Low Voltage Alarm

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evapilotaz

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Is it common the low voltage alarm light to be lit up when at 1000RPM's?

Once slightly above 1000RPM it turns off.

The amp meter still showed the alternator was still working but I do not recall the amps it was putting out at the time. I wanted to see if the light would turn off during run-up. So the typical run-up is advance throttle to 2000 RPM and check Oil Pressure/Temp, pull Carb heat and look for slight RPM drop, the normal stuff. When I did this the Low voltage light turn off, Next the Mag check at 1700Rpm check and the Low voltage Light remained off. Once I backed down to 1000RPM it was on.

So I got clearance for take off and advance the throttle to full. During ground run I looked at the voltage light it was off, Checked oil pressure/Temp all the normal checks during ground roll. Everything looked good So I rotated at rotation speed. Low voltage light was off during the remainder of the flight.

During the flight I was thinking how would I handle if I lost all electrical. So I was going over emergency procedures in my head.

Anyway is this normal behavior for the low voltage light to do this?
Its a C172N and I have 10 hours in this particular airplane and I do not recall it doing this before.

Thanks of advance.
 
Maybe, but I'd be concerned that it's a bit weak.

It's normal to have low voltage at idle.

Can you see any pulsation in the lights or the ammeter? That's a sign that the alternator has blown a diode in its rectifier, and will fail to charge when another one goes.
 
It would appear you have a bad battery. The light should be off when there is nominal battery voltage in the system either 12 or 24. When the engine is at low RPM, the voltage supplied by the alternator is less than nominal. At that point the meter should be reading the battery. At higher RPMs the alternator provides higher voltage to charge the battery. I would service the battery-remove, fill the cells, charge and replace. Keep an eye on it and get a handheld readio. That's what I needed when I lost electrical in my plane.
 
Immediately after startup, it isn't unusual. Once you goose the accelerator the first time and get the alternator on line, it shouldn't drop off again.
 
evapilotaz said:
Is it common the low voltage alarm light to be lit up when at 1000RPM's?
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In something like a 172? Yes, especially with a large electrical load (e.g., lots of lights on). IIRC, the books for most planes in that class say if the light goes out above 1500 RPM, it's good to go.

That said, if that light in my plane normally went out at 1000 RPM with the "usual" load, and that level to get it to go out started to go up slowly over a number of flights, I'd be getting my mechanic to look into it even before it got to the minimum book level of, say, 1500.
 
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cgrab said:
It would appear you have a bad battery. The light should be off when there is nominal battery voltage in the system either 12 or 24.
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In every light single I know, that light is set to light up about halfway between battery voltage and alternator voltage, i.e., at about 13 volts in a 12/14 system or 26 volts in a 24/28 system. Thus, you see it on when you turn the master, but then it goes out when the engine is started and turning fast enough to get the alternator pumping out enough juice to run the electrical system.

The principal reason for the light is to alert you when the alternator quits and the bus voltage drops to the battery level. That way, you can take appropriate action while you've still got enough battery left to get you down even IFR/night. Setting the warning to less than battery voltage (12 or 24, as appropriate) would delay that warning until you were very short of battery power, thus defeating its primary purpose.
 
Following the Checklist After Startup you check for the low voltage light to go out and it did. I think when I turned on the avionics is when the voltage light came on. I just never notice this before so that is what drew my attention. Normal maybe so.

Thanks
 
That alternator should be able to handle daytime loads with the avionics on. The low-volt light shouldn't be on anytime. It's an alternator, after all, not an old generator. Way more power at low RPM than any of those ancient generators.

A couple of predictions here: (1) It's probably a Kelly-rebuilt alternator, and (2) they (or whoever had it apart) used too much grease in the rear bearing. Grease gets flung out of there and ends up on the slip rings, where it attacks the carbon brushes and forms an electrically resistant scum on the slip rings. That reduces the field current so that even when the regulator is trying to deliver full field it can't, and the low RPM at idle can't generate enough power to satisfy the indicator light. I've seen this exact same thing too many times to count. Someone washing the engine with solvent might also have squirted a lot into the alternator, too, which can also gum things up.



Dan
 
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