Avionics Bus Dropping Offline?

[Mich48041]

A bad ground can cause strange things to happen.

[444TX]

Without detailed circuit architecture it is impossible to offer real help. There are just too many variables.

I believe you have a high resistance connection somewhere. Likely where your busses, alternator B lead, master power, master contractor, or etc. are connected. The alternator seems to not be getting full battery voltage when the engine is running. This is why you have the high charging voltage.

The battery is not low. 12.9 volts when not running is OK. A fully charged automotive battery is 12.6 volts and an odyssey PC680 is 12.85 volts.

There is the remote possibility of a ground issue, but would look at other first.

You could power everything up, like the plane is running and hook a battery charger to the alternator B lead at the alternator to simulate the circuit under real conditions. If you can reproduce the problem then check voltage at each major junction. Everything 12 volt should be about the same voltage.

If the meter is connected to the battery negative everything that is ground should have very little to no voltage on it. If there is more than .5 volt (I know a general high number) at any grounds there is a ground problem.

You stated you had the engine running, with the master off, and the alternator was charging. This is not good. It is not a good idea to have the alternator running with the battery off line. I would recommend that the master switch be a dual pole switch that also shuts off the field or control lead to the alternator and to be sure that the alternator does shut down when the field or control circuit is disconnected. If not you may not be able to shut down the electrical system with the engine running.

Also, do you have any high voltage protection. My guess is no. If no, it would be a good idea to install something for high voltage protection.

GM

[ssokol]

I pulled the EMS logs for yesterday's flight. It appears that about four minutes after startup (about a minute before takeoff) the voltage climbed above the 14.1 v set point on the alternator.

At the same time, the log shows the value from the amperage shunt - which measures amps into and out of the battery - went to zero.

From that point the voltage goes up to / bounces around at 14.6 - 14.8 for the next 16 minutes. Amperage reads zero all this time.

At 2:02:50 PM, about 20 minutes after startup, the voltage suddenly jumps to 15 and then bounces between 15.1v and 16.9v. At 2:03:28 I pulled the power to the avionics bus, ending the log.

If I recall correctly - and that's a bit of an if, as I was focused on safely operating the airplane - I popped the alt field breaker to try to switch to battery-only before I shut down the avionics. If so, I don't see any indication that doing so made any difference in the situation: voltage remained high, battery amperage remained zero.

If anyone cares to look at the log, I have it posted here:

https://falkenavionics.com/files/voltage_issue.log

First 13 characters are the timestamp. Voltage is "vbat". Amperage is "ishunt" (multiply by 1000 to get amps).

[ssokol]

Between work and the weather snap here in Texas I haven't had a chance to fight with this for a while. Here's where things stand:

I have replaced all of the potential bad actors in this farce: the automotive relay, the PC680 battery, and the Denso internally regulated alternator. The three replacements are in verified working condition. Replacing them did not solve the problem.

The system appears to work as expected when safely sitting on the ground powered by the battery. The avionics master switch grounds the automative relay. The automotive relay feeds power from the main bus to the avionics bus. All the avionics are happy.

Things do not work as expected when the engine and alternator are on. Snapping on the avionics master switch does nothing. The relay doesn't click. The bus does not power up. But other, odd stuff does happen...

I have a stand-alone volt meter (not the one in the EFIS) that I'm using to measure voltage at various places in the system. With the engine off and the master contactor on (so battery only) the master bus shows 12.7 volts between the master bus bar and the negative terminal on the battery. With the master off, it show 0.0v. So far, so good.

When I start the engine, I can see the voltage step up from battery voltage of 12.7 to alternator voltage of 14.2. The voltage then begins to climb rather quickly. When I ran it this afternoon, it climbed to 15.6 within one minute.

So I shut down and started measuring things with the engine off, and heres's where it gets really weird...

I disconnected the master bus (the bus bar on the panel) from the system - it's not connected to the battery or to the alternator.

• With the master contactor open (off), I measure 0v between the master bus bar and the negative terminal on the battery.
• When I turn the master switch on, I see about 0.7 volts between the master bus bar and the negative terminal of the battery.
• When I unground the magneto, the voltage jumps to over 6v.
• When I disengage the alternator field breaker it jumps to about 8v.

This is with the bus bar completely disconnected from the feed. There should be no path for power to get to the bus. But there's power there. So how?

The master contactor on this airplane has two outputs - the high amperage feed to the starter relay and the (now disconnected) to the bus. That's it.

Weird, eh?

One other oddity that may or may not matter here: for reasons I've never understood, the starter on this airplane won't budge if the alternator field breaker is open. That one is not new - it's been that way since I got it.

[EDIT: Update the above - found where the power to the starter switch comes from the alternator field breaker.]

A friend and neighbor with an RV will be coming over to take a look this weekend. I would appreciate any suggestions as to what we should check.