What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Master Left ON - Question about Charging

Ok, I flew out of town to visit family. When I returned to the plane 2 days later, battery completely dead. So, I decided to prop start the plane. Wasn't even sure if I could because my plane uses slick start system and I have never tried to prop start it. Anyway, it fired right up. However, never made any electricity. I then put a battery charger on it for a while and it started and alternator made electricity. My thought was that since the battery was completely dead, the p-lead never had any power to it to turn the alternator on. The pilot that was there with me said that shouldn't matter. That is the only thing that makes sense to me. Any thoughts??
 
Unless it is a permanent magnet generator, it will need excitation (12v) to create a field. Sounds like it worked as it should.

I belong to the same club btw.
 
"P" lead has nothing to do with the electrical system. It's strictly for the ignition system.
The reason it didn't charge was that with a completely dead battery the alternator had no voltage to "excite" the coils. It doesn't need much. But it does need some voltage.
Once you put a charger on it, you "excited" the field and everything back to normal.

Oops, Looks like Alton types faster than me.
 
Last edited:
After the engine was running, a flashlight battery would have been enough to jump start the alternator. No field voltage, no magnetic field and, no output voltage. Like others said above.
 
The second question is how badly you nerfed your battery by letting it drain completely flat - in my experience the PC680 only tolerates that abuse about twice, and then gives up the ghost.
 
The second question is how badly you nerfed your battery by letting it drain completely flat - in my experience the PC680 only tolerates that abuse about twice, and then gives up the ghost.

You killed the battery - it is just not yet done dying. Replace it.

Also note that flying with a suspect battery translates to little electrical reserve.

Carl
 
Thanks for the input. So my concept was right, need a little current to make alternator work (even though it doesn't have to do with the p-lead). Anyway, I have a pretty cool very light battery that will charge a battery (or provide the small amount of electricity needed) via the aux cord. Note to anyone interested: Your backup spare battery charger does not help when it is sitting on your nightstand 150 miles away!!!
 
Not charging a dead battery before flying is what I call "serious alternator abuse", and foolish. The alternator is there to keep the battery topped off and to carry the aircraft electrical load, asking it to charge a fully depleted battery (which will take everything the poor little alternator can put out) and carry the aircraft loads at the same time, well, you're just asking for trouble.

Treat a dead battery like bad weather, sometimes you just gotta take a delay to do the right/safe thing.
 
I ran my PC680 stone cold dead nearly 3 yrs ago by leaving the master on. (imagine that! :)) I charged it with a good charger that had an AGM setting for charging glass mat type batteries. It recovered fine, and I have flown it without trouble ever since.
 
Not charging a dead battery before flying is what I call "serious alternator abuse", and foolish. The alternator is there to keep the battery topped off and to carry the aircraft electrical load, asking it to charge a fully depleted battery (which will take everything the poor little alternator can put out) and carry the aircraft loads at the same time, well, you're just asking for trouble.

Treat a dead battery like bad weather, sometimes you just gotta take a delay to do the right/safe thing.

Yep, I ran the engine to charge a flat battery and saw the poor little 30 amp alternator putting out 49 amps in an attempt to refill the dead battery. The alternator was dead within the hour....

Charge the battery before starting the engine.
 
The operator I rent from when I need extra seats has a strict replacement policy when this happens. I discovered it once because the Hobbs was off by 23 hours and then no juice during preflight checks.

Not sure if trickle charging it and then flying it would be acceptable. As Walt mentioned taking off and using the alternator to charge it doesn't sound right.

Minimum... full charge and test before flying with it. Better to replace it. That's what? Half a tank of 100LL. You are worth it :)
 
Yep, I ran the engine to charge a flat battery and saw the poor little 30 amp alternator putting out 49 amps in an attempt to refill the dead battery. The alternator was dead within the hour....

Charge the battery before starting the engine.

Not an expert, but I am skeptical of this. I seem to recall from my Aeroelectric Connection education that alternators are incapable of putting out much more amperage than they are rated for (unlike voltage, where a faulty alternator can result in runaway voltage).

Any others with more knowledge than me care to comment?

erich
 
Not an expert, but I am skeptical of this. I seem to recall from my Aeroelectric Connection education that alternators are incapable of putting out much more amperage than they are rated for (unlike voltage, where a faulty alternator can result in runaway voltage).

