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  • Re: Backup battery switch?

    by » 4 years ago


    Hi Tim, this would be ideal to keep the power lines short and neat with a remote pilot to drive the power relay which you could install firewall engine side. However the condition to follow this would be to use a sealed relay, rated 20A in order to avoid surprises when you really need to switch to backup (probably insert a routine check to see if all is ok) as while the starter relay is non critical (if it fails, you simply don't start the engine while you're on the ground) the backup relay is critical as if you need it it means your gens are dead and need your battery to restart and land asap. So this relay should be A-grade (we've used particular sealed relays from Saia-Burgess that can be fully immersed in water and withstand a wide range of temperatures) as if installed engine firewall side heat and cold and pressure variations cause condensation inside non sealed relays and we don't want glitchy contacts due to oxidisation in time (the backup battery relay would not be used as often as the start relay). Generally speaking, automative relays (plug-in, fast-on type) do fine in normal atmospheric conditions and "internal" environments but I don't suggest using them in aircraft engine compartments in general for the above reasons.
    The backup battery is fine and I suggest this solution (Dynon and other EFIS manufacturers already provide backup batteries for their systems) as runtime is critical once in backup (auxiliary power) mode.

  • Re: Backup battery switch?

    by » 4 years ago


    At what coil voltage does the relay drop out? Could you potentially have a condition where there is some voltage in the battery but you cannot feed it to the ECU? while I haven't done it yet, I would want to make sure to understand all possible failure modes of generators, battery and components.

  • Re: Backup battery switch?

    by » 4 years ago


    good point Geoffrey,
    considering this it would be best to go with a DPST toggle for the backup switch and forget the relay.
    Rob

  • Re: Backup battery switch?

    by » 4 years ago


    Typically relay coils still work at 10V so well under the standard 12.6V a resting battery outputs, but it's best to test each relay with a bench PSU to see at what voltage it will not be able to trigger the relay (the voltage required to keep the relay triggered is less then what is required to trigger it).
    As mentioned using a simple DPST switch is on one hand more secure but on the other you have to bring the (4) AWG 12 wires right up to your panel. While the starter wires are only loaded temporarily, the backup power will be loaded for the duration of the emergency (until you land). Not that a bigger deal, just more tefzel wiring and if done properly it should be fine (AWG 12 is enough to feed the 10A pumps and ECUs). Some voltage drop is expected (a charged lead acid battery "rests" at 12.6 (plateau) not at 13.8 (charge float limit) and the whole system should work fine until well under 12V (did not get a definite reply from engineers). Better turn off all unneeded avionics when in this situation and have a seperate backup battery or get avionics which have one already as optional.
    As an additional note, if your battery is not able to toggle the relay coil, don't think it will be able to power the ECUs and power hungry pump (s). You must be certain to have a fit battery at all times, electronic injection system is built around a continuous power source hence the 2 Gens and Rotax's refusal of LiPoly or LiIon substitute batteries as these involve additonal charge cutoff and regulator electronics inside the battery itself which are an additional point of failure.

  • Re: Backup battery switch?

    by » 4 years ago


    I had no problem running 12g to the DPST switches. Tefzel M227/16-12-9 is less than 3mm outside diameter so it is easy to work with.

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