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Most gliders are fitted with a single 12V7AH battery; some have 14V 7AH; some have a reserve....
Once you start adding GPS systems, horizons (which can draw 1.5A on start and 1A in a steady state), Mode C transponders etc then electrical endurance for these long flights (where you may be more reliant on additional instruments) does becomes a issue.
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Let's go back to this bit for a moment. Filser's TRT600 LAST draws about 0.2 A at 12 V.
If your AI draws 1 A, is it unreasonable to expect you to fit a transponder to fly in cloud? In fact, is it unreasonable to expect you to fit a transponder to fly at all?
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OK lets look at the practically:
The typical power budget for a glider is as follows:
GPS - 150mA (it's common for most competition ac to be fitted with 2)
Flight Director/PDA/Audio Vario(based on cambridge 302) - 1A
Radio - 500mA (RX) 1.8A on (TX) - Based con Filser ATR600
Normally steady state = 1.650A;
Max on radio Transmit ~3A.
A fully charged (new) 7AH battery will provide ~ 7Ah for 1 hour or
1 Ah for 7 hours (rated at 10 Hr rated as most 7AH batteries are)
As a ball park figure, and based on these figures most 12V 7Ah batteries will last about 5-6 hours before the voltage begins falls to a point which begins to be an issue and probably about 7 hours before equipment begins to fail. This is normally about right for club flying, although many aircraft are fitted with an additional 2v cell to provide a greater potential, and some are fitted with 2 batteries, to ensure that there is sufficiant voltage available for a full flying day.
For the competition pilots amonst us, and for those whose aircraft are equipped for cloud flying:
add into this a secondary GPS/Logger at about 150mA
an AH at 1.5A start/1A sustained
and a typical transponder at between 400-800mA (not 0.2A as you suggest - See the full technical manual for the TRT600 at
http://www.filser.de/handbuch/trt600instmane.pdf.
Lets just assume that there is an additional load of 150mA for a competition glider which draws an additional 1.5A when the transponder is fitted and the AH is switched on.
The maximum current drawn could be as high as 5.3A but likely to be about 2.3 A.
In fact 2.5A is used as a design guide.
Whichever way you look at it, an additional 400/700mA load due to a transponder on such a fragile power budget IS significant. It can mean the difference between systems working at the end of a 7 hour competition flying day or not working. And it is more important to maintain a workable supply to the Radio/Logger/GPS at the end of a flying day, than fit additional instrumentation, when there are procedures in place mitigating the need for it (calling on 130.4)
...Oh and as a PS....I design glider electrical systems!