Runaway Battery Voltage

Anyone ever seen this? I was flying along today and all of a sudden my bus voltage shot up to 22 volts on my 12 volt system. I immediately killed the alternator, but no change in voltage. I loaded up the system (turned everything on that there was to turn on. That gave me about a half-volt drop, but still really high. All my avionics are good to 24 volts, so no issue there.


I was way out in the middle of South Dakota so I continued home. Over about 45 minutes the voltage slowly dropped to about 16.5 volts.

Was I looking at thermal runaway? Or maybe just a bad indicator? I have the PC925 and I didn't think it was possible for it to produce that high a voltage.

-Matt
402BD

MartinPred said:

I was flying along today and all of a sudden my bus voltage shot up to 22 volts on my 12 volt system. I immediately killed the alternator, but no change in voltage.


-Matt
402BD

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The fact that there was no change in voltage makes me suspect a false reading. Or that the switch/breaker for the alt is inop.

Are you sure the alternator was not still putting out -------- could be an internal gremlin that both drives the voltage up and prevents the field from being shut off.

Internal or external regulated alternator??

Thats exactly why i put another master relay on the battery lead leaving the alternator,just in case.That way i know for sure the alternator and battery are isolated.

ND 55

It's the ND 55. So I suppose I could have a bad internal regulator combined with a bad lead. I'll check it out.

Thanks,

-Matt
402BD

flyer174 said:

Thats exactly why i put another master relay on the battery lead leaving the alternator,just in case.That way i know for sure the alternator and battery are isolated.

Click to expand...

A pullable circuit breaker in the B-lead circuit back to the battery/main bus will accomplish the same function. My plane has a 40A breaker (35A alternator) to kill the alternator's output if needed. These breakers are available up to 50A for under $25.

Or just spend $35 on a B&C Over-Voltage trip circuit....

http://www.bandc.biz/pdfs/OVM_wiringdiagram.pdf

It will either trip the Field Breaker (ext regulator) or disconnect an added Alternator Contactor (int. regulator)

Either way, the excess voltage will not be present for long, and certainly not long enough for you to see it on a voltmeter.

It will react much faster than you can pull a breaker...

If your alternator was really off-line, there is no way that I am aware of that a 12v battery can produce 22v.

Strongly suspect a false reading.

What kind of voltmeter do you have?

22volts

Believe your readings, I was in an rv that was reading 16 v then went ballistic to 30 volts , thinking this all can't be true from a car alternator then smoke ,not a good feeling strapped in a cockpit. Oil from the governor pipe leaking a week earlier on to the alternatorhad caused this to happen and cutting the field didn't stop the over voltage .

Thats why I still prefer a good externally regulated alternator that you can positively shut of the field current to if necessary (the aircraft units have built in OV protection).

It absolutely defies logic to put 20K+ in most panels and then use the cheapest possible source of electrical power to supply them.

It was the alternator.

I called the Odyssey tech support line and spoke to one of their engineers. Obviously there's no way the battery could produce 22 volts on its own. But if it's exposed to high voltage, the battery will absorb the energy and then dissipate it. This explains why I was still getting a high voltage reading after I killed the alernator.

So I installed a new ND 55 and everything's working as advertised. The cutoff circuit worked as it should have and the battery doesn't appear to have sustained any damage. It's still holding a charge just fine and there's no evidence of out-gassing or thermal runaway. But it's a good thing that I caught the over-voltage right away and was able to kill the alternator. So some sort of automatic over-voltage protection is definitely something to consider. I have the standard 60-amp circuit breaker on the alternator but that was no help in this scenario.

For what it's worth my panel costs no where near $20k. I have mostly steam gauges with an Apollo GX55, a Blue Mountain EFIS Lite, and a iPad--all of which had no problems handling 22 volts. And it was very convenient to be able to go to the local CarQuest for a replacement alternator. But I could see the value of a better source of power for a high-end panel.

-Matt
N402BD