Battery switching without tears

On Monday, April 6, 2020 at 11:23:21 AM UTC-7, Dan Marotta wrote:
Tom,

Isn't the rotary panel switch in a Schleicher glider a make before break?

OK, I'll stop posting to this thread now.Â* My solution, which I've
relied on for years without ever losing a connection or blowing a fuse
seems to be too simple to be acceptable.

On 4/5/2020 7:45 PM, 2G wrote:
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

--
Dan, 5J

Dan,

Think about it: if it were MAKE before BREAK I wouldn't have had this problem and sure as hell wouldn't post a solution to a non-existent problem.

On Monday, April 6, 2020 at 9:40:36 PM UTC-7, 2G wrote:
On Monday, April 6, 2020 at 8:05:20 AM UTC-7, kinsell wrote:
On 4/5/20 8:00 PM, 2G 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.

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


There sure are high currents when you power up the panel. Current into
the cap is capacitance times dv/dt, where capacitance is large and dv/dt
is near infinite. Do the math. The switch may survive for a while then
fail due to the repeated surges. The fact the Klixon didn't trip
doesn't mean anything, fuses and breakers can take huge overloads for
brief periods without tripping.

There's better ways of doing this with diodes than just putting on a
monster cap.

Dave

Remember how they come up with these current ratings. It is based on heating of the contact - a very short current pulse (μsec) that results in virtually no heating.

I flew with this mod today and it worked perfectly - no glitch in the LX9000 QNH.

Tom

Well then, I must be really confused.Â* If the switch was a BREAK before
MAKE, the equipment would be momentarily without power.Â* Some devices
can survive that, perhaps with internal capacitance, but others will
lose power momentarily.Â* This may or may not be an issue.

Inrush current seems to be a big bugaboo to some people and, in some
cases it is, but what is the time constant associated with the inrush?Â*
How much current are we talking about and for how long? People keep
talking about the high battery "charging" the low battery during the
milliseconds that they are in parallel. Theoretically, yes, practically,
hogwash.Â* The heat required to blow a fuse or burn a wire does not rise
instantaneously; there's counter EMF to reduce the current...Â* So many
details that meant a bunch when you were taking a test back in school
but, practically speaking, don't mean squat in this case.

So, switch your batteries and protect your circuits however you wish.Â*
I'll stick with my make before break switches.Â* I'll even report back
when something fails due to their use, but don't hold your breath.

On 4/6/2020 10:43 PM, 2G wrote:
On Monday, April 6, 2020 at 11:23:21 AM UTC-7, Dan Marotta wrote:
Tom,

Isn't the rotary panel switch in a Schleicher glider a make before break?

OK, I'll stop posting to this thread now.Â* My solution, which I've
relied on for years without ever losing a connection or blowing a fuse
seems to be too simple to be acceptable.

On 4/5/2020 7:45 PM, 2G wrote:
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
--
Dan, 5J
Dan,

Think about it: if it were MAKE before BREAK I wouldn't have had this problem and sure as hell wouldn't post a solution to a non-existent problem.

--
Dan, 5J

On Monday, April 6, 2020 at 9:36:21 PM UTC-7, 2G wrote:
On Monday, April 6, 2020 at 8:02:53 AM UTC-7, jfitch wrote:
On Sunday, April 5, 2020 at 7:02:13 PM UTC-7, 2G wrote:
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

That gets you to 2A - still less than the panel draws with the PTT pressed I'd bet. The time constant of the Klixon is quite long compared to the inrush current (if the impedance is low). Do you have a high bandwidth current probe for your O'scope? The peak current may depend mainly on the arcing characteristics of the contacts in the switch.

It's too bad they can't figure out how to make an LX9000 that can take a momentary interruption in power - many others have figured out how to do this.

The scope and probe are 100 MHz. I don't have a current probe (would have to use a series resistor which would change it from "without tears" to "many tears." Remember, the capacitor is providing the energy to the instruments when the switch is between break and make, so no current is required from the battery when it makes.

The issue with the switch will not be switching between two batteries. It is switching on the first battery, charging the capacitor. DC contact rating for a switch is determined from two things: heating of the contacts and arcing resistance during switching. The latter is generally more critical, which is why usually the AC rating is higher. A rotary switch of this type is particularly susceptible as it is not "snap acting".

Inrush current seems to be a big bugaboo

Seem to have time to kill. Lets try for some numbers.

Here's the fuse data sheet
https://www.littelfuse.com/~/media/a..._datasheet.pdf

I use a 7.5 amp fuse on each battery.

Assuming the battery full and empty voltages differ by 1 volt.
The current with both batteries on the bus also depends on the wiring resistance. If there is 0.1 ohms, then the fuse current is 10 amps.
at 25C, that could blow it in a few seconds.

The I2T rating for the fuse is 60. That is measured over 8 milliseconds. So I think that says the fuse will blow quickly with 86 amps.

Seems like unless you have really good wiring or a big charge difference, quickly switching make before break should be ok.

That matches your experience with the Stemme and also the Walmart run at the recent contest where the switch was left with both batteries on the bus for a while.

Good practice (checklist discipline) holds that all switches be off
before turning the battery on.Â* At the end of the day, all equipment
should be switched off before turning off the battery.Â* The switch can't
suffer much arcing under those conditions.Â* As said elsewhere, the
voltage difference between two batteries being switched between, is not
much more than 1 volt and the time that they are in parallel is probably
less than 100 mSec using a make before break switch.

