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Date: Thu, 21 Dec 2006 14:52:39 -0500
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From: "Dan Schaefer" <dfs155@adelphia.net>
Subject: [LML] Re: electric attitude indicators
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Hamid, you are correct in your post regarding what a battery's "AH" rating 
really means - however, there's a little bit more to the story that you 
didn't touch on.

In almost every case, the ampere-hour rating test given a battery by the 
manufacturer, whether C/1 or C/20 (or whatever), is usually continued to 
some voltage lower than it's rating - like 10 volts on a nominal 12 volt 
unit or 18 to 20 volts on a 24 volt unit to squeeze out the very best 
numbers for the marketing department. The terminal voltage on a lead-acid 
battery under load will decline in a fairly linear manner as the discharge 
continues, unlike a NiCad that holds a reasonably flat discharge voltage 
until the end when the voltage falls off the table.

One mfg. I dealt with (on the B-1 - I was the Rockwell engineering manager 
for replacing the NiCad primary batteries with sealed lead acid RG units) 
insisted his 28 volt aircraft battery met the 25 AH specification but when 
we looked closer, we found he was fudging a bit on the test and had set the 
lower discharge voltage limit at 18 volts. In reality, his battery was 
somewhere in the 18 AH range when normal test limits were set (in this case, 
our spec. considered discharged at somewhere around 22 volts).

If you're relying on the name-plate AH rating to keep critical systems up 
for a given span of time, you need to know how low a voltage said systems 
can tolerate and continue to function and what your battery rating is in the 
real world.

Regards,

Dan Schaefer