Fuel pump vs voltage issue

Rupester · #1 11-06-2014, 10:26 AM

Yesterday on departure climb, I had a low voltage warning for the first time.
The voltage showed 12.5V at 2500rpm and normal climb attitude. (alarm is set at 13.0V) I looked around for anything unusual electrically, and noticed the fuel boost pump (older AFP version) was still on. I turned the fuel pump off, and the voltage returned to 13.0; warning light went out. Amp indications were absolutely normal thruout... i.e. when the fuel pump was switched on there was not a large jump in amperage, but there was a 0.5 V drop each time. (My ammeter measures current going into the battery from the alternator.)
I turned the boost pump on and off a couple of times during the flight and each time saw the same voltage drop with no perceptible change in amp draw.
Any ideas what's going on?

Doug · #2 11-06-2014, 01:35 PM

Looks to me like the alternator is not charging at all. Bus voltage should be closer to 14.1-14.2 Volts and the current initially a high charge rate after start and tapering off within a few minutes to close to zero after the battery is charged.
Your voltage drop is consistent with the battery only supplying current and the current is the battery discharge.

n82rb · #3 11-06-2014, 01:51 PM

fully agree with doug. did you by chance turn on any other electric load to see what the voltage and amperage levels were? I would guess it you turned a high load such as pitot heat, or landing light you would see very large drop also.

with a good alternator/voltage regulator you should see 13.8 or so no matter what you turn on. (within reason )

bob burns
RV-4
N82RB

Rupester · #4 11-06-2014, 02:06 PM

Didn't try pitot heat, but should have.

Add'l Info: I flew for 1.6 hrs, after which the Odyssey Optimizer charger showed the battery as fully charged. The voltage held at 13.0 or so the entire flight. If the alternator was putting out zero, I would have expected the buss voltage to sag.

krw5927 · #5 11-06-2014, 02:32 PM

Quote:

Originally Posted by Rupester

Didn't try pitot heat, but should have.

Add'l Info: I flew for 1.6 hrs, after which the Odyssey Optimizer charger showed the battery as fully charged. The voltage held at 13.0 or so the entire flight. If the alternator was putting out zero, I would have expected the buss voltage to sag.

Any weird noise coming through the audio system? Might have a diode going (gone) bad in the alternator rectifier.

Could also be a failing voltage regulator as well.

What is "normal" system voltage in your plane? I would guess something above 13.0?

Rupester · #6 11-06-2014, 08:41 PM

No particular weird noises. Yes ... System normally runs somewhere above 13volts.
Battery is a PC680; any possibility these symptoms could indicate a flaky or failing battery?

Doug · #7 11-07-2014, 04:24 AM

Can you measure the battery voltage across the terminals? It is possible that the displayed voltage may be offset low if you have isolation diodes between the bus and the display sensor.

Also, what charge/discharge currents are you seeing in flight? The battery current in flight should be very close to zero if it is fully charged.

BillL · #8 11-07-2014, 04:59 AM

Quote:

Originally Posted by Doug

Can you measure the battery voltage across the terminals? It is possible that the displayed voltage may be offset low if you have isolation diodes between the bus and the display sensor.

Also, what charge/discharge currents are you seeing in flight? The battery current in flight should be very close to zero if it is fully charged.

Possible causes in likely order:

bad connection in voltage sensing wire back to alternator.
bad ground connection back through alternator.
loose alternator belt
bad regulator
failed diode(s) - noise

Give me a call Terry if you wish to discuss.

Good Luck, this should be straight forward, but take some time to check carefully.

Rupester · #9 11-07-2014, 09:02 AM

Thanks to all .... today is a hangar day .... will be investigating possible sources.

skylor · #10 11-07-2014, 01:59 PM

Quote:

Originally Posted by Doug

Can you measure the battery voltage across the terminals? It is possible that the displayed voltage may be offset low if you have isolation diodes between the bus and the display sensor.

Also, what charge/discharge currents are you seeing in flight? The battery current in flight should be very close to zero if it is fully charged.

If your aircraft has an Aeroelectric Connection type e-bus architecture and uses the B and C Specialties rectifier as an isolation diode, you will see nearly 1 volt voltage drop on the e-bus at full rated load due to the forward bias of the diode. If your engine monitor gets its power input from the e-bus, this can explain why the voltage readings are low.

Furthermore, if your e-bus also supplies your electric boost bump, turning it on increases the e-bus load which causes additional voltage drop, plus alternator voltage can drop slightly under increasing loads as well.

In my RV-8, I have replaced the B and C rectifier with a high quality Shottkey Diode that has a forward bias of .4V under load which is much better.

Skylor
RV-8