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Zavatson, Christopher J (US SSA) wrote:
Hamid,
Where did this come from?:
For the batteries in question here, the very
definition of the nominal battery
capacity designation is the number of amps that can be discharged for
one hour.
This definition is provided in the battery manufacturer’s literature –
see Gill and Concorde websites.
I was not aware that this discussion was restricted to Gill and
Concorde batteries. While the definition of "Amp-Hour" is the number
of amps that a battery can deliver for one hour, the industry standard
is to report this at C/20 or "when discharged over 20 hour" rate.
While individual manufactures may choose to use different rates, be it
1C rate or C/6 or even C/100 rate, the industry standard is C/20 and
that is what one need to assume unless one reads the data sheet and
verifies that it is something else. If you looked up the specific data
for your particular manufacturer, you are way ahead of most of the
crowd who have installed a "25Ah" battery -- the one that has "25" in
its name.
Note that the ‘25’
in Gill ‘G-25’ does not mean 25 Ah, just as the ‘25’ in
Concorde’s ‘RG-25XC’ does not mean 25 Ah.
I suspect that that is news to many listers, most A&P's and almost
all certified bird pilots.
The Gill G-25
is rated at 18 Ah
A quick calculation using typical numbers indicates that a battery
rated at 18Ah at a 1C rate, would be rated at about 25Ah at C/20 rate.
It is just speculation on my part, but I wonder if the the G-25 at one
time used to be rated as 25Ah till they decided to change the reporting
standard to more conservative numbers.
All that said, even using the battery's 1C rate as specified by the
manufacturer, if you are planning on any of the following:
-other than a brand new battery
-other than a fully charged battery (like one that was used to start
the engine 30 minutes ago)
-operating at other than the specified temperature (around room
temperature)
-loading with anything more than the 1C current drain
you better figure out how much you need to derate the battery. A
simple division of the 1C capacity by your current consumption is going
to give you a wildly optimistic number. My point remains, when you do
all of that, you may come up with a number that may surprise you.
Regards,
Hamid
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