Alternator check?

[karmarepair]

Comrades, now that I've deleted the magnetron primary ignition and replaced it with a SDS electronic rig, I'm TOTALLY alternator dependent, and so I'm adding an alternator check to my runup checklist. What voltage should I be seeing at what RPM? I have hydraulic brakes, so I MIGHT be able to hold a full power runup, and my hunch is I don't get ANY charging until at least 2000 RPM. I'm thinking 13.2 volts minimum at 2700 RPM? The SLA battery I'm using likes a fairly high charge voltage, so I wonder if that's enough.

[Bryan Cotton]

That seems pretty low. We typically get to 14.4-14.6 on our stock Aerovee system, if we are up for longer than an hour. We have an EarthX battery and it has been working for XC. At home we plug it in.

Why did you delete the Magnetrons? After a year and 125 hours, I'm a fan of the system. If I was going to delete something it would be the secondary - if I could find something that uses less power.

[daleandee]

Bryan is on point with thinking your readings are too low. I'm totally electrically dependent also (dual ignition Corvair) and I use an Odyssey PC-680 battery. Odyssey says that my battery should be charged at 14.3-15.0 with the sweet spot at 14.7. My readouts (I have two) show 14.8 & 14.7. Checking with a VOM confirms the 14.7 gauge is correct. Of course with the Corvair I use a belt driven 20 amp Dynamo but it's still a PM alternator.

Something I did that you might consider (you may have already done this) is because mine is electrically dependent the two ignitions are wired to the battery side of the master contactor. The reasoning is that if the contactor were to fail, or even the alternator/regulator I'd still have battery power to the ignitions to keep the fire burring while searching for a place to put it down.

Dale
3.0 Corvair/Tailwheel

[BRS]

I'm a bit confused. You checked voltage at 2700 rpm. But was the battery fully charged from the drain of starting? You won't get full voltage untill the battery gets a full charge. What does your regulator normally regulate at? How many amps were you pulling at that 2700 rpm. How much of that was the ingnition and how much of that was going towards charging your battery?

[daleandee]

But was the battery fully charged from the drain of starting? You won't get full voltage untill the battery gets a full charge.

I'm going to politely disagree with this. Your voltage reading is showing what is coming out of the regulator. If you want to know battery voltage you will see it before engine start or when shutting down the alternator in flight. At very low RPM your voltage may show what the battery voltage is if the alternator is not putting out more than the battery voltage. But at 2700 RPM you are seeing the voltage the regulator is sending out to the system.

[Bryan Cotton]

I'm going to politely disagree with this. Your voltage reading is showing what is coming out of the regulator. If you want to know battery voltage you will see it before engine start or when shutting down the alternator in flight. At very low RPM your voltage may show what the battery voltage is if the alternator is not putting out more than the battery voltage. But at 2700 RPM you are seeing the voltage the regulator is sending out to the system.

But if you are pulling lots of amps out of the system, there will be a voltage drop. Our system shows ~13.2 at runup, and we don't see mid 14s until flying for a while. This would correspond to the battery state of charge, and the amount of current being pulled out of the system. On climbout, 3200 RPM we see something like 13.8.

We shut down the secondary ignition shortly after takeoff and leave it off until the pre-landing checklist. The sooner the battery is replenished from the start and other electrical loads it carries on the ground, the better.

[BRS]

I'm going to politely disagree with this. Your voltage reading is showing what is coming out of the regulator. If you want to know battery voltage you will see it before engine start or when shutting down the alternator in flight. At very low RPM your voltage may show what the battery voltage is if the alternator is not putting out more than the battery voltage. But at 2700 RPM you are seeing the voltage the regulator is sending out to the system.

You don't have to be polite to disagree as it's not impolite to disagree. Some people on the other hand are (not you)..... Anyway.

Perhaps I was imprecise. When I said 'battery voltage' what I really was thinking was 'charge voltage'. Good for calling me out. With that said I stand by my assesment but I'd better be more clear about what I'm thinking as the typed text is not always true to my thoughts.

The flow of current (my sonex as an example with a revmaster) is from the alternator ---> regulator ----> battery. Between the regulator and battery is where I measure voltage and current. It's the same voltage right on the battery positive post. With the regulator/alternator switched off, there exists only battery voltage. Something like 13.3v on an EarthX battery. But at the exact same positive battery post, once the alternator is switched on, the voltage begins to climb to the regulator voltage. This is as long as other power consumers (lights, ignition, pumps, etc) don't exceed available alternator current. So when I speak of battery voltage what I really mean is the voltage on the positive battery post, regardless of it's charging state. Thus if you have an alternator overloaded that voltage might be 12.7 volts or it might be 14.5 volts if the alternator is putting out full current and is not over loaded. It's this voltage that a solid state ignition will normally see.

To the origanal question of powering an electronic ignition, I'd be concerned with how long the battery, without the alternator, can sustain the ignition and any other required items to keep the enging going.

Keep in mind, the airframe (pumps, lights, radios, etc) all (normally) run on the alternator current not the battery voltage.

[karmarepair]

I seem to have a knack for lighting the fuse on little explosions <grinning>

Context: I'm the third owner of this Not Yet Flying airplane. The Second Owner (my EAA Chapter) and I have spent hundreds of bucks on new coils, new wires, new plugs, new trigger magnet to try and make the magnetron ignition work to no avail. My current theory is the PMs on the flywheel itself got demagnetized somehow, and pulling the engine and replacing the flywheel MIGHT have fixed this problem, but I had that thought after I'd sent Ross at SDS my money. The SDS ignition looked to me like the quickest safe way to get this plane in the air.

But, I'm now Alternator Dependent.

I need a GROUND check to make sure my Alternator and Rectifier/Regulator are doing their job. At present, the only tool I have is voltage. I have NOT yet run the engine up to 2700 RPM, so I'm speculating what the voltage would be. At 2100 RPM, it's about the same as with the engine off, about 12.7V.

Dale, thanks for the hint to power the ignition straight from the battery.

[daleandee]

Perhaps I was imprecise. When I said 'battery voltage' what I really was thinking was 'charge voltage'.

Your post makes it clear that you know what's happening. The voltage meter will give you what is available on the system. That voltage will change due to loads on the system. My Odyssey tested at 12.83 at last test and generally charges at 14.7. I've never managed to over load the PM alternator in cruise but at idle with the avionics, transponder, landing lights, radio, ignitions, & strobes on I can sometimes get a low voltage warning but a couple hundred RPMs brings it over 13 ...