You overall analysis is good, Shaun, but the actual current draw will
not be the same as what the mfr says and is often high. Transponders,
e.g., are spec'd at max reply rate of 500/sec. On my 2 electric
gyros, xpondr, GPS map/comm, and monitor draw is less than 1/2 the 11A
you've estimated, based on actual measurement. Thus, even the SD-8
vacuum pad alternator could be enough add'l power if one shops also
for current draw on avionics and is content with minimum goodies on
the panel.
Regards,
Fred F.
Shaun Simpkins wrote:
>
> The 16AH hour minimum quoted on the Europa factory schematic is Rotax'
> recommended minimum battery size for the 914 (section 9.4.14).
>
> Note also that available power is not the full 18A; according to section
> 19.5, the maximum loading from the fuel pumps, TCU, servo motor, and
> warning lamps is 8A.
> Typical for each pump is 2A; the TCU, probably about 0.15A; the rest are
> intermittent. So at best, we have perhaps 15A available to us. Airmaster
> CS prop loads are also intermittent. Another consideration is the battery
> charging capacity of the system. I believe the FAA rule of thumb is that
> 20% of the alternator capacity should be reserved for charging. Your
> personal specs may differ, but this is another 3.6A off the top.
>
> Thus, the 18A alternator is able to power about 11A of external loads with
> charging reserves. That's what basic electric gyros, VFR nav/comm, xnpndr,
> and engine monitor will eat up.
>
> Incidentally, this thread was last active in late 2000. Check out the
> archives. Many are running heavy electrical loads on the factory alternator,
> but probably with very small charging reserves.
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