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Backup Batteries

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cjensen(at)dts9000.com
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Posted: Sun Apr 20, 2008 3:00 am Post subject: Backup Batteries

Curious question about the backup battery for a Mid-Continental LifeSaver attitude indicator with back up battery.  While the back up battery is rated for one hour, actual flight testing, when the battery is new, showed that a duration of nearly two hours were available, though the battery seems to die after about three years---disappointing for a +$100 battery.



By way of configuration, the backup battery is fed by plane power to keep charged.  If the plane electrical power is lost, the AI reverts to the battery to keep its gyro spinning--all very conventional.  The question is "is there a credible scenario whereby the electrical power in the plane is not only lost, but actually becomes an "electron sink" and pulls those lifesaving electrons out of the backup battery for the AI, shortening its useful duration?  Is this incredible?  Is diode isolation needed on the electrical lines to the backup battery?



Chuck

Velo XLRG

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nuckolls.bob(at)cox.net
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Posted: Sun Apr 20, 2008 4:52 am Post subject: Backup Batteries

At 06:52 AM 4/20/2008 -0400, you wrote:



 	  Quote: 	
		  



Curious question about the backup battery for a Mid-Continental LifeSaver 

attitude indicator with back up battery.  While the back up battery is 

rated for one hour, actual flight testing, when the battery is new, showed 

that a duration of nearly two hours were available, though the battery 

seems to die after about three years---disappointing for a +$100 battery.


	


    They're doing the same thing you would be expected to

    to when sizing your e-bus loads in relation to the

    size and condition of you ship's battery. The 1 hour

    endurance rating should be up to and including the

    service life limit of the battery . . . which in this

    case is the 3 years that you've cited.



    New out of the box, would would expect the battery's

    capacity to be higher than at end of life.



 	  Quote: 	
		  By way of configuration, the backup battery is fed by plane power to keep 

charged.  If the plane electrical power is lost, the AI reverts to the 

battery to keep its gyro spinning--all very conventional.  The question is 

"is there a credible scenario whereby the electrical power in the plane is 

not only lost, but actually becomes an "electron sink" and pulls those 

lifesaving electrons out of the backup battery for the AI, shortening its 

useful duration?  Is this incredible?  Is diode isolation needed on the 

electrical lines to the backup battery?


	


    Any designer worth his/her salary would have included

    reverse power isolation as part of the product. I.e,

    all system failure modes should have been addressed

    in the design such that if you install per the

    manufacturer's instructions, it will perform as

    advertised.



    So yes, your question is certainly valid  . . . for

    which the manufacturer of the product should answer,

    "no sweat . . . that's built in."



    It's unfortunate that internal backup batteries tend

    to be pricey. The attention paid to selection, installation

    and maintenance of a backup battery is more rigorous

    than for the ship's battery that at least gets "tested"

    for internal resistance every flight cycle when you crank

    the engine.



    But if your project has an e-bus, then you've done

    the planning and provided for an un-interruptible

    power source. Stacking another $100 battery on top

    of this may not offer a good return on investment.



    Bob . . .

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