I should have made the above clear up front.

On 4/7/2020 10:44 AM, jfitch wrote:
On Monday, April 6, 2020 at 9:36:21 PM UTC-7, 2G wrote:
On Monday, April 6, 2020 at 8:02:53 AM UTC-7, jfitch wrote:
On Sunday, April 5, 2020 at 7:02:13 PM UTC-7, 2G wrote:
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
That gets you to 2A - still less than the panel draws with the PTT pressed I'd bet. The time constant of the Klixon is quite long compared to the inrush current (if the impedance is low). Do you have a high bandwidth current probe for your O'scope? The peak current may depend mainly on the arcing characteristics of the contacts in the switch.

It's too bad they can't figure out how to make an LX9000 that can take a momentary interruption in power - many others have figured out how to do this.
The scope and probe are 100 MHz. I don't have a current probe (would have to use a series resistor which would change it from "without tears" to "many tears." Remember, the capacitor is providing the energy to the instruments when the switch is between break and make, so no current is required from the battery when it makes.
The issue with the switch will not be switching between two batteries. It is switching on the first battery, charging the capacitor. DC contact rating for a switch is determined from two things: heating of the contacts and arcing resistance during switching. The latter is generally more critical, which is why usually the AC rating is higher. A rotary switch of this type is particularly susceptible as it is not "snap acting".

--
Dan, 5J

On Tuesday, April 7, 2020 at 9:32:07 AM UTC-7, Dan Marotta wrote:
Well then, I must be really confused.Â* If the switch was a BREAK before
MAKE, the equipment would be momentarily without power.Â* Some devices
can survive that, perhaps with internal capacitance, but others will
lose power momentarily.Â* This may or may not be an issue.

Inrush current seems to be a big bugaboo to some people and, in some
cases it is, but what is the time constant associated with the inrush?Â*
How much current are we talking about and for how long? People keep
talking about the high battery "charging" the low battery during the
milliseconds that they are in parallel. Theoretically, yes, practically,
hogwash.Â* The heat required to blow a fuse or burn a wire does not rise
instantaneously; there's counter EMF to reduce the current...Â* So many
details that meant a bunch when you were taking a test back in school
but, practically speaking, don't mean squat in this case.

So, switch your batteries and protect your circuits however you wish.Â*
I'll stick with my make before break switches.Â* I'll even report back
when something fails due to their use, but don't hold your breath.

On 4/6/2020 10:43 PM, 2G wrote:
On Monday, April 6, 2020 at 11:23:21 AM UTC-7, Dan Marotta wrote:
Tom,

Isn't the rotary panel switch in a Schleicher glider a make before break?

OK, I'll stop posting to this thread now.Â* My solution, which I've
relied on for years without ever losing a connection or blowing a fuse
seems to be too simple to be acceptable.

On 4/5/2020 7:45 PM, 2G wrote:
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
--
Dan, 5J
Dan,

Think about it: if it were MAKE before BREAK I wouldn't have had this problem and sure as hell wouldn't post a solution to a non-existent problem.

--
Dan, 5J

If my power switch was a break-before-make (also called a shorting switch) I would not have had a problem and certainly wouldn't have posted a fix to a non-existent problem. But it isn't. It was easier for me to put in a capacitor than replace the switch. It also protects against any switch bounce while switching.

People seem to be overly concerned with inrush current. High inrush current would only happen if there were a substantial amount of power supply capacitance in the LX9000 that needed charging. If that were the case it would handle a millisecond battery switch over without any problem, but it doesn't.. Also, there is no inrush current during battery switching as the caps in the LX9000 are already charged.

Tom

I put a power resistor in the circuit to keep the current surge down. I undersized the capacitor so if I mess up on the with rotation I can lose power.. Typically I'll shut off some non-essential equipment to lower the draw if I really don't want a computer reset.

Andy

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

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 Tuesday, April 7, 2020 at 8:06:46 PM UTC-7, Andy Blackburn wrote:
I put a power resistor in the circuit to keep the current surge down. I undersized the capacitor so if I mess up on the with rotation I can lose power. Typically I'll shut off some non-essential equipment to lower the draw if I really don't want a computer reset.

Andy

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

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.

You didn't say what the resistor size was. There is already a resistor in the circuit - it is called the internal resistance of the battery,wiring resistance, switch contact resistance and the equivalent series resistance (ESR) of the capacitor.

On Tuesday, April 7, 2020 at 8:58:53 PM UTC-7, 2G wrote:
On Tuesday, April 7, 2020 at 8:06:46 PM UTC-7, Andy Blackburn wrote:
I put a power resistor in the circuit to keep the current surge down. I undersized the capacitor so if I mess up on the with rotation I can lose power. Typically I'll shut off some non-essential equipment to lower the draw if I really don't want a computer reset.

Andy

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

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.

You didn't say what the resistor size was. There is already a resistor in the circuit - it is called the internal resistance of the battery,wiring resistance, switch contact resistance and the equivalent series resistance (ESR) of the capacitor.

I think it was only a few Ohms, but it was a big sucker so it could take the current. I figured bet to play it safe so you know where you are dissipating the energy. Probably unnecessary, but I am a belt + suspenders kind of guy.

Andy