Battery Contactor -- Failures?

[MichaelB]

I coming into our RV10 build late and was not around for the planning of the electrical panel and ordering of the panel several years ago. Having some electrical experience I took on the final wiring for the project and now that it is complete, I have a concern.

I believe that our wiring plan came from a modified ZR12 ? plan. I've attached how everything is currently wired. My concern is that the way things are now if we were to have a battery contactor failure we would loose all electrical power.

I'm not sure of the resilience of the things, but I'm hesitant to place all faith in a mechanical relay.

Does anyone have any input as to whether this looks correct or not? I can provide more information about the panel if necessary, but it is a G900 panel, Dynon backup etc.

What I'm thinking about doing is running adding another battery contactor with a lead to the essential bus, adding a diode between the essential bus and the main power bus, and then running the backup alternator to the essential bus instead of the main bus.

Any input would be appreciated.
Thanks,
Mike

[Bill.Peyton]

I am just completing an RV10. I chose to incorporate two batteries and a dual bus system. I have one alternator and two contractors. I am using a blue sea technologies dual battery coupler to maintain the second battery. The garmin systems have dual power inputs, so it's easy to configure a system. I used the VPX pro system for the main power bus and used a fuse block for the essential buss power. Setting up a dual buss IMHO is a must for an all electric plane. I have heard of two contractor failures over the past few years. IMHO a second alternator is not worth the complexity for the small benefit gained. A second battery provides provides plenty of safety margin. The question you have to ask yourself if you have dual alternators is, if my primary alternator fails, am I going to continue to fly to my destination or land and fix the issue. My answer is land, find and fix the problem. Based on that scenario, the second alternator does not add any safety benefit. BTW you are twice as likely to have an alternator failure with 2 alternators than one.
Hope this helps in your thinking process

[RVG8tor]

I have my endurance bus feed from the main bus, but with a flip of a switch I can feed the endurance bus from a hot battery buss. The hot battery bus is always hot, A relay allows power from a 15A fuse to supply power to the endurance bus. A diode stops the flow from the endurance bus to the main bus if both have power. I think this is part of the Z-12 plan from Aeroelectic.

Other things I have on the hot battery bus are some lighting circuits and a GPS. As to your fear of total electrical failure, you battery relay and you alternator would both have to fail. The alternator would still provide power you just don't have the battery to buffer things but you should still have power. Just a thought

Cheers

[Bill.Peyton]

Actually, if the contactor fails and no voltage is provided to the field of the alternator, the alternator will not produce voltage. The same as would happen if the voltage regulator failed.

[RVG8tor]

Originally Posted by Bill.Peyton

Actually, if the contactor fails and no voltage is provided to the field of the alternator, the alternator will not produce voltage. The same as would happen if the voltage regulator failed.

I was thinking of this as a battery relay failing open inflight after the alternator was running. I am not expert by any means but since the alternator would be supplying power to the main bus and the source of field power comes from the main bus wouldn't the alternator continue to operate. I thought I read this on the Aeroelectic Forum when someone had a similar question a few years ago.

Cheers

[Mark Dickens]

Originally Posted by RVG8tor

I was thinking of this as a battery relay failing open inflight after the alternator was running. I am not expert by any means but since the alternator would be supplying power to the main bus and the source of field power comes from the main bus wouldn't the alternator continue to operate. I thought I read this on the Aeroelectic Forum when someone had a similar question a few years ago.

Cheers

I recently had a similar conversation about this with Bob Nuckolls and he pointed out that alternators need a battery in the loop to prevent them from large voltage excursions. The battery acts as a stabilizer and a shock absorber. I'm no expert either, but the way I understood what he was saying is that an alternator by itself is unstable. This is why Cessna had the master/alternator field switches ganged so that if you turned off the master, you automatically disconnected the alternator field. Whether the alternator would continue to power itself through the field circuit, I assume it would, but based on what I was told, that's not a desirable situation.

[MCA]

Mike,

If you're following Bob Nuckolls' methodology, then the E-Bus as you have it labelled is not really an E-Bus. It is just another switched bus. I recommend going back and studying the purpose of an e-bus and the associated wiring - you'll see that it provides a backup power path in case of a battery contactor failure.

Also, I'm not sure I follow the purpose of the avionics/e-bus switch you have in there now.

I'd suggest some modest changes based on Bob's wiring diagram. Perhaps there are some reasons for the way you have it now but they are not clear from what you've shown. Hope that helps.

[Bill.Peyton]

Once the field looses energy it will not produce any substantial voltage. Once the field voltage is lost, there will be a little residual magnetism that remains in the field windings, and if there was no load on the alternator, you might see 10 volts at the output. It would not power your avionics.

[BobTurner]

Back to the OP's question:

Having a single battery contactor with no alternatives is standard in tens of thousands of Pipers and Cessnas. Their service record is pretty good. Of course these aircraft also have non-electric backups. If you have a few independently powered backups you can have an equivalent amount of safety. Whether or not that's enough is up to you to decide.

[bret]

Back in the 70s when I was learning mechanic stuff, one of my back yard mechanics told me I could test the altanator by pulling the battery lead off, if the car still ran, it was good. Bad now days with ECMs and everuthing else sensitive to voltage spikes and what not, but the output would back feed the to the field and charge the rotor windings.