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Date: Sun, 23 Dec 2007 12:07:41 -0500
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From: Hamid Wasti <hwasti@lm50.com>
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Subject: Re: [LML] Re: Glass Panel Backup
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Bill Wade wrote:
>  If anything happens to the alternator or the other battery I know I 
> have a conservative (and easy to figure) 30 Amp-hours before voltage 
> drops to 10V.

You do realize that 30AH does not means 1A for 30 hours or 60A for 0.5 
hours? The capacity of a battery goes down as you increase the current. 
Yes, I am talking CAPACITY, not just the time. In fact, the parasitics, 
which cause this accelerated decrease in capacity, increases as the 
square of the current. So as you increase the load, especially things 
like the hydraulic pump, pitot heat & landing lights, you may find that 
the battery does not last quite as long as you were counting on.

You may find it quite instructive to do a test on the ground (on jacks 
so you can move the gear) and see how long the battery actually lasts. 
The only down side to that is that after you torture your battery like 
that, you really ought to consider replacing it.

And when you are doing this test, remember to retract and lower the gear 
a second time to account for a missed approach or go-around.

Regards,

Hamid