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Battery switching without tears



 
 
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  #21  
Old April 6th 20, 02:42 AM posted to rec.aviation.soaring
2G
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Posts: 1,439
Default Battery switching without tears

On Saturday, April 4, 2020 at 11:45:25 PM UTC-7, Mark Morwood wrote:
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?


No, there is enough parasitic drain to take of that.

Tom
  #22  
Old April 6th 20, 02:45 AM posted to rec.aviation.soaring
2G
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Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 12:36:09 AM UTC-7, John Foster wrote:
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?


The glider, an ASH 31 Mi, is already wired with a battery selector switch for the avionics, and is the way to go. You definitely don't want to parallel Pb and LFP battery's accidentally.

Tom
  #23  
Old April 6th 20, 02:47 AM posted to rec.aviation.soaring
2G
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Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 3:15:06 AM UTC-7, 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.


As you can see from the installation photo, the physical size of the cap is not an issue.

Tom
  #24  
Old April 6th 20, 02:52 AM posted to rec.aviation.soaring
2G
external usenet poster
 
Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 8:18:00 AM UTC-7, 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.

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


That would require changing the switch on the panel. The course of least resistance (hence the title "Battery switch without tears") was to simply add a capacitor. It also avoids shorting of two dissimilar batteries.

Tom
  #25  
Old April 6th 20, 02:54 AM posted to rec.aviation.soaring
2G
external usenet poster
 
Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 8:45:30 AM UTC-7, 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.


I have no idea what the inrush current is. The time period of this current is so short that it doesn't trip my Klixon circuit breaker.

Tom
  #26  
Old April 6th 20, 02:57 AM posted to rec.aviation.soaring
2G
external usenet poster
 
Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 9:19:23 AM UTC-7, Martin Gregorie wrote:
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


Schottky diodes are unnecessary and I would not recommend them do to their voltage drop. The cap works just fine and isn't invasive (it is wired in parallel to the existing voltage bus).

Tom
  #27  
Old April 6th 20, 03:00 AM posted to rec.aviation.soaring
2G
external usenet poster
 
Posts: 1,439
Default Battery switching without tears

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.


These capacitors are intended for power supply applications and can handle high currents (note the size of the connector posts), although there aren't high currents in my panel.

Tom
  #28  
Old April 6th 20, 03:02 AM posted to rec.aviation.soaring
2G
external usenet poster
 
Posts: 1,439
Default Battery switching without tears

On Sunday, April 5, 2020 at 11:25:37 AM UTC-7, jfitch wrote:
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.


Schleicher parallels two sets of contacts of that 4-throw, dual pole switch..

Tom
  #29  
Old April 6th 20, 07:26 AM posted to rec.aviation.soaring
krasw
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Posts: 668
Default Battery switching without tears

On Sunday, 5 April 2020 13:15:06 UTC+3, Tim Newport-Peace wrote:
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.


Wait, don't all other instrument "see" the capacitor in LX9000 as well? It does not matter where the capacitor is physically, it still supplies current to the whole circuit (which is closed, apart for batteries for a short while)?
  #30  
Old April 6th 20, 10:29 AM posted to rec.aviation.soaring
Tim Newport-Peace[_6_]
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Posts: 65
Default Battery switching without tears

At 06:26 06 April 2020, krasw wrote:
On Sunday, 5 April 2020 13:15:06 UTC+3, Tim Newport-Peace wrote:
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.


Wait, don't all other instrument "see" the capacitor in LX9000 as well?

It
does not matter where the capacitor is physically, it still supplies
current to the whole circuit (which is closed, apart for batteries for a
short while)?

The Schottky Diode mentioned is there to prevent this. It isolates the
instrument and it's capacitor from the rest of the instruments.

12v----diode-----instrument/capacitor.



 




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