Battery switching without tears

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 the avionics battery before launching). The switch over seemed to go okay, but then I noticed that my LX9000 was giving me unbelievably short glide distances. It turns out that the QNH altitude had been reset to the altitude at the time of switching. This was unacceptable, so I resolved to do something about it before this season. The simplest solution was to add a capacitor to the avionics power bus. The capacitor supplies power as the power selector switch is moving, and breaking, from the avionics battery, and connector, 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 = I * t / C or C = 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 situation) and t is 0.1 s, then C = 0.2 F (200,000 μF). The capacitor would 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 size of the capacitor to 50,000 μF. I ended up finding a suitably sized 39,000 μF capacitor rated for 25 V. A smaller capacitor could by used if the current drain is lower, which is likely for most gliders.
https://www.digikey.com/product-deta...301-ND/6928303

I installed the capacitor yesterday and monitored the bus voltage during switch-over with an oscilloscope, which was anti-climatic: there was no detectable 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 seen at:
https://flic.kr/s/aHsmMo9rN7

Nice, I have the same issue and have been thinking along these same lines.

Thanks for the report. I had wondered about doing the same thing. Just curious, did you include a resistor across the capacitor to make sure it discharges when the master is switched off and no loads are left on?

On Saturday, April 4, 2020 at 7:23:44 PM UTC-6, 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 the avionics battery before launching). The switch over seemed to go okay, but then I noticed that my LX9000 was giving me unbelievably short glide distances. It turns out that the QNH altitude had been reset to the altitude at the time of switching. This was unacceptable, so I resolved to do something about it before this season. The simplest solution was to add a capacitor to the avionics power bus. The capacitor supplies power as the power selector switch is moving, and breaking, from the avionics battery, and connector, 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 = I * t / C or C = 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 situation) and t is 0.1 s, then C = 0.2 F (200,000 μF). The capacitor would 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 size of the capacitor to 50,000 μF. I ended up finding a suitably sized 39,000 μF capacitor rated for 25 V. A smaller capacitor could by used if the current drain is lower, which is likely for most gliders.
https://www.digikey.com/product-deta...301-ND/6928303

I installed the capacitor yesterday and monitored the bus voltage during switch-over with an oscilloscope, which was anti-climatic: there was no detectable 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 seen at:
https://flic.kr/s/aHsmMo9rN7

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

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

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

Interesting. Many years ago Volkslogger had a similar issue and it is worth
examining their solution. Only the instrument in question (LX9000) needs
protecting, not the entire Avionics Bus.

They added an Electrolytic Capacitor, same as you did. But they also added
a Schottky Diode in series between the instrument/capacitor and the
supply.

The capacitor now only has to maintain the instrument in question and not
everything on the Supply Bus. In your calculation, the value of I is
greatly reduced (0.6A instead of 2A) and therefore the value of C is also
reduced, making for a much smaller capacitor.

On Sat, 04 Apr 2020 18:23:41 -0700, 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 the avionics battery before launching).

Some years back I had a momentary disconnection problem with the XLR plug
supplying power to my panel: it carries a common ground plus a positive
line from each of the two batteries I carry - one supplies flight
instruments, the other drives radio and T&B.

I added small capacitors (18mm diameter x 35mm long, 35-50v, so 1000-2000
uF capacity or thereabouts for each capacitor - thats a guestimate since
I can't read their markings without major surgery), one for each battery,
and with a 0.1 ohm resistor in series with it.

This solved the problem completely, but my current draw is lower than the
yours: 500 mA for flight instruments and 200 mA with T&B running and
radio on and receiving.


--
Martin | martin at
Gregorie | gregorie dot org

On Sunday, April 5, 2020 at 3:36:09 AM UTC-4, John Foster wrote:

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery.. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

That often works, but the engine battery will be fast-charging the panel battery while both are connected in parallel, so you don't want to leave the switch in that position for very long. If there's a 5 amp fuse between the engine battery and the panel, you don't want to blow it.

John DeRosa's excellent tutorial details several good options for the two-battery issue. Starts at slide #95:
http://derosaweb.net/aviation/presen..._Practices.pdf

Cheers,
...david

On Sunday, April 5, 2020 at 9:22:59 AM UTC-4, David S wrote:
On Sunday, April 5, 2020 at 3:36:09 AM UTC-4, John Foster wrote:

I'm no electrician, but wouldn't it work just fine if you wired both avionics and engine battery in parallel, with a switch (on-off) to each battery. That would mean two switches. Turn the engine battery switch on before turning the avionics battery switch off. Or am I missing something here?

That often works, but the engine battery will be fast-charging the panel battery while both are connected in parallel, so you don't want to leave the switch in that position for very long. If there's a 5 amp fuse between the engine battery and the panel, you don't want to blow it.

John DeRosa's excellent tutorial details several good options for the two-battery issue. Starts at slide #95:
http://derosaweb.net/aviation/presen..._Practices.pdf

Cheers,
...david

Actually, slides 119 - 123 provide additional options.

http://derosaweb.net/aviation/presen..._Practices.pdf


Page 97 shows the caps.

Page 98 shows why not to switch 2 in parallel for longer than it takes a fuse to blow. (Murphry says it will be the fuse on the good battery;-)

Page 120 shows the diodes. Note that the K2's have more volts to start with, so the diode drop is less of an issue.

I run like page 120, except with only one diode. The theory is that with the flat discharge curve of the K2's, I mostly run on the non-diode batt until is gets near empty and then transition to the second K2. It's automatic and I can watch the bus voltage to see how it's doing. SH wires the front battery in parallel with one of the mid batteries, so if the diode is on one of those, it runs with the vanilla ship wiring. There is a second connector on the diode battery for charging. (The second mid-battery gives me a switchable third, but so far haven't needed it.)

I also built an 'ideal' diode (page 121) but decided it I'd rather keep it simple and see the discharge progress in the bus voltage.

John's pictures make this easy. Aside from the one-diode, two-K2 plan, has anybody got another that is not in his collection?