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Dual Master Contactors with Single Battery?

A-Ron

Member
I have a dual alternator, one battery system.

I am using the VPX-PRO which has two internal electrically independent banks, which is great, but there are still a couple of single-point failure components upstream of the VPX... the master switch and the master (battery) contactor. I'm not too concerned about the master switch, but I am concerned about an inflight failure of the master contactor which would shut down the VPX. I have backup circuits for the alternators' field and Vsense wires. My question is, instead of creating an Essential Bus to power a few necessities, which adds complexity, why not just have two master contactors? Is this possible with one battery? Could I just wire two master contactors in parallel that "close" when the master switch is flipped on, and if one "opens" the other is there to keep the circuit energized?

If this is possible can someone guide me to a wiring schematic?
 
I have personally know of two contractor failures, mine being one of them. Mine failed on the ground. I dissected and found the failure mode to be pitted contacts. I have two batteries, two busses and two contractors. The main bus is a VPX, and the secondary is a fuse block. I replaced the main contractor with a Blue Seas L9012 contactor.
If I were to install 2 contactors I would use a single pole double throw (spdt) as the master switch. Parallel the two contactors and actuate each contactor with each pole of the switch. If one fails, just switch to the other one.
Simply paralleling the contactors would not allow you to recognize that one failed.
 
Failure of the master contactor is unlikely. If it does fail, that failure will likely occur during startup, not during cruise. Even if the battery contactor does fail during cruise, the electrical system might continue to be powered by an alternator, depending on how the aircraft is wired. The voltage may be unstable without a battery connected.
Yes, it is possible to connect two battery contactors in parallel. Use a double pole master switch, one pole for each contactor. And yes, if one contactor fails, the other will still provide a current path. Each contactor should be tested independently annually, otherwise you will never know that one has failed.
Having two contactors is just as complex as having one contactor plus E-Bus. Two contactors are heavier than one. And two contactors are heavier than one contactor plus one E-Bus relay.
I am not necessarily recommending two contactors, just saying that it is feasible.
 
Just another data point...

40+ years of flying light aircraft. Three master contactor failures, all of which resulted in a total electrical failure. Two in IFR conditions. All were in certified aircraft.

This failure mode is rare but it does happen.
 
Aaron,

Adding a second master switch and solenoid in parallel will address this one issue, but sorry to say the single master switch and solenoid are not, in my opinion, your main single point failure risks.

If you plan on a backup mode that has just one or the other alternators feeding your panel (no battery connected) I recommend you fully test this mode. Be well instrumented so you mitigate frying expensive avionics. A neighbor just fried a radio by inadvertently quick cycling the alternator switch with the engine and panel up. I only turn off the primary alternator to do a periodic test to make sure the standby alternator picks up. On my designs it is impossible to have an alternator feed any avionics without one or both batteries on line.

Carl
 
I have a second identical contactor right next to my primary which serves my ground power plug (which has already saved my bacon once). Obviously not useful in the air, but on the ground (where most failures would occur). It can be swapped over in just a couple of minutes. The spare has already been cannibalized out on a road trip once to get a buddy home. At the 10 year point I?ll probably swap positions of the two to balance the wear and tear.
 
Aaron,

Adding a second master switch and solenoid in parallel will address this one issue, but sorry to say the single master switch and solenoid are not, in my opinion, your main single point failure risks.

If you plan on a backup mode that has just one or the other alternators feeding your panel (no battery connected) I recommend you fully test this mode. Be well instrumented so you mitigate frying expensive avionics. A neighbor just fried a radio by inadvertently quick cycling the alternator switch with the engine and panel up. I only turn off the primary alternator to do a periodic test to make sure the standby alternator picks up. On my designs it is impossible to have an alternator feed any avionics without one or both batteries on line.

Carl

Thanks for the input Carl. I am using a VPX which supplies power to both voltage regulators to both B&C alternators. So, if my master contactor fails, I lose the VPX and both alternators so the above scenario you describe isn't possible in my setup unless I'm not understanding something.

Scenario:

Master contactor fails in flight causing both alternators to go down (actually using only one at a time), but both are now unavailable. Battery now offline too.

- Engine still running as I use magnetos
- Both EFIS (Dynon HDX) automatically switch to their BU batteries.
- EFIS(on BU batteries), powers GPS antennas, both ADAHRS, ARINC, AP Panel, and KNOB Panel.
- I lose GPS(including NAV Radio), ADSB, Trim motors, Com Radios, Audio Panel, Transponder, AP Servos, Pitot Heat.

- I now switch the alternator offline
- Switch to Master Two (second contactor) and "poof", main battery is back online
- Switch the alternator back online
- Continue flight to destination with FULL capability

I guess my thinking is, for the weight of an extra master contactor and a few inches of cable(I could use some aft CG in the 10), and with the VPX having two independent buses (one will get me on the ground if the other fails), then why not?

Unless I'm missing something, this seems like a pretty darn good plan.


Thank you so much for everyone's input, much appreciated!
 
SNIP....


I guess my thinking is, for the weight of an extra master contactor and a few inches of cable(I could use some aft CG in the 10), and with the VPX having two independent buses (one will get me on the ground if the other fails), then why not?

Unless I'm missing something, this seems like a pretty darn good plan.


Thank you so much for everyone's input, much appreciated!

