Battery switching without tears

Wouldn't a make-before-break switch solve the problem in a much simpler
fashion?Â* That's what my Stemme uses and switching between main and tail
batteries is a non event.

On 4/5/2020 4:02 AM, Tim Newport-Peace wrote:
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.




--
Dan, 5J

On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 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

What is the inrush current when you first switch the power on? Must not be enough to blow the fuse, but that'd be something I'd want to O'scope with a current probe.

On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote:
Wouldn't a make-before-break switch solve the problem in a much simpler
fashion?Â* That's what my Stemme uses and switching between main and tail
batteries is a non event.


Depends on what happens between make and break.

There will be some amount of surge current from the full battery to the empty one. If the current times the time exceeds the fuse IT, you can blow a battery fuse.

Long wires out to the tail and quick switching could make this less likely.

On Sun, 05 Apr 2020 09:10:53 -0700, stu857xx wrote:

On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote:
Wouldn't a make-before-break switch solve the problem in a much simpler
fashion?Â* That's what my Stemme uses and switching between main and
tail batteries is a non event.


Depends on what happens between make and break.

There will be some amount of surge current from the full battery to the
empty one. If the current times the time exceeds the fuse IT, you can
blow a battery fuse.

Long wires out to the tail and quick switching could make this less
likely.

So would a schottky diode inline with each battery. It will cost you a
whole 0.25v voltage drop and even make the switch unnecessary if you
don't mind drawing from both batteries at once. I power my logger that
way, but split the panel feed so one battery runs flight instruments and
the other does radio and T&B.

Backups? My nav system is a PNA thats good for at least a couple of hours
on internal battery and my backup vario is a Borgelt B.40 thats good for
8 hours plus off the fresh PP3 dry battery it uses for an alternate power
source.



--
Martin | martin at
Gregorie | gregorie dot org

On 4/5/20 9:45 AM, jfitch wrote:
On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 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

What is the inrush current when you first switch the power on? Must not be enough to blow the fuse, but that'd be something I'd want to O'scope with a current probe.


Yep. High enough current might eventually erode the switch contacts, or
even damage the capacitor.

Well, when I had my LAK-17a, I had a make-before-break and I never blew
a fuse.Â* The idea is to flip the switch, not see how slowly you can keep
it in the "make" position.Â* I would imagine the "make" time is of the
order of no more than a couple of thousandths of a second.Â* Some body
with a scope could measure that time and let us know.

On 4/5/2020 10:10 AM, wrote:
On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote:
Wouldn't a make-before-break switch solve the problem in a much simpler
fashion?Â* That's what my Stemme uses and switching between main and tail
batteries is a non event.

Depends on what happens between make and break.

There will be some amount of surge current from the full battery to the empty one. If the current times the time exceeds the fuse IT, you can blow a battery fuse.

Long wires out to the tail and quick switching could make this less likely.

--
Dan, 5J

You guys must pass the salt like this:
https://www.youtube.com/watch?v=b3PgxWaQMaQ



On 4/5/2020 11:03 AM, kinsell wrote:
On 4/5/20 9:45 AM, jfitch wrote:
On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 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 / CÂ*Â* orÂ*Â* 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

What is the inrush current when you first switch the power on? Must
not be enough to blow the fuse, but that'd be something I'd want to
O'scope with a current probe.


Yep.Â* High enough current might eventually erode the switch contacts,
or even damage the capacitor.

--
Dan, 5J

On Sunday, April 5, 2020 at 10:03:31 AM UTC-7, kinsell wrote:
On 4/5/20 9:45 AM, jfitch wrote:
On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 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

What is the inrush current when you first switch the power on? Must not be enough to blow the fuse, but that'd be something I'd want to O'scope with a current probe.


Yep. High enough current might eventually erode the switch contacts, or
even damage the capacitor.

The commonly used rotary switches are already being run over spec in modern panels. Schleicher and most others are typically using something like the NKK MRY106 or equivalent, these are called a 2A switch but that is the AC rating, DC rating is 1A. A modern panel uses something like 1.5A, and that is before you key the PTT switch.

When I redid my panel I used an MRT23 which is 3A, and paralleled the contacts for a 6A total. Rotary switches with a higher rating are normally much larger physically.

On Sunday, April 5, 2020 at 1:01:09 PM UTC-5, Dan Marotta wrote:
You guys must pass the salt like this:
https://www.youtube.com/watch?v=b3PgxWaQMaQ


delightful, but not quite the same

A 27 at a recent contest had two separate switches plus an old guy to implement make and eventually break. The result took a Walmart run to get a replacement fuse.

The diode seems a more certain alternative.

On Sun, 05 Apr 2020 12:15:53 +0000, Martin Gregorie wrote:


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.

Correction: still don't how big the capacitor is but the current limiting
resistor in series with it is 10 ohms (just measured it) so the capacitor
won't charge or discharge more at than 1.2 amps. The resistor is a big
wire-wound resistor in a rectangular ceramic case with the capacitor
taped onto it, so is both mechanically and electrically strong.

But, as I said, since this fixed the problem thats enough to prevent
vario and radio resets if I smack the XLR connector powering my panel. Nav
is fine - got its own internal battery.

The same setup may well sort out a break-before-make switch too, with the
advantage that fitting it across the +12v and ground lines on the panel
side of the switch can most likely be done without disturbing anything
that's already in your panel and is both inexpensive and easy to make and
install.


--
Martin | martin at
Gregorie | gregorie dot org