nuckolls.bob(at)aeroelect
Guest
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 Posted: Sun May 17, 2020 5:01 pm Post subject: EXTERNAL PWR with MASTER/BATT SWITCH? |
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At 03:41 PM 5/17/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "supik" <bionicad(at)hotmail.com>
Bob, thank you for the long story. Actually I used your scheme with the OV protection and the GPU is wired to be plugged in directly on to the battery through the EXT PWR Contactor.
If I am not wrong, Marc Ausmann's scheme connects the GPU to the battery as well. This is what I am planning for..
But with your scheme there is no need to turn on the MASTER SWITCH.. |
Yes. I do that so that the external power
can also be used to charge a battery
in-situ without powering up the rest of
the airplane.
| Quote: | | C172 and SR22 -both require to turn on the MASTER to close the BATTERY and EXT PWR CONTACTORS at the same time. Thus you are not able to charge your battery unless you leave the MASTER ON (stupid?) or you connect your charger directly with the battery poles or take the battery out.. |
Yeah, that theme goes back quit a ways. See
exemplar 1974 wiring attached. 1962 C185
does about the same thing. With the battery
master off, you can crank the airplane but
you cannot charge the battery. In the 74
172 drawing we see the avionics bus
disconnect relay (item 4) which unhooks
radios while cranking and/or when ground
power is hooked up.
Hinky ground power sources have been around
since day-one but the philosophy behind their
use is not an industry wide consensus.
| Quote: | CAUTION
In accordance with the manufacturer’s recommendation, external
power should not be used to start the airplane with a dead battery
or to charge a dead or weak battery in the airplane. The battery
must be removed from the airplane and battery maintenance
performed in accordance with the appropriate AMM procedures. |
Yup, this is a legacy hang over from the
days of wild-n-wooly ground power hooked
to a dead and perhaps trashed flooded battery.
A lot of potential for spectacular
messes in the battery box. It's still generally
true . . . a totally discharged battery
should be recharged with a smart charger,
while preferably but not necessarily situated
outside the airplane. NOT by a 400A ground power cart
or 60A alternator.
| Quote: | ----------------------------------------------------------------------------------
Dead battery is clear but why not charge a weak battery if your charger is safe..?
Or is it that the intelligent charger might not provide enough amps to close the
EXT PWR Contactor when plugged in through the EXT PWR Socket? |
This is why YOU need to become the system
integrator and craft your own procedures
that meet design goals while minimizing
risk. What you suggest is perfectly doable
if you understand how NOT to let it devolve
into a bad day.
Remember that cookie-cutter airplanes are generally
owned and operated by individuals with just enough
knowledge to avoid doing something really stupid . . .
and even then it doesn't always work out. The
POH for most airplanes are crafted as much to minimize
lawsuits as opposed to educating the attentive
and thoughtful pilot.
Bob . . .
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