nuckolls.bob(at)aeroelect
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 Posted: Wed Aug 31, 2016 9:55 am Post subject: Z13/8 with EarthX battery and dual electronic ignition |
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I’m building an RV-14A that will have a full glass IFR panel and dual electronic ignition (still undecided about which brand of ignition but for discussion purposes let’s assume it’s not P-mags). I’d also like to use the EarthX ETX680 battery for the significant weight savings. Would you be comfortable using Z13/8 for this mission?
A few specific questions I have:
1) Is the SD-8 with the parallel R/C circuit & PMR1C-14 regulator guaranteed to self-excite even if the alternator is brought online without a battery connected?
The circuit (Z25) has been tested on numerous
airplanes . . . I'll be testing it further
on my drive stand . . . but yes . . . the SD-8
will come up without a battery when wired as
shown.
One possible failure mode is the physical connection to the battery, but my real concern is the unknown failure modes of the EarthX technology. It’s more complex than a lead acid battery due to the BMS and newer chemistry and has a short track record thus far, so compared to an Odyssey PC680 I feel there’s a greater chance that the EarthX may simply shut itself off or fail in a manner that is equivalent to having the battery completely disconnected.
During normal cruise operations, stress
levels on internal electronics is a small
fraction of that experience when cranking
the engine. We don't have enough data about
internal workings to do the classic MTBF
study . . . but there's a lot of these devices
in service . . . the risk is small. Even PC680s
have gone "open". I have on my bench now
that will go to Wichita on my next trip . . .
need to machine the top cap off to inspect
the inter-cell connections.
In this scenario I would assume the main alternator will no longer function reliably . . .
why do you assume this? Alternators have
been demonstrated to run self-excited after
start up . . .
so I need high confidence that the SD-8 will come online and produce 12-14 V, especially with both ignition systems depending on it.
Your choice . . . but the p-mag features
offer an exceedingly robust failure modes
effects analysis . . . but even LightSpeed
system operating directly from a battery
bus are on a par with the p-mags . . .
Operationally would you recommend having both alternators on all the time for normal flying? Or would you normally have the SD-8 off and only switch it on if the main alternator fails?
I think that's what is described in the 'Connection.
It seems one potential downside to having both alternators on all the time is if there’s an overvoltage condition it may trip the field breakers for both alternators. You would not know which one failed and you’d have to reset one CB at a time to see which one works. Not a really big deal but it adds troubleshooting complexity.
Actually that's not so. The LR series regulators
are selective ov trip. If the main alternator runs
away, field voltage will be greater than 1 volt.
If the field voltage is LESS than 1 volt, the
LR3 ov trip is suppressed.
Further, with the limited output from an SD8,
you're not likely to EVERY see an OV event
if the SD8 is the only alternator running unless
system loads are very light . . . and the main
alternator is inop.
But it's a moot point . . . leave SD8 off until needed.
Z13/8 shows both electronic ignition systems connected to the Main Battery Bus. This seems ok for Pmags, but for ignition units without their own generating capability, I’m thinking it would make sense to move one of them directly to the + battery terminal with a fusible link.
Do as you wish . . . but the wiring as shown
is exceedingly robust . . . your getting wrapped
around the axles of exceedingly remote probabilities.
Z13/8 shows the main battery lead, battery bus lead, and fuselink for the SD-8 all connected to the same terminal on the battery contactor. While I’m sure this is a pretty robust connection, it’s still a potential single point failure that could result in loss of your battery and both alternators (along with Main bus, E-bus, and Batt Bus) all at once.
. . . I think I'm more concerned about colliding
with an inboud meteor than I am for contactor
stud failure.
I know I could alleviate some of my concerns by going with an Odyssey PC680 and installing magnetos or Pmags, but the Odyssey battery is significantly heavier and the Pmags are still a concern for long-term reliability (I’m skeptical about how any electronic device can handle the heat exposure that is inherent in the Pmag design and have an MTBF of more than several hundred hours). I could also add another smaller battery and have a full dual batt/dual alternator architecture, but this adds more weight, complexity, maintenance, expense, etc, so I prefer not to go that route unless necessary.
The p-mags don't really operate that hot.
Modern electronics are not that fragile.
One of the first temperature tests accomplished
by a friend of mine is to throw the thing into
the chamber at 100C to see that there are no
weak compoents or joints in the build.
If it were my airplane, I'd go with Z13/8 as
published with p-mags. If you go any other
ignition system, run from battery bus on
separate switches. Should you ever get down
to SD-8 only operation, turn off one ignition
system. Run pumps off the battery bus too.
Bob . . .
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