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czechsix(at)msn.com
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 Posted: Tue Aug 30, 2016 10:10 am Post subject: Z13/8 with EarthX battery and dual electronic ignition |
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Hi Bob,
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) 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? 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. In this scenario I would assume the main alternator will no longer function reliably 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.
2) 2) 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? 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 wouldn’t 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…
3) 3) 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.
4) 4) 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’m thinking about moving the SD-8 fuselink from the battery contactor over to the MAIN BATTERY BUS fuseblock stud. This would look the same to the electrons but eliminates the battery contactor stud as a single point failure for the entire system. Thoughts?
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.
Thanks,
Mark Navratil
Spring Hill, KS
RV-8A N2D built/flew/sold
RV-14A #140017 finishing wings
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user9253
Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan
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| Posted: Tue Aug 30, 2016 6:14 pm Post subject: Re: Z13/8 with EarthX battery and dual electronic ignition |
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The Rotax 914 fuel injected engine has two alternators: the permanent magnet one and the larger electromagnet (wired field) one. The permanent magnet dynamo is completely separate from the aircraft electrical system and is dedicated to powering a fuel pump only. No matter how the aircraft electrical system fails, the fuel pump will keep working as long as the engine is running.
The SD-8 could be wired similar to the Rotax dynamo.
An AGM lead battery is unlikely to short out. Is it possible for the EarthX to short out and bring down the whole aircraft electrical system?
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