Hi all

I'm really confused by the backup battery switch. I think the explanations I can find is not helping me, and comparing the Rotax user guide with the plane's makes it even worse.
This is from the plane's checklist.

11. LANE switch circuit A = on
12. LANE switch circuit B = on
14. Backup battery switch = off
15. Start power switch = on
16. Starter button = press until engine start
17. Start power switch = off
18. Backup battery switch = on
19. Increase RPM to cca 2000/min

As you can see the power switch and backup battery need to be opposite. In the Rotax manual, the backup battery switch is not touched.
But here the backup battery switch need to be on. Otherwise the plane is not charging the battery during flight (learned that the hard way when flying the plane home from the factory). In my mind that indicates that the backup battery switch is actually reversed!

Can someone explain what the switch actually does, and how you are supposed to use it during startup?

Kind regards, Claus

  • Re: Backup battery switch?

    by » 5 years ago

    It looks like an Error to me! ... What Plane is this?

    The Battery should be wired to ALT-B through the Master/Battery Switch.
    Master/Battery ON, Battery Charges!

    The Batt Backup Switch will normally remain OFF at all times.
    It is used to keep the engine running long enough to get you on the ground in one piece in the rare event of a double Alternator/Regulator failure.

    If ALT-B fails, The Engine continues to use ALT-A to supply the ignition, and leaves you the Battery for the instruments.
    If ALT-A fails, The Engine will take over ALT-B, and leaves you the Battery for the instruments.
    If Both ALTs fail, The engine dies, you activate the Backup Switch to get a restart and get yourself on the ground as soon a possible.
    The battery has limited capacity so shutting down all unnecessary electronics/lights will be a good move.
    Land quickly, as your remaining flight time is now measured in minutes.

    The engine will be using the main battery to power the ignition with the Backup Switch ON.
    Don't plan on staying airborne for more than 30 minutes on a good day. Plan to touchdown in under 15 minutes.
    Get on the ground Immediately.
    Declaring an Emergency is not warranted but a "PAN" call certainly is.
    You want to land NOW, without delay, on the nearest runway, even if the airport has no facilities.
    A deserted airport is 100 times better than a corn field.

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

  • Re: Backup battery switch?

    by » 5 years ago

    Hi Bill

    Thanks a lot for your reply.

    It is an ATEC 321 Faeta.

    I'm then wondering if there are two mistakes.

    1. I know that the battery is not charged if backup battery switch = off. I do not know if that is right or wrong. But if the switch is reversed, then it is actually on.

    2. It really sounds like the switch is rotated 180 degrees. It makes so much more sense to have it off all then time and if you need "backup battery" you turn it on. This is not the case now.

    Do you know if the battery is actually charged or not if the backup battery switch is on in a normal installation? We did fly with the switch in on and then the instruments started to die after 45 mins. But that was a user mistake. Alt A and B still worked, but no charge to the battery.

    What do you think I should do? Reverse the switch as number 1, and then try to figure out if the battery is connected in the right way. And still wondering why it need to be flipped during start of the engine.

    Again. Thanks for helping.

  • Re: Backup battery switch?

    by » 5 years ago

    I think there is more wrong than a reversed switch.
    The Battery Backup Switch should NEVER need to be ON.
    It is the Emergeny Backup Power source for the Ignition.

    The Battery Backup switch connects the battery to the Ignition directly.
    If you have no Lane Alarms, Then ALT-A is powering the Ignition.
    If the Backup Switch is ON, then ALT-A will be charging the Battery. Bad!
    ALT-A is the smaller ALT (10Amps), ALT-B (30A) is intended to charge the battery.

    If the Instruments are draining the battery, that indicates that ALT-B is not connected to the Main Buss.
    Is the Battery Switch ON and actually connected?
    Do you have a Blown Fuse (30A) that you don't know about?

    If this is a new plane, and never known to have been woring correctly, The factory needs to be involved.
    It should never have been delivered in this state.

    Charging the battery with the Ignition ALT-A is a disaster waiting to happen.
    It is only sized to power the ignition, Nothing More!

    Have a close look at your Electrical diagram and be sure you understand the system.
    I can only go off what you are telling me. The true story may be entirely different.

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

  • Re: Backup battery switch?

    by » 5 years ago

    It is a new plane, and no electrical diagram available. I have requested one, but I do not think I will get one.

    I have been studying the installation wiring diagram in the Rotax installation manual. It is pretty much clear, expect where the current flows when charging.
    I can see pin 3 on X3 is shared between the star power switch and a 12V fixed wire through F2. Is this the positive side from ALT B when charging?

    The whole thing is still confusing. When I say the switch is on all the time, I'm sure it actually means it is off all the time.
    When we flew with the backup battery switch off (meaning on), the battery slowly died. It is only a 8Ah battery, and it took 45 mins for it to start failing. That could perhaps indicate that it was "charging" from ALT-A, but the load consumption was higher than what it could deliver. Just before landing, I tried to switch the backup battery switch on (meaning off) to get power to the flaps. This worked, but I now think I turned the charging back to ALT-B and it did provide power to the plane's circuits again. We could then operate the flaps and landed safely. So back to the idea of maybe the only mistake is the reversed switch.

    I have not seen any separate fuses anywhere, but I have not looked for them either. According to the Rotax diagram they should be located in the engine compartment.

    We do have a shunt installed for the G3Xs. This is located in the cockpit, but I'm not aware (yet) how it is connected. I believe it is connected directly on the wire coming from the battery. It used the standard "crimp" connectors you use in car. When I had to change the two wires (showed negative value when charging!), one "connector" did fall off. To my knowledge, if this connection breaks the engine will keep running, but if the ALT-A+B will fail, then I cannot switch to backup battery. It must run through the shunt too. I really need to solder them to the connector to make sure this will not fail.
    Do you agree. The shunt should be mounted on the wire from the battery, before or after the main switch?

    I think I have to start measuring around the plane and check if the installation is like what Rotax recommends. I just need to find the way the charge current flows.

    Kind regards, Claus

  • Re: Backup battery switch?

    by » 5 years ago

    The contents of this thread are a perfect example of the need for a publication exposing the thinking behind the design and operation of the engine, as discussed elsewhere. Ambiguities and gaps in our understanding of the way the engine is intended by the designers to work is a safety hazard.

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