar neighbor flies to Mexico a lot and had a dead battery (Concorde) in his Comanche when he tried to come home.   He said it was way low on voltage and he doubted it could be recharged, but they managed to get it jump started and flew it home.   He says the battery has been fine since.

So I think it depends on the condition of the battery, your charging system, etc., etc.

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Either way I wouldn’t attempt it. The alternator will destroy itself trying to charge a dead battery. Then you have to replace that too. 

Your alternator is a POS if it can’t handle a max charge. There is a CB that protects it anyway and electronics from a runaway voltage regulator (50-70 amps IIRC). It will get hot but if flying it will be cooled. Boats have had high capacity alternators (up to 120amps) for years, at some point they just can’t achieve enough amperage to get to full charge.
I’d be more worried about the 40-50 year old wiring.


Tom
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It charges, but as Byron said it's VERY hard on your alternator. Nor would you be taking off on a airworthy (discharged) battery. If the alternator fries shortly after takeoff you'll have nothing.

Always charge the battery up first and then start up with an APU preferably to prevent further discharge and only depart when the alternator output is showing a normal amount of charging showing the battery is charging and back up near normal. That might take a few minutes after startup at a fast idle.

I learned that the hard way, at night even, before I knew anybetter. Really stupid!! But you don't have to be.


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Not sure if hypothetical or not, but this was my weekend 700nm from home. Convenience light bumped and left on.
Battery showed 7.6V and wouldn't even flip the master solenoid. We hooked up external power, it fired up and I flew home.
It cranked right up back at home. I'll see how it looks over the next couple weeks, but I suspect I'll have to replace it since it had a full drain.
 

I've had the happen too - without access to an APU in Mexico. Luckily mine wasn't totally drained. It was my wife's leg to fly home. She flipped the master on and we could see the battery was very drained. If we had tried to crank with the starter it would have been totally drained. So instead I got out and hand propped it. We sat there for probably 1/2 hr watching the alternator output come down to near normal. I knew our last hour and a half would be IMC to an IMC approach so I wasn't going to depart without ensuring the battery came back up in charge.

The battery was pretty new and came back up pretty good but I expect it's longevity has been compromised. But The rotor on my main alternator died about 1.5 years later - I am sure that episode took its toll. But down south I didn't have the options we have here in the US.
The alternator only puts out 10-20 amps normally with a healthy electrical system on my 28v system and 20 amps is only after startup.


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5 hours ago, ArtVandelay said:


Your alternator is a POS if it can’t handle a max charge. There is a CB that protects it anyway and electronics from a runaway voltage regulator (50-70 amps IIRC). It will get hot but if flying it will be cooled. Boats have had high capacity alternators (up to 120amps) for years, at some point they just can’t achieve enough amperage to get to full charge.
I’d be more worried about the 40-50 year old wiring.


Tom

Alternators put out 60Aor 70A but they are not rated for continuous duty at that rating.  Ask Hartzell or Plane Power if they are. They are designed to  Replenish the battery  from the starting draw and to supply inflight loads.. a dead battery will overheat and burn the rectifier diode pack. I just changed one last annual on my Plane Power alternator. The battery had a weak cell and it failed the capacity test.  The diode was burned in the rectifier bridge.  

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The charging system has to be able to support a continuous load of 80% of its rated output.   So a 70A alternator has to be able to sustain a continuous 56A load.    If your normal load is that much, then turning some things off will allow the excess to charge the battery.    Most ammeters are wired to show the battery charging current, so if the charging current isn't too crazy, the breakers aren't popping, and you don't have everything electrical turned on at once, you'll probably be fine.    This is another area where having LED stuff instead of older technologies helps, and upgraded avionics may also help, to reduce the continuous load on the charging system.   Monitoring the charging current can tell you whether the battery is coming back or not.

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See if I can summarize a few things....

1) 2 days of having a light on... pretty well drains the battery.

2) There are a few improvements available for adding timers to circuits to turn the light off in the future.

3) Charging the battery before use is the best route...

4) Charging the battery in flight is... sub optimal.  Tough on all parts of the system...

5) Not charging the battery First may have some rules regarding AW, for a reason...

6) Some batteries were built by a less than reputable manufacturer... and its ability to come back to life may be non-existent...

7) So... if you have a one battery system... and the battery doesn’t charge in flight... Alternators require a minimum voltage to be able to produce the magnetic field required to start charging....

8) a Generator is magically better for running with no viable battery...

9) Flying in IMC with an unknown battery condition seems to be highly undesirable...

10) Flying in VMC with a fully manual M20C... batteries are optional...   :)

 

PP thoughts and summary only...

Best regards,

-a-

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thank you for all your stories. Of course, it is a torture to your battery an alternator to crank your engine with just external power. But it may be the only alternative.

But the main Point of my posting is still not clear:

The Cessna guy was of the opinion that after cranking with external power and quickly removing the external power, then it may happen that your battery has still pretty low voltage so that the battery Solenoid cannot be Held in closed Position -> battery is detached from the electrical bus and does not Charge. I think, he also told me that in his Cessna the Battery relay is ONLY energized by the battery - not by the alternator. Of course, with enough voltage  in your battery, the battery relay stays closed and THEN the power from the alternator reaches the battery, charges it and you can Forget my question.

His solution: Don't remove the external power too fast. Keep it connected for a certain time and let the alternator do ist work. Maybe 3-5 minutes will be sufficient?

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Nico,

Thanks for clearing up the key point of your question...

I covered half of it, and not the other half...

