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What systems do you want to back up? How much current do they require? How long do you want to back them up for?
Can't see this would be needed. The 912is will not have a charging problem at all. Its unlikely you'll ever max out its output by anything you install for usage, less you got some real fancy radar stuff. Stick with a good oddessey battery or similar, an you'll have some extra money in your pocket for the $100 dollar hamburger. Think you'll otherwise complicate a very simple system. Your Dynon or E.I.S. will always tell you your charge rate @ a glance. Just thoughts from a 701 that's never had a battery problem. Safe air,, Pat
You can't make an informed decision until you know how much energy you need. It sounds like all you are concerned with is keeping the engine going so that probably means ECU + fuel pumps. Find out what they consume and compare it to any battery you are considering. It looks to me like the 2 amp and 4 amp devices you are looking at are designed for EFIS etc. not fuel pumps which might draw 4 or 5 amps (just a guess). I believe the Rotax 912iS already has dual (independent) alternators. If that is the case I would think that a decent sized battery like an EarthX dash 36 model would give you triple redundancy. I'm sure the Rotax monitor will tell you if you lose an alternator. If that happens shed unnecessary loads and head home. Seems to me that triple redundancy should be good enough, and an additional battery wouldn't serve much purpose. Where do you stop? Why just two batteries? Why not three or four? Why not add a wind generator and some solar panels. What does Rotax advise? If they are good with a single battery, I would be too.
Put in a low voltage pizo to get your attention. Remember you only have one crankshaft, camshaft , prop , radiator, ecu. How redundant are we really?
I like the idea of a switching system with battery back up - whether you build it yourself or purchase off the shelf. For the type of flying I plan on doing - VMC, remote, single ship with electronic ignition and electric fuel pumps for the UL350iS it makes sense to me.
I'd certainly go for the second battery (battery backup) of some sort versus dual alternator...
well part of the process is education...I didn't realize a Rotax engine would keep running with battery removed so I feel a lot better about only having one (battery). Nevertheless I'm going to do some investigating on the TCW Tech backup, to see if the benefits may outweigh the cost and weight gain. Thanks to all for responding.
A "regular" Rotax has an ignition system that generates its own power. Your 912iS does not. Your 912iS must have power for the ECU and for the fuel pumps. This power can come from battery or alternator. These comparisons to the carbureted Rotax engines are not valid. Your ignition system is completely different.
Check out the dual battery setup on the Viking website. Viking uses an ECU and electric fuel pumps, so you have the same issues. Their system is real simple and easy to wire. With the dual batteries you can run on battery #1, #2, or both. Normal operation is to fly on battery #1 on the outgoing leg and #2 on the return, that way both batteries are kept at full charge. Only one battery is charged at a time, so there are no alternator issue and you always have a fully charged extra battery if you fail to catch the discharge on the battery you are currently running on. Pretty much the only time you would want both batteries on at the same time would be if you needed the extra juice for a cold weather start.
Here is a snip from the website.