As most RV-10s need a battery mounted aft, when you add the second master contactor just modify for two PC-625 batteries instead of the whatever big one you are using. Run both batteries in parallel. Now you have a more robust setup and it solves your ?I lost the battery? problem? with no pilot action. The two PC-625s provide for a very good W&B (assuming you are using a standard engine and Hatzell BA prop).

I replace one PC-625 every three years, so no battery is more than six years old. This gives me some assurance of battery reserve (a design element). The pulled batteries go on to a second life in tractors and such. I never use any sort of ticket charger or ?battery minder?. For extended ground panel operation I use a real regulated power supply connected to each avionics buss.

But per my email, if you add the second battery the door is open for a far more fault tolerant power distribution than this if you want it.

Keep in mind there are other failure modes besides a master solenoid that can make a battery go away. I recall reading about a twin (two engines, two alternators) loosing all power when a high resistance contact melted the common bus tie. Single point failures go beyond specific components failing. For you, the second alternator protects against only one failure mode, the loss of the primary alternator.

Carl
 
As most RV-10s need a battery mounted aft, when you add the second master contactor just modify for two PC-625 batteries instead of the whatever big one you are using. Run both batteries in parallel. Now you have a more robust setup and it solves your ?I lost the battery? problem? with no pilot action. The two PC-625s provide for a very good W&B (assuming you are using a standard engine and Hatzell BA prop).

I replace one PC-625 every three years, so no battery is more than six years old. This gives me some assurance of battery reserve (a design element). The pulled batteries go on to a second life in tractors and such. I never use any sort of ticket charger or ?battery minder?. For extended ground panel operation I use a real regulated power supply connected to each avionics buss.

But per my email, if you add the second battery the door is open for a far more fault tolerant power distribution than this if you want it.

Keep in mind there are other failure modes besides a master solenoid that can make a battery go away. I recall reading about a twin (two engines, two alternators) loosing all power when a high resistance contact melted the common bus tie. Single point failures go beyond specific components failing. For you, the second alternator protects against only one failure mode, the loss of the primary alternator.

Carl

Thanks again Carl. I like your recommendation of using two batteries. Using this setup, would you use two master switches, both being "on" during normal operations? Or just one switch?

Also, above you mentioned an email. Did you send me one? Cant seem to locate any email from you. Thanks again.
 
carl,you don't think that the solenoid is the major single point failure concern. there is a post with 3 of these failures on one pilot. what do you know of that is likely to fail more than 3 times in a pilot's lifetime?
 
carl,you don't think that the solenoid is the major single point failure concern. there is a post with 3 of these failures on one pilot. what do you know of that is likely to fail more than 3 times in a pilot's lifetime?

Sorry Bob, we are looking at systems failure risk through very different lenses. I don?t look at a single component like a master solenoid and try to figure out how to add a backup if it fails. I assume every element in a power distribution system can fail, be it a component, a battery, an alternator, a ground or any connection. If you assume something will fail, determine the impact to your ability for continued IFR flight. Now decide if you can accept that impact or not.

In other words, I like to design in ?graceful degradation?. I have not seen any single battery system that meets this design criteria.

For day VFR stuff, do as you like.

Carl
 
Thanks for the input Carl. I am using a VPX which supplies power to both voltage regulators to both B&C alternators. So, if my master contactor fails, I lose the VPX and both alternators so the above scenario you describe isn't possible in my setup unless I'm not understanding something.

Scenario:

Master contactor fails in flight causing both alternators to go down (actually using only one at a time), but both are now unavailable. Battery now offline too.

Loss of a contactor will only disconnect the battery from the system, the alternators should continue to operate normally for the most part (a little extra ripple on the buss) as well as the rest of the electrical system. I have no idea how the VPX will react to this situation but with std breakers it would not be an issue.
 
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Loss of a contactor will only disconnect the battery from the system, the alternators should continue to operate normally for the most part (a little extra ripple on the buss) as well as the rest of the electrical system.

FWIW, I accidentally tested this during my Phase I, during some turbulence. I was switching between my primary and secondary alternators checking output between them, and in a decent jolt of turbulence I hit my master contactor switch rather than my primary alternator field switch. At that point I was operating on my Plane Power 60 amp alternator only, with no battery on the bus. Everything was stable for the 10 or so seconds it took me to figure out what I had done and correct it. I pulled the Skyview data after the flight and the voltage did show some minor fluctuations but held stable remarkably well. I do have a load-heavy electrical system with full glass panel and electric-only fuel pumps, so perhaps the decent load on the alternator helped.

Your results may vary, and I'll repeat that I did not do this intentionally - but the test turned out well.
 
Does anyone use a SPST Master switch?
I am completing a G3X retrofit and considering abandoning the Van?s DPST Master/Alternator switch for a straight Master switch with a separate Alternator Field switch.
 
A good friend of mine had a master contactor fail in flight. Fortunately he was IFR in VMC. At first he was seeing voltage fluctuations so he power cycled the master, and then of course the alternator didn't come back online. So while yes it is possible for a contactor to fail open and the alternator would keep doing its thing, its likely it would go offline in the process of diagnosing the problem.
 
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A good friend of mine had a master contactor fail in flight. Fortunately he was IFR in VMC. At first he was seeing voltage fluctuations so he power cycled the master, and then of course the alternator didn't come back online.

Curious what the thinking was here...what was cycling the Master supposed to fix (what did he think was going wrong)? Or was he just trying something to try anything in the hopes that it would make the "problem" go away?
 
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