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Feedback on dual batt dual alt electrical design

echozulu

Well Known Member
Designing this for an IFR RV-10 with SDS EFI.

I started off with Nuckoll's Z-101 design combined with Leok's diagram I found on VAF. I had initially intended to go with dual alt and 1 battery and adapted the Z-101 design to this:
1altdualbatt_liiceg.jpg


But it looked like however unlikely it may be, a battery failure would result in the alternator keeping everything online, and while it should be more than capable of the task, if the alternator had a hiccup or something like that there is no way to bring the backup or primary alternator back online.

So I moved on to adapt the design I had into a dual battery dual alt design like this:
dualaltdualbatt_mf8gzz.jpg


This should allow the engine electronics to remain powered no matter battery or alternator failure, and also provides a way to energize the backup alt in the scenario of a VP-X failure. I moved everything off of the E-bus in the second design as I intend to use an ETX900 as the primary and an ETX680 as the secondary, so either should have enough capacity to keep the main bus fed for 20-30 minutes and I can leisurely shed load through the VP-X if I need to.

One thing that I need to add to these diagrams is diodes on the two backup alternator field wires.

Interested in any feedback or suggestions.
 
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The relay coils need diodes across them to protect the controlling switches from arcing and sparking. Banded end of diode connects to positive.
 
How is the Aux battery charged when the Alt Buss is off?

Both battery contactors/relays would be closed during normal operations which should permit charging to both batteries from any alternator.

E-buss switch would remain off unless there was a need to bypass VP-X or isolate primary battery due to a fault of some kind.
 
Earth-X

Very similar design as your second drawing. I used 2 - ETX-900 batteries to keep them redundant. The 900 is only .8 lbs heavier and gives you a little more electrons in reserve. Plus the 10 seems to need weight aft. Also have a electron dependent engine.
 
Designing this for an IFR RV-10 with SDS EFI.

...dual battery dual alt design

... Interested in any feedback or suggestions.

Re Dual Alt, Dual Batt power schematic of post #1.

By battery failure do you mean the battery goes open circuit? An open battery is a benign failure in flight discovered at the next preflight. I suppose to have a battery go open and an alternator failure on the same flight would be an acceptably low probability.

Alternators:
  • You might ask on the Aeroelectric List about the concept of feeding an alternator field from dual diode-isolated sources.
  • Are the alternators internally or externally reguated? Curious, I guess it doesn't matter.
  • IMO it makes sense to turn on both alternators in flight. Plane Power uses 14.2 and 13.6 V for primary and backup alternators respectively and B&Cs can be set to something similar.
  • Is there something to gain by powering the backup alternator thru the ECB box in addition to thru the engine bus alternate feed switch? The ECB box is an SOP for both alternator fields so you added an alternate feed for the backup alternator field but if the engine quits that's one more switch to think about. It would be great if engine-out switch flipping was reduced to only two, Backup Pump on and Fuel ECU as required.
  • Both alternator B leads are on a single stud making that stud an SPOF.
  • Why not connect the backup alternator B lead to the aux battery?
  • The backup alternator should be periodically stress tested in flight by turning off the main alternator.

If your engine bus will have separate power connections for its primary and alternate feeders that eliminates an SPOF in the form of the connection at the bus. I see your schematics show separate connections but I don't know if you intend to construct it that way. Z101 rev B shows a single connection to the engine bus which makes that stud an SPOF.

IMO it makes sense to turn on both engine bus feeds in flight. The alternate feed should be turned off periodically to stress test the primary aka diode feed.

Fire-in-the-cockpit-both-masters-off scenario:
  • Engine runs on aux battery capacity. Do you plan to periodically capacity-test the aux battery?
  • Backup alternator feeds the fire and, depending on the position of the Pri/Off/BU Alt Switch, its field current reduces the endurance of the aux battery.

Bob Nuckolls believes Z101 is as reliable as Z14. If you want dual batteries you might use Z14 as a template; it needs an engine bus though. BTW there's a rear battery addendum sketch for Z14 in a metal airplane.

A failure analysis of your electrical power system could/should prevent surprises. I did a failure analysis of my Z101-based electrical power system in case it's instructive; link.

P.S. High-current solenoid-operated switches that have double-spreading contacts are conventionally called contactors versus relays.
 
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An overly complex/capable electrical system may be prone to more points of failure, and be more difficult to understand adequately making it less reliable than a simpler system.

I’m not a big fan of dual battery systems for several reasons. IMHO, one larger high quality battery changed out at regular intervals is as good as it gets for a single engine sport plane. A second backup alternator is a good thing for cross country trips, also over-voltage protection and low voltage alarms to give immediate warning of a condition that will lead to running out of electrons.

Bevan
 
By battery failure do you mean the battery goes open circuit? An open battery is a benign failure in flight discovered at the next preflight. I suppose to have a battery go open and an alternator failure on the same flight would be an acceptably low probability.
SNIP

A well maintain battery (or two) should be the most reliable component in a power distribution system. Getting power from the battery(s) is the issue.

Assume any single component (a relay, a master solenoid, a master avionics switch, an alternator, a ground wire, common buss connection, VPX, etc.) will fail. Does your system have the redundancy for continued IFR flight? It not, then either mitigate the risk or fly day VFR.

It is difficult to do this mitigation with a single battery system.

