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  • Re: Backup battery switch?

    by » 5 years ago


    I have never heard of a fusebox connector getting hot but I agree, most aircraft have less than 20A continuous load. Anyone have this problem?
    I have been warned to be careful: some people are putting the X3 connector on crooked (cross threaded). And the grey rubber seal being left out.

  • Re: Backup battery switch?

    by » 4 years ago


    Hi Claus & all,
    just to follow up on your discussion, we are about to install a 912is on a VL3. I have analysed the official technical wiring and had some chats with some mechanics to get a clearer picture. All I can say is that there is not a large installed base of 912is here in Italy yet, but in any case there is support from certified Rotax people. I immediately questioned the start "mechanism" and "backup battery" (which should be prioperly called "emergency/auxiliary battery power" as this appeared the most critical. All these connections pass through the 3 "power" pins as shown on the rotax manual diagram Page 28 (Ed1 R3 E). The schematic shows:
    Pin 1 - through Backup *dual pole switch* to +12V of battery and the second switch connects the 2 grounds together (airframe and EMS)
    Pin 2 - to pin 3 through "Start Power" dual pole switch and the second switch connects both grounds together
    Pin 3 - to pin 2 through "Start Power" switch and to +12V battery through 30Amp fuse

    The 2 ground connections are also necessary during startup (and in AUX/Backup operation to make the pumps work) as this avoids ground differences during high current absorption (the starter relay gets power from the 12V battery respect to airframe ground while the EMS ground is connected to gen A "floating" from the airframe ground" so a potential difference is generated as soon as the generator starts generating and battery is cranking amps on a different ground) and equalizes temporarily the grounds until the engine is started and idleing and able to power the ECU units (and charge the battery).

    Generator B (grey cable) can generate 30Amps and is used to charge the battery during normal conditions after the engine has started (initially it powers the ECUs as it is able to energize/output more power at lower rpms respect to gen A) and it is connected to the normal AIRFRAME ground (-> explains why the 2GNDs must also be shorted during startup - important). When the voltage on Gen A is sufficient it takes over powering the engine electronics and Gen B just charges the battery and powers avionics. It is important to monitor indipendantly the power output of Gen B as maximum output should not be exceeded as the EMS does not monitor the current output on Gen B. If it fails, avionics will be powered solely by the battery and the battery will not be charged. If instead Gen A should fail, Gen B will be switched to powering the engine electronics (ECUs etc) and will not charge the battery. If both gens fail, what isn't really explained is that as the battery is between airframe ground and +12V Buss and the EMS uses it's own ground, it cannot continue to function so it will power off and so will the engine. The pilot must flip the AUX power/Backup battery switch in order to unify the 2 grounds and bring the +12V to the emergency input of the EMS/ECU electronics (pin 1) and so boot the ECUs and power the pumps and then restart the engine. A battery failure in such a system is evidently inadvisable, as your flight time depends on your battery capacity if both Gens are dead. A minimum of 16Ah battery is suggested by Rotax, better if you have an 18Ah.



    I've seen that some aircraft manufacturers have attempted an installation of a simple timer relay to simply startup. This solution may seem simple but it is not as the time which the start power connection is maintained can vary signficantly with the engine temperature as I have discovered. Currently only the 912is certified EMU by Stock Flight Systems appears to correctly control this time and "automate" start procedure as it monitors the rpm speed and various temperatures and also suggests the proper throttle setting related to the current engine temperature. It's a good unit but comes with a price (around 3000USD). To keep things simple and effective the 2 switch (one pushbutton and one spring loaded DPST) mechanism presented by Rotax is the best (and cheapest). Some have used "diesel" engine rotary switches successfully but I believe the seperate pushbutton and switch system is still a winner.

    The only variant we have been pondering on is the 2 switch pump system: in the shown Rotax schematic both pump A and B have a user accessable switch. We think it would be safer to connect the main pump directly through a 10AMP breaker, which is usually ON and not easily flippable. The pumps won't turn on until the Start Power switch is toggled anyhow, so it won't drain any power when you switch on the Master. The AUX pump switch can be present as normal to check the pump's operation (pressure difference). If needed, the main pump can be interrupted by pulling the breaker, but only if you want to for diagnostic purposes, as with the AUX/Backup Battery switch.

    Regarding your initial battery charging problem which you have prorbably solved by now, if you look at the schematic you will see that if you CLOSE the Backup switch you are bridging the 2 grounds together too (if the manufacturer of your aircraft has respected at least some of the Rotax specifications) so it may be a grounding fault. Be sure your 2 regulator grounds are not connected in normal conditions using an ohm meter - their grounds should only be shorted in the starter moment or with backup battery switch flipped. Regulator B (grey plug) charges the battery in normal conditions - be sure it's ground is connected to the airframe ground and so to the battery minus. About the the LED lighting problem, the output of the ECUs drive normal filament lamps so the "bleed" current is high enough to dimly light a low current LED. To avoid this you need 2 x 1K resistors, one in series and one in parallel to the LED as shown in the schematic.

    In the next few weeks we'll be completing the cockpit and wiring so we're hoping to test run it pretty soon and see how the FlyBox EFIS behaves reading the CAN data (I would have installed a Dynon but the client had a good offer on this EFIS system). Fingers crossed!
    WiringSchematic.png (You do not have access to download this file.)

  • Re: Backup battery switch?

    by » 4 years ago


    My interpretation of the wiring is exactly as Luke has described. In my case, I installed the Stock Flight Systems EMU along with a solenoid. As Luke says, the EMU will close the solenoid switch upon EMU power up and stays closed until the engine reaches 1,500 rpm (I think that's the right number). For my backup switch, I installed a second solenoid which joins the airframe and ECU grounds as well as providing direct battery power to pin 3 (with an inline 30A circuit breaker). This functions virtually identical to the EMU solenoid but instead of opening at 1,500 rpm, it stays closed to keep the grounds common and power to the engine. The second solenoid was handy for keeping the high current lines neat and tidy and not having to install a high current switch on the panel. If anyone believes this to be of poor practice, please let me know! I also installed a TCW IBBS backup battery for my EFIS, EMU and COM so I can devote full battery load to the engine instead of avionics in case the unfortunate every happens. My battery is 16Ah and I'm thinking of upgrading to something more robust.

  • Re: Backup battery switch?

    by » 4 years ago


    Sounds like your backup solenoid is always closed with the master ON? Not sure if you intended to say this but the backup switch should never be closed during normal operation.
    It is emergency or ground maintenance use only and by leaving it closed you are defeating the redundancy of the system.
    I recommend a simple toggle switch for the Backup; avoid extra failure points, parts & complication if you can

  • Re: Backup battery switch?

    by » 4 years ago


    Thanks for the reply, Rob. And no, the solenoid is only closed when the backup switch is activated (closed). I've tested all the circuits and they operated as described in the installation manual as well as the schematic you and the good folks at Skytek provided. The only difference is that I replaced the backup switch in your diagram with a solenoid and added a remote solenoid power switch.

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