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Battery Size For 912 iS?

in 912iS Technical Questions 7 years ago

  • Re: Battery Size For 912 iS?

    by » 7 years ago

    [quote]Is there any data on the likelihood of a double alternator failure?[/quote]
    Probably much less likely than a total engine failure.


    [quote]Can the engine be started in the "backup battery" emergency mode?[/quote]
    EVERY Engine start is in a "Backup" Battery Start.
    "Start Power" and "Backup Power" are identical except "Start" passes through a relay that removes the Start Power from the ECM Bus once the generators come on line.
    Backup Power attaches the battery directly to the ECM Bus.

    ~ ~ ~

    The "Lead Acid Equivalent" only applies to high Current/ Starting applications.

    A 16Ah Lead Battery will deliver 16Ah if discharged at 0.8A over a 20 hour period.
    If the same battery has a 400CCA rating, that means 400 amps for 30 seconds before it drops to 7.2volts.
    400A x 30 Seconds = 3.3Ah. About 20% of the 20-hour rating.

    A lead acid battery with a 400CCA rating would typically be in the 30-35Ah range.
    So the 12Ah Lithium battery is the 36Ah "Lead Acid Equivalent" in the 30-second 400A contest.
    This is all fine for starting, which is a battery's typical intended use.

    But this is not what our concern is.
    If an auto battery becomes depleted, you just pull to the side of the road.
    In an aircraft, it gets a little more exciting.

    Unless you can stay in the air for another 20 hours after the generator dies, The 20 hour Ah rating means little.
    What you want is more like the 1-hour rating which is never specified.
    Assume it to be about half of the 20 Hour rating just to be safe.

    A 16Ah rated battery (8Ah) is going to last about 40 minutes on a 12 amp draw.
    A 12Ah rated Battery (6Ah) is going to last 30 minutes.
    If you need a 30-minute fuel reserve, you should have at least 30 minutes of battery to go with it.
    Are you ever more than 30 minutes from the nearest airport?

    The Ah rating is all that matters when you are trying to get on the ground without undue delay.
    You are not going to be worrying about over discharging either battery.
    You will discharge it until it the engine quits or you land, whichever comes first, no matter what.

    Buy all the Amp Hours you can get, and only expect to use half of them.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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  • Re: Battery Size For 912 iS?

    by » 7 years ago

    Bill, Thanks for the very informative discussion about Lithium ion batteries. I can only hope that every owner of an 912iS engine reads the thread. You have cleared up some issues I had about the ratings of Lithium battery specifications. In my case I had a areovoltz 16 cell before the Rotax battery instruction. I then changed to the Earth-x 36C. However I had this nice 16 cell so I mounted it behind the pilot seat on my Kitfox and wired it in as a standby emergency battery. It does not have the wire size to be a starting battery but has wire size to support engine operations. I connect the 16 cell to the main buss with an appropriate circuit breaker type switch which can put the standby battery in parallel with, or in the event the main battery is depleted , the mains switch can be turned off to isolate the depleted battery and the ECU source will continue with the emergency battery. I realize this arrangement complicates the issue however after 40 years of working on complicated avionics systems I am quite comfortable with the procedures involved. My EFIS has provisions to monitor two 12v sources so I see the main battery status as well as the standby battery status.The standby 16 cell is idle throughout the flight unless called for. Thanks again for the great thread.Richard
      
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  • Re: Battery Size For 912 iS?

    by » 7 years ago

    Almost forgot!
    Thanks to Geoffery for starting the thread
      
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  • Re: Battery Size For 912 iS?

    by » 7 years ago

    Yes good thread...
    My Aircraft came with an 11Ah lead acid !! It has been "fine", however I am trying to buy and EarthX EXT900 (16AH) for months. The issue is finding a way to freight it here...everyone just sees the word "lithium" and panics.

    I think the LED light to show a battery issue is a great idea too.
      
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  • Re: Battery Size For 912 iS?

    by » 7 years ago

    Glen,
    The EarthX Dealer Search says you have 3 dealers nearby.

    Downunder Stol Viking Aircraft
    64 Williams St.
    Christchurch, New Zealand 7630

    Precision Motorbikes Limited
    807 Jones Rd.
    Rolleston, New Zealand 7675

    Motomox Ltd
    25C Peraki St.
    Kaiapoi, South Island 7630

    They will likely have to order it.
    While not permitted as Cargo on Passenger Aircraft, They can be transported on Cargo Only Aircraft in a 70% discharged state.
    EarthX ships batteries every day.
    They will know the requirements to get you your battery,

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
    Report
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