Battery switching without tears

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

Switches are generally rated according to interrupt capacity, and that's why the AC and DC ratings are different. In the immortal words of a friend of mine in the business, "DC just doesn't switch worth a ****". As the contacts open, an arc is struck. With AC current, the arc self extinguishes (for small switches) when the current goes to zero. The arc persists longer with DC, causes more contact wear, hence a reduced rating.

A "1A DC" switch is not going to fail or in any way be stressed by keying the mic and momentarily running 2, or even three or four amps. Try not to turn off the master while you are talking on the radio :-).

My panel has a CN2, CNv, radio, flarm. I have the standard issue Schleicher rotary switch, no modifications. Switching between batteries is simple: turn the knob. No drama, no instrument problems.

Evan Ludeman

On Monday, April 6, 2020 at 8:33:37 AM UTC-7, Tango Eight wrote:
Switches are generally rated according to interrupt capacity, and that's why the AC and DC ratings are different. In the immortal words of a friend of mine in the business, "DC just doesn't switch worth a ****". As the contacts open, an arc is struck. With AC current, the arc self extinguishes (for small switches) when the current goes to zero. The arc persists longer with DC, causes more contact wear, hence a reduced rating.

A "1A DC" switch is not going to fail or in any way be stressed by keying the mic and momentarily running 2, or even three or four amps. Try not to turn off the master while you are talking on the radio :-).

My panel has a CN2, CNv, radio, flarm. I have the standard issue Schleicher rotary switch, no modifications. Switching between batteries is simple: turn the knob. No drama, no instrument problems.

Evan Ludeman

No, a 1A DC switch will probably not instantly fail at 2A. An engine redlined at 7000 will probably not instantly fail at 8000. A glider rated at 5.5G will probably not instantly fail at 7G. Still, designing something to routinely operate out of spec isn't considered good practice in my shop. And the switch might fail with an inrush current of 20A, while switching, after awhile.

CN, like most other manufacturers designed to the reality of switching power sources in operation. LX, apparently, did not. I have no problem with switching my break-before-make rotary switches on a full panel of (non LX) electronics - nothing resets.

The continuation of this thread is proof that we are quarantined.

On Monday, April 6, 2020 at 11:33:37 AM UTC-4, Tango Eight wrote:
Switches are generally rated according to interrupt capacity, and that's why the AC and DC ratings are different. In the immortal words of a friend of mine in the business, "DC just doesn't switch worth a ****". As the contacts open, an arc is struck. With AC current, the arc self extinguishes (for small switches) when the current goes to zero. The arc persists longer with DC, causes more contact wear, hence a reduced rating.

A "1A DC" switch is not going to fail or in any way be stressed by keying the mic and momentarily running 2, or even three or four amps. Try not to turn off the master while you are talking on the radio :-).

My panel has a CN2, CNv, radio, flarm. I have the standard issue Schleicher rotary switch, no modifications. Switching between batteries is simple: turn the knob. No drama, no instrument problems.

Evan Ludeman

Case in point to illustrate Evan's statement: I've tried to use a small water heating tank to use excess solar power to heat some water. Its heating element is rated 120VAC 15A. It has a thermostat with an internal switch to match. I fed it about 50VDC 6A from solar panels. That switch died in short order. Also note that we're warned not to disconnect solar panel connectors while the sun is shining and the power is being used, because the arc will damage the contacts in those connectors.

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.

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

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

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