 

Let’s say your battery is so bad it isn’t going to charge... (worst case)

1) Starting using a GPU provides full voltage to the plane and alternator...

2) The alternator needs full voltage to generate the magnetic field that is required to get the alternator to start producing electricity...

3) Is there a solenoid related to the alternator? I have a switch for the field wire... it might operate a solenoid...

4) there are many solenoids in the plane... If these decide not to operate, the effect might be temporary...

5) If the alternator temporarily goes down, and there is no ‘full’ voltage to restore its magnetic field... the alternator won’t be able to come back on line...

6) Leaving a charging cart connected to your battery is not a controlled method of charging your battery...

7) An uncontrolled flow of electrons into a dead battery is probably hazardous to the remaining health of the battery... 

8) Leaving a charging cart connected for longer than just starting the engine... leaves the plane in an unknown condition... and the pilot will not see what is going on with the battery...

9) If the charging cart has some restriction to current flow, that would be interesting... a high tech smart cart!

10) highest probability of returning the battery to normal requires overnight charging...  A fast charge to see the battery is working normally takes about an hour...

11) filling a battery in minutes... risks boiling the insides out... this essentially defines going from the pan, jumping into the fire...

12) find out what solenoid you have that can turn off your alternator... this will answer your specific question.

13) hard to say what the voltage is going to be between the alternator and the dead battery... and what the battery switch solenoid is going to do...

14) Cessna guy that says leave the cart attached for a while is good advice for a battery that is low... but really not good advice for a battery that has been drained...

 

PP thoughts only... no shortcuts for proper battery maintenance...

Best regards,

-a-

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I looked at the wiring diagrams for a C-172N and a M20J. If the alternator is generating power, it will hold the battery relay closed even if the battery is dead. This makes sense since you wouldn't want the battery out of the circuit as it acts as a big capacitor to dampen out voltage transients.

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It depends on whether the alternator is 'self exciting.'   If the battery is really dead then the alternator won't work if it is not self exciting.

Late automobile alternators with built in regulators 'self start' and charge with out a battery input.  Not sure old school aircraft alternators will do that, so that is potentially the problem.

 

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An standard 3-wire alternator with external regulator like most airplanes use does indeed need battery power to start up. This would come from the external power cart. After it starts up and is producing voltage, the external battery should no longer be needed since the alternator output will feed back through the regulator to excite the field.

This should really only be an emergency procedure. It will take 3 or so flight hours to charge the battery. If you have an airplane loaded with accessories and avionics and turn everything on, it may put a strain on the alternator. Also, you should know why the battery died before attempting this so you don't take off with an electrical issue or a defective battery that might overheat. I'm not recommending this, but only trying to answer the original question about what will and will not work.

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2 hours ago, skykrawler said:

It depends on whether the alternator is 'self exciting.'   If the battery is really dead then the alternator won't work if it is not self exciting.

Late automobile alternators with built in regulators 'self start' and charge with out a battery input.  Not sure old school aircraft alternators will do that, so that is potentially the problem.

 

Most GA alternators require external power to start.   Old-school generators could (and should) hold some residual magnetism in the field shoes, enough to start the process and start generation, but alternators typically don't do this.    Some of the brushless generators (i.e., actually an alternator) in big things like airliners will self-start, but they're huge and would never fit in a Mooney...well, not under the cowl with an engine in there, anyway.

Older Mooneys with generators should be able to self-excite, and if they can't then reflashing the field coils should restore enough residual magnetism to renew that capability.

 

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  • 2 years later...

If it starts right up and if the charging starts high but comes down steadily then I would say you have a battery that just got drained and is probably still good and a good alternator. If the battery won't hold a charge then you need to replace it and if the alternator can't charge the battery even from being mostly dead then you'll have to replace it sooner or later also.

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I can tell you from experience with a C-210 if you jump start a dead battery, then takeoff, as soon as you try to put the gear up, everything goes dead.

It goes dead because the amperage required to raise the gear is in excess of what the alternator can make after normal running loads are subtracted.

A Mooney as it’s direct electric drive and not hydraulic may work, or might not.

An alternator requires electricity to power the field in order for it to make electricity.

Best if possible to remove and charge the battery, if it’s a flooded battery it’s likely it will boil out acid if the alternator has to charge it in flight. I had that happen, last Gill battery I had.

I now never turn off the strobes, if I get stupid and leave the master on, I’ll probably notice the strobes as I walk away.

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23 hours ago, A64Pilot said:

I now never turn off the strobes, if I get stupid and leave the master on, I’ll probably notice the strobes as I walk away.

There are a few simple rules:

1) Alway open the hangar doors fully.  Mark wheel lines and have permanent chocks.

2) Never leave the towbar on the plane.  You are either using it, or not.

3) Never let go of the gear lever until you have verified its down

4) Always leave a beacon on, always look back at your plane to admire it and check master off.  (does not help with cabin lights though).

Aerodon

 

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I've done it many times and everything was fine. But once it wouldn't charge after starting. I couldn't figure it out. So I asked around. Someone told me if it's dead dead, when you start it with the GPU, on some airplanes (it happened to be a Baron) there isn't any voltage in the battery to "excite" the alternator and it won't kick on. So I gave the batterys a slight charge, started with the GPU, and went on my merry way. My brother leaves the master on more often than me. I liked the strobe idea above.

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So if you have dual alternators do they share the load charging a drained battery thus less demand and wear on each? Or does one take the majority of the load?  I notice at idle my belt driven alternator outputs a charge where as the engine mounted one does not until i get the rpm’s above 1000. 

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