Alternator failure is the most probable risk, but if you have a well maintained battery(s) the outcome severity tends to remain manageable - just land before battery exhaustion.

Carl
 
This is why a second (backup) alternator (see note) is a better choice than a second battery. It’s an unlimited supply of electrons over time. A battery is a heavier, limited reservoir of electrons with an unknown quantity which diminishes over the its life. SNIP

Bevan

You assume the only power distribution failure risk is a loss of alternator. Again, while an alternator failure is most the most probable risk, the outcome is more than manageable. For example, a twin engine plane with two alternator and two batteries ended up with a dark panel, at night. The alternators were just fine. The batteries fed a high resistance common buss bar connection that melted. Another incident that happened to a neighbor’s Mooney was a gear motor that stuck on. It fried taking the electrical system with it. It was a clear night and with ATC talking to the pilot via a cell phone the Mooney was able to do belly landing. A dozen alternators would not have helped.

The Achilles Heel for single battery, one or two alternator systems is a failure of a component to get power from the battery to the panel. As I previously stated, single battery designs are challenged to mitigate this risk.

That said, just adding a second battery and leaving everything else the same provides no gain (other than weight). Here thoughtful design pays dividends.

Test - how many people are still flying with a single Avionics Master Switch? Or for that matter, all the avionics on a single buss?

Carl
 
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Re Dual Alt, Dual Batt power schematic of post #1.

By battery failure do you mean the battery goes open circuit? An open battery is a benign failure in flight discovered at the next preflight. I suppose to have a battery go open and an alternator failure on the same flight would be an acceptably low probability.

I think here I'm protecting against any number of issues that could cause the battery to not output power, including wiring, contactors, installation etc. I realized in my original 1 batt dual alt design that the battery shorting to ground for example might leave me with no ability to start the backup alternator, and an extra battery is cheap. My understanding is while generally an alternator carrying the load by itself is no issue, any momentary increase of current above alternator capacity could lead to the alternator shutting off.

Alternators:
  • You might ask on the Aeroelectric List about the concept of feeding an alternator field from dual diode-isolated sources.
    Is there a general issue with this configuration? I hadn't seen any mention of issues with diode isolated alternator feeds before.
  • Are the alternators internally or externally reguated? Curious, I guess it doesn't matter.
    External.
  • IMO it makes sense to turn on both alternators in flight. Plane Power uses 14.2 and 13.6 V for primary and backup alternators respectively and B&Cs can be set to something similar.
    Using a VP-X, I don't think it's possible to have both on at the same time, although I personally don't see any issue with both running at the same time, one just won't be doing anything.
  • Is there something to gain by powering the backup alternator thru the ECB box in addition to thru the engine bus alternate feed switch? The ECB box is an SOP for both alternator fields so you added an alternate feed for the backup alternator field but if the engine quits that's one more switch to think about. It would be great if engine-out switch flipping was reduced to only two, Backup Pump on and Fuel ECU as required.
    Engine out should be two switches only in this config. Backup pump On and E-Buss On. The backup alternator going through the E bus switch protects against a VP-X failure. A failure of the VP-X in this design should still leave you with two batteries and one alternator that can keep the engine running indefinitely.
  • Both alternator B leads are on a single stud making that stud an SPOF.
    I think I can wire it to different physical locations. For example, one can be wired to the battery contactor and the other to the starter contactor. Electrically identical and prevents a SPOF.
  • Why not connect the backup alternator B lead to the aux battery?
    I could. Wiring it to the Aux battery contactor should be electrically identical to this diagram. I didn't want to wire to the battery side of the contactor because taking the battery off the bus for any reason would also take the alternator off.
  • The backup alternator should be periodically stress tested in flight by turning off the main alternator.

If your engine bus will have separate power connections for its primary and alternate feeders that eliminates an SPOF in the form of the connection at the bus. I see your schematics show separate connections but I don't know if you intend to construct it that way. Z101 rev B shows a single connection to the engine bus which makes that stud an SPOF.

IMO it makes sense to turn on both engine bus feeds in flight. The alternate feed should be turned off periodically to stress test the primary aka diode feed.
I don't think I would have the alt feed on in normal operations, but maybe I'd turn it on before startup to test it. Alt-feed on, verify ECU operation, batts on, alt feed off, proceed with startup.

Fire-in-the-cockpit-both-masters-off scenario:
  • Engine runs on aux battery capacity. Do you plan to periodically capacity-test the aux battery?
    The plan is for both batteries to always be on during normal operation.
  • Backup alternator feeds the fire and, depending on the position of the Pri/Off/BU Alt Switch, its field current reduces the endurance of the aux battery.
    This will take some more thought. I think one possible solution is to turn everything on the VP-X off and this could be accomplished with on screen menus I think. Another possible solution is to turn the E-bus switch into a three position switch so I can choose to energize the E-Bus without switching on the backup alt.

Bob Nuckolls believes Z101 is as reliable as Z14. If you want dual batteries you might use Z14 as a template; it needs an engine bus though. BTW there's a rear battery addendum sketch for Z14 in a metal airplane.

A failure analysis of your electrical power system could/should prevent surprises. I did a failure analysis of my Z101-based electrical power system in case it's instructive; link.

Super helpful since I based most of my design off the Z-101.

P.S. High-current solenoid-operated switches that have double-spreading contacts are conventionally called contactors versus relays.
 
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