Any others with more knowledge than me care to comment?

erich

In systems with external regulator, the regulator is part of the above equation, meaning that if the regulator will put out more field voltage than the alternator is "designed" for, the alternator will at least temporarily :eek: put out higher amps than rated. Since there is a mix and match arrangement for these two components in HB craft, I could see this happening easily.

For internally regulated alternators, it could be that the field voltage is limited in some way by design to ensure only rated output but that is simple speculation on my part.
 
Yep, I ran the engine to charge a flat battery and saw the poor little 30 amp alternator putting out 49 amps in an attempt to refill the dead battery. The alternator was dead within the hour....

Charge the battery before starting the engine.

Not an expert, but I am skeptical of this. I seem to recall from my Aeroelectric Connection education that alternators are incapable of putting out much more amperage than they are rated for (unlike voltage, where a faulty alternator can result in runaway voltage).

Any others with more knowledge than me care to comment?

erich

The engine monitor was displaying 49 amps. It came back down to a lower level in a short period of time but I suspect the very heavy initial load is what killed the externally regulated alternator.
 
Odyssee claims their batteries are capable of recovery from deep discharge, But, the charging current must be carefully regulated - not likely from an aircraft alternator.
 
Not an expert, but I am skeptical of this. I seem to recall from my Aeroelectric Connection education that alternators are incapable of putting out much more amperage than they are rated for (unlike voltage, where a faulty alternator can result in runaway voltage).

An alternator is rated for X amps at it's rated voltage. When you battery is dead, it is at a lower voltage and thus the alternator will put out more current. This is just basic ohm's law, where the voltage differential is higher and thus the current is higher. You do eventually run into magnetic saturation, but that will depend on a variety of factors so it's not unexpected that you could do 1.5X rated current into basically the dead short of a discharged battery.

This doesn't mean that the windings, diodes, etc are happy with this additional current though.

Years ago a Diamond DA42 crashed when both engines failed on takeoff. The batteries had been discharged, it was jumpstarted, and then flown. When the pilot retracted the gear, this was enough electrical draw to overpower the alternators, and the engines stopped because they needed electrical power to run (Thielert Diesels). So if you do ever jumpstart or hand prop a plane, make sure the flight you are about to take requires *no* electrical power to do safely or you may be in for a surprise when your overstressed electrical system goes on vacation.

--Ian @ Dynon
 
Last edited:
Odyssee claims their batteries are capable of recovery from deep discharge, But, the charging current must be carefully regulated - not likely from an aircraft alternator.

Having discovered the battery in my wife's Miata was flat, I thought I'd recharge it. We've got an intermittent low-current load that will draw down the battery during winter months when the car isn't being driven. The battery is a PC680. It has responded very well to a 10 amp recharge current from a not-so-smart charger. After two weeks of sitting, the battery had retained its charge and is cranking the engine with ease.

No, I don't plan on testing this again - I AM planning on installing a battery cutoff circuit in the ground wire to ensure we don't discharge the battery again next winter.
 
In our experience with many individual Odyssey batteries in many different airplanes, they're pretty hit-and-miss when it comes to recovering from a master left on. Sometimes they do, sometimes they don't. If it happens, we usually try charging them overnight and see if they come back to life.

That said, if the engine depended on battery power, I would replace the battery immediately. Not worth taking the chance! (I wouldn't fly in an airplane that required battery & alternator power to run even with a new battery, but that's just me...)
 
Dead Battery

Most certified aircraft have small-even D cell packs- to provide a current to excite the alternator. Should our amateur craft have something similar?
 
A bit of thread drift, but......

....since it was brought up, here's Odyssey's take on a deeply discharged battery:

http://www.odysseybattery.com/documents/ProceduretorecoverdeeplydischargedODYSSEY.pdf

I completely killed mine at Oshkosh last year (master on). Took it to the emergency repair station, put it on a charger for a couple hours, got it to hold voltage under a load, got the plane to start, and increased voltage all the way back to Omaha. Luckily it has never missed a beat since. Does worry me a bit about the alternator. I'll see what happens.

YMMV

BTW-I ran around like a mad-man when the battery was on the charger looking for a replacement. Spruce had several PC680's on hand at their booth. In the event you need one during Air Venture. :)
 
Last edited:
Greg,
I haven't seen a battery pack to support the alternator in any certified plane. But now you have me curious. What brand and model have you seen that are rigged this way?
 
Is Greg thinking of the Diamonds that don't have vacuum pumps and use a bunch of AA batteries as a backup for the electric AI if the electrical system fails? Far from common, but it exists.
 
Back
Top