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Rotax 912iS Electrical Charging System

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cluros(at)gmail.com
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Posted: Sat Sep 08, 2018 6:37 pm Post subject: Rotax 912iS Electrical Charging System

This question is specifically in relation to the Rotax 912iS stock electrical system. Lane A and B voltages are rock steady at 14 V, but the volt sensor on the Dynon Skyview is seeing 13.5 - 13.7 V. The aircraft is wired exactly as per Rotax installation instructions and follows their wire size recommendations. I'm going to stick a voltmeter on the battery tomorrow and see what the actual voltage at the battery is while the engine is running but if it confirms the Dynon reading I'm concerned the battery is never going to be charged.



The strange thing is the Dynon Skyview default for this engine's voltage output has 13.5 in the middle of the green band. For a Lycoming or Continental I put the green band at 14-14.5

Is anyone here familiar with the 912iS? If this is how the system is designed, what are the implications for an Odyssey PC680 that spends most of it's waking life at 13.5 V?

Thank you all,

Sebastien

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nuckolls.bob(at)aeroelect
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Posted: Sun Sep 09, 2018 8:54 am Post subject: Rotax 912iS Electrical Charging System

 	  Quote: 	
		  

 The strange thing is the Dynon Skyview default for this engine's voltage output has 13.5 in the middle of the green band. For a Lycoming or Continental I put the green band at 14-14.5 	




    An SVLA battery should be run at no less than 14.0

    and no more that 14.8 with 14.4 being the nominal

    "target". Anything below 14.0 has a high probability

    of failing to top off the battery after a significant

    discharge. This has nothing to do with the make/model

    of the engine.



 

  	  Quote: 	
		  Is anyone here familiar with the 912iS? If this is how the system is designed, what are the implications for an Odyssey PC680 that spends most of it's waking life at 13.5 V? 	


    If true, that battery is seriously under-served for

    charging.



    Your proposal to investigate battery terminal

    voltage while in flight is a good move. Keep in

    mind just because a certain voltage is recorded

    at one place in a system, it's not a sure bet

    that the same voltage will be recorded elsewhere

    even tho both points are connected with copper

    pathways. There are subtle but significant voltage

    drops in system wiring. Further, if the readings

    are taken with two different instruments, it's

    possible that they may not be calibrated to the

    same accuracy.



    Irrespective of engine, the voltage regulator's

    prime directive is TAKE CARE OF A BATTERY. Given

    that there's a lot of copper monkey-motion between

    the regulator and battery, the regulator's electronics

    can only ASSUME that it knows what battery voltage

    is.



    For SVLA batteries, voltage in cruising flight with

    a fully charged battery should be in the 14.2 to

    14.6 range.  If the alternator is off for any

    reason, the battery voltage will fall quickly to

    something under 13.0 volts. when the battery falls

    to 11.0 volts, it has probably less than 5% of

    it's maximum capacity remaining. 



 

   Bob . . .

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