Main Battery Low voltage

As suggested, I used the Rotax MMH procedure to check my generators. Refer to attached summary page for the findings. Generator B is the one I'm most interested in as it supplies the aircraft systems and rechargers the battery. Battery is not currently charging properly.

Gen B Voltage: Two of the phases are slightly higher than Rotax but the third phase is lower. ie. 20% less than the other two. Is this a problem?

Gen B Current: All three phases very similar and close to the Rotax value. (I was unable to measure the current at high rpm at it was too windy standing so close behind the prop).

Gen B Resistance:  All three phases very similar and close to the Rotax value. (I was using a Fluke multimeter which dioes't has a lot of sensitivity, but definitely none of the phases were open circuit.

To me this seems like the Gen B is functioning correctly. Any thoughts?

When I ran the engine with Gen A unplugged and the 'battery backup switch' turned on the Dynon screen displayed Lane A = 13.9V, Lane B = 13.9V and display voltage 12.1V. I don't understand this. Does this mean that Regulator B has two separate outputs?

I'd really appreciate it someone could tell me where the problem is and how to fix it.

Thanks

Rod

Rod

With Gen A disconnected, Gen B will be commanded to provide power to the EMS, and will not provide power to the airframe bus. Closing the Em Battery Switch provides 12v + power from the airframe bus to the EMS via terminal 1 on the X3 connector on the bottom of the fusebox.  It also ties the airframe ground to the engine ground via a second pole on the same switch. So it would seem like the lane voltages would be similar to the airframe bus voltage with Gen A disconnected and the Em Battery Switch closed.  That said, I’ve not tried this and it’s possible the backup battery switch circuit is diode protected at the fusebox so that power can only flow to the EMS, and not back to the airframe. You could try this same test with the start power switch instead of the EM battery switch if your aircraft is equipped that way. There is only one output from Regulator B. 

My next steps would be:

1. Go back to the stator B plug, disconnect it, and check each lead to engine ground. There should be no continuity from any lead to ground.  Careful your fingers don’t touch the probes, or you can get a false reading. If you happen to have access to an electrical resistance tester that operates down at the 50 volt level, that would be a better test, but an ohm meter is what most use.  You are looking for a leak to ground on one or more of the stator leads, and this may or may not show up at the low voltage an ohm meter provides.

2. With the engine running, and having reached 2500 RPM for a few moments, check the output voltage of regulator B at the fusebox (hopefully around 13.9V). Note that voltage and compare it to your airframe bus voltage. If these are not similar, check the voltage at terminal 3 of the X3 connector at the bottom of the fusebox using the ground plate of regulator B as a reference.  If the EMS has already assigned Gen B to the airframe, this voltage should be the same as the output of regulator B.  You see where I’m going with this, you are following the circuit from the regulator output back to the airframe bus to see where the problem might be.

3. Check the resistance between the regulator B ground plate and the airframe.  This should be 0 ohms or close to it.  

4.  When Gen B is providing power to the EMS, the ground path is directly from the regulator to the fusebox.  However, the airframe ground connection to Gen B is via the grounding plate at the regulator. Yes, these are essentially the same, but there are a couple extra connections involved in the latter.  Check the condition of the connections at the regulator B ground plate.  This ground plate can get hot under some conditions.  Often there are only two connections at this ground plate, the one coming from inside the fusebox, and one going to the airframe ground bus.  

Thanks, Jeff, for such a detailed reply. Great suggestions, I'll continue investigating as you suggest.

cheers

Rod

Rod

I forgot to say that if you decide to test the voltage at terminal 3 of the X3 connector as I suggest in step 2 of my previous post, you need to find a place to do it without disconnecting the X3 connector.  Removing this connector disconnects the airframe power bus from the output of regulator B, and you should not do that while the engine is running. Terminal 3 of the X3 connector is the 12v + output from regulator B to the airframe power bus, so it will be energized from the battery anytime the master switch is on, but should see a voltage rise when the engine is running and generator B has been assigned to the airframe by the EMS.  Good luck. 

Hi Jeff,

Over the weekend I did the checks your recommended and confirmed that the generators, regulators and fusebox are all producing exactly what the MMH manual says to expect. What a relief, as I was expecting this to be an expense fix. After all this tested it turned out to be a poor bolted joint at either the shunt or the main 30A fuse. After cleaning the terminates the Bus voltage rose from 12.3V to 13.2V, which is the difference between the main battery charging or not charging. I thought I checked all these connections 12 months ago but obviously not well enough.

Thanks again for your excellent explanation of the system and instructions on what to check.

Cheers

Rod