Alternator non-failure and Dynon

So, today I was flying north and about an hour out of Reno I noticed on my Dynon D120 that my battery voltage fell below 12.5, and was discharging 5-6 amps. I monitored it for a few minutes, and nothing changed. My immediate thought was alternator failure, or a blown fuse. Not wanting to end up with a problem away from home, I aborted the trip and turned around and flew back to Reno. Enroute the voltage continued to drop and the amperage continued to show discharging (though I did shed some electrical load by turning off some non-essential stuff).

Back on the ground, I pulled the cowl and put my voltmeter on the battery (Odyssey). Lo and behold, it showed about 13.8 volts! Seems not very discharged to me, considering that I flew at least an hour with the Dynon indicating discharge on the ammeter and low voltage (I was down to 11.2 by the time I landed). More checking with the meter and the voltage at the buss is 13.7, on the wire to the D120, while at the very same time the D120 is giving me 12.2 volts as a reading. I've spoken to Dynon and they are scratching their collective heads and maybe will have some insights to share with me tomorrow. Or maybe I'll be sending the box in for inspection.

A couple other bits of info. I did not try to restart (it was in the hangar before I pulled the cowl and getting cold outside so I didn't want to pull it back outside). On the return portion of the flight, the ammeter was pretty much steady (discharging) but the voltage seemed to fluctuate up and down by several tenths of a volt with no change (that I'm aware of) in the electrical draw.

If anyone else out there has any thoughts as to what might be going on here (or seen a similar problem with the D120), my ears are open.

Thanks,
Greg

Are you sure your meter is working? A fully charged Odyssey should only show about 12.8 volts. I've had a couple of alternator failures in flight (actually the wire connection failed) and saw voltages in the range of 11.5-12.5 after the failure.

Alan,

Pretty sure its good (it's never showed any signs of failure before), but worth a check with another meter tomorrow. If that is indeed the case (bad meter), then a failed wire or other alternator issue appears likely.

Thanks!
greg

13.8 V on the battery is odd. Highest I can get is 13.0 V and that may be reading high a bit (voltmeter error).

Given that the battery voltage is suspect, not much I can say except that low buss voltage and a discharge in amps are consistent with an alternator system issue.

I had this problem last year and the field wire broke...thus I was not energizing the alternator field. Easy to find as soon as I got the cowl off.

My guess is that the Dynon is not the problem. Put a voltmeter on the output of the alternator (careful with wires and stuff) and compare that with the Dynon. If identical, then you may have isolated it to the alternator if the field wire is intact.

Caveat: I am not an alternator expert.

My D120 always reads a little less than the actual battery voltage. Dynon said somewhere that there was a diode inside that caused this.

My D180 is the same. I was concerned because the P680 battery only showed 11.9 volts on the Dynon but read 12.6 when using meter or on the charger.
Phelps

I have a AFS2500 and never show more then 12.1 volts on a 680 battery. Depending on length of time since the last flight I show 11.9 to 12.1 prior to start. I don't think its just a Dynon issue. At first this surprised me since I know more about boat electrical systems then aircraft systems. A boat battery is considered 50 percent discharged at 12.2 volts and that is the normal minimum you ever want to see to insure a full service life from a marine battery.

George

Most of the modern EFIS's have now been designed with multiple power inputs for redundancy. The simplest way to do this is to diode the various input feeds and then join them at the input to the actual EFIS power supply (all internal to the EFIS box). Most of the diodes will give you at least a .5 volt drop, so by the time you are measuring the actual "bus voltage", you are well below the actual value at, say, the battery terminals. If you want to see actual battery voltage, you need to pick it off upstream of the EFIS/EIS instrument, or mentally add whatever drop you get through the instrumentation.

Now Greg, in his first post, stated that he saw the voltage drop, plus a discharge on the ammeter - this indicates that something changed, so it's definitely something to look into. But lower than expected voltage as displayed on an EFIS - in itself - is not necessarily a problem.

