At 01:23 05 April 2020, 2G wrote:
On one of my flights last year I had to switch between my avionics
battery
=
and engine battery when the avionics battery voltage dropped too low (I
had=
left the master on after the last flight and could only partially charge
t=
he avionics battery before launching). The switch over seemed to go okay,
b=
ut then I noticed that my LX9000 was giving me unbelievably short glide
dis=
tances. It turns out that the QNH altitude had been reset to the altitude
a=
t the time of switching. This was unacceptable, so I resolved to do
somethi=
ng about it before this season. The simplest solution was to add a
capacito=
r to the avionics power bus. The capacitor supplies power as the power
sele=
ctor switch is moving, and breaking, from the avionics battery, and
connect=
or, or making, to the engine battery (this is called a "break before
make"
=
switch. But how big of a capacitor to use? The basic equation involved
is:

V =3D I * t / C or C =3D I * t / V

where V is voltage, I is current and t is time.

Translation: the bigger the capacitor the smaller the voltage drop. If
the
=
requirement is to keep the voltage drop to 1 V, the current is 2 A (my
situ=
ation) and t is 0.1 s, then C =3D 0.2 F (200,000 =CE=BCF). The capacitor
wo=
uld also have to be rated for 16 V, min. That is a pretty big capacitor,
so=
I decided I could tolerate a larger voltage drop (4 V), which cuts the
siz=
e of the capacitor to 50,000 =CE=BCF. I ended up finding a suitably sized
3=
9,000 =CE=BCF capacitor rated for 25 V. A smaller capacitor could by used
i=
f the current drain is lower, which is likely for most gliders.
https://www.digikey.com/product-deta.../399-14301-ND=
/6928303

I installed the capacitor yesterday and monitored the bus voltage during
sw=
itch-over with an oscilloscope, which was anti-climatic: there was no
detec=
table drop in bus voltage. Apparently the bread-to-make time is very
short,=
perhaps a millisecond. Haven't had a chance to fly with it yet, but
should=
be able to soon. The scope waveforms and capacitor installation can be
see=
n at:
https://flic.kr/s/aHsmMo9rN7