Paul

We all get to know our planes after a while, and I don't think the actual number is as important as knowing what is usually seen both before engine start and once the alternator is on and working normally.

In this case, I'm scratching my head a bit. Lots of things don't add up to a logical explanation. Please let us know what you find.

Paul,

Agreed, my voltage dropped significantly. Normal in-flight conditions have the Dynon reading about 14.2V when the alternator is charging the battery, and somewhere around 12.5V without the alternator running but avionics on. I'm headed out to the hangar this afternoon (when it warms up) and will check the battery with a different voltmeter. If the Dynon is correct and a new voltmeter confirms this, then it's clearly an alternator issue and I'll be looking in more detail at the wiring runs for a break.

Cheers,
greg

OK, the answer (or so it appears). My digital voltmeter seems to be not working, thus the funky reading on the battery voltage. A second (analog) meter gave me about 12.5 volts on the battery today (as opposed to 13.5-13.8 on the digital one), which is closer to being in line with the Dynon reading.

Second, and original, part of the problem was the dropping voltage and discharging of the battery in flight. Turns out that the alternator field fuse was blown, and after replacing it I am getting the alternator charging the battery again. So obviously it was not a direct short circuit. The mystery to me is why this blew after 450 hours of service. Apparently Continental engines have this occur not uncommonly, and replacing the fuse/resetting the breaker is SOP for those engines.

Chalk up another "experimental airplane learning experience" for me.

Cheers,
greg

Cool!

Well that was easy...

Frank

Glad to hear it was cheap fix. The fuse blowing seems kind of odd though; have you rechecked the field wire and its connections? What kind of alternator?

Alan,

Yes, I checked the field wire and connections including a resistance check. Everything looks good as far as I can see. I will be flying it this weekend and looking to see if it happens again. Plane Power alternator.

Greg

You might want to keep an eye on the voltage. According to Plane Power's instruction sheet it has built-in crowbar protection. This works by tripping the 5a circuit breaker (or fuse in your case) if over-voltage is detected. Could be you had a momentary over-voltage event?

Input (and offer of assitance at the aeropuerto) deleted by poster...didn't read page two of the posts, with the fix (duh!) Glad you got er dun Greg!

Cheers,
Bob

Agreed Alan. I will be keeping a close eye on things for the next couple flights. Not sure what would have caused a voltage spike, though (hoping the alternator is not failing in some manner....)

Greg

Brushes wear fast

at altitude and in our engines with higher vibration. I installed a new 70 amp Nippondenso alternator a few years ago. After about 500 hours the brushes are worn to the replacement point and it started to vary the voltage more. Now I plan to replace the brushes every other year. It is easy.

Here is one link that discusses replacing the brushes

http://www.sparkys-answers.com/2010/10/changing-brushes-in-denso-alternator.html

Any idea on the best quality brushes?

The PP field can easily draw 4 amps, you may consider a 7.5 amp fuse to eliminate nuisance fuse blowing (or circuit breaker popping).

A side note. Yesterday I took out the battery in my voltmeter and replaced it. I had not thought about that before since all my old voltmeters were analog. Turned out that the 9V internal battery was only putting out about 6.5V! The replacement battery made the voltmeter work properly - no more creeping upward with time, it is now rock steady when measuring battery voltage. So I learned something about electronics yesterday as well!

cheers,
greg

I had an alternator go out on my 7 last weekdend. The first indication was strobe noise in the headset. This I never heard before in 800 hours. Did not register at first what might be wrong. Next morniong tried to start and battery just about dead. It finally started and when I checked battery voltage on RMI monitor, It indicated 11.4 volts....oopsss. Checked field voltage to alternator..11.4 volts. Replacement alternator solved problem. I think a stand alone volt meter or low voltage light is probably more valuable than a ampmeter..IMHO Apparently the Whelen strobe power supply does weird things at low voltage.

Ed Booth, RV 6,7,10,9A