Battery switching without tears

On Wednesday, April 8, 2020 at 12:58:39 PM UTC-4, Martin Gregorie wrote:
On Wed, 08 Apr 2020 08:57:43 -0600, kinsell wrote:

Runs an embedded linux, they're not storing any data during normal
operation, so baffling why that's so hard to fix.

When does it commit data to non-volatile memory, i.e. does in
intentionally buffer it and only commit at intervals or as part of
shutting down?

A common problem is forgetting to put /tmp in RAM.
Leaving it on a uSD will eventually end badly.


FWIW The same problem occurs with Raspberry Pis.

As you may or may not know, these run a Debian Linux clone as their OS
and use SD cards for non-volatile memory by default. Much of the time
people can get away with simply pulling the plug when the Pi appears to
be idle, but if you do that while the Pi is flushing its caches to its SD
card or, more rarely, the card is in the middle of a wear-levelling
process, then the SD card will become corrupted and possibly permanently
damaged if it was wear-levelling when the power vanished. Which is why
everybody soon learns to shut the Pi down with a 'sudo stop' command
before powering it off. I think its worse with SD cards simply because
their internal controllers and cheap, rather basic and have no power
buffering. Use an SSD instead or, even better, a hard drive and the
problem largely goes away because ext4 is a journalling filesystem, so
has built-in recovery.

I agree this is a tricky problem, and maybe best solved with some sort of
cheap'n cheerful UPS. Here's a suggestion along those lines:

Pimoroni sell the PowerBoost 1000 Charger, a small and fairly cheap UPS
circuit ($US 19.39 from Amazon), which you connect between a 5v power
supply and the device you want to power via a UPS socket. You also
connect a suitable sized 1S (3.7 volt) Lithium-ion battery to it - the
sort used to power small RC models would be fine - and there's a power
buffer for any UPS-powered device that doesn't have an internal battery.
Its good to supply up to 1000mA provided that the battery is rated for
that current. Some soldering is needed.

It comes with a selection of sockets, but they're all sat loosely in
place on the board so you can solder the ones you want on, sling the
others and solder any permanent connections you need.

I have one but haven't used it yet - I'm planning to make a PDA from a 4"
touch screen and a Pi Zero WL and use this Pimoroni plus an RC model 1S
LiPO battery to power it. Add a case made from epoxyboard and it should
be ready to rock'n roll.

Another Pi solution is this one with superCap: https://juice4halt.com/
Beware the window before supercap completely charged (should finish
shortly after boot)...

Enjoy,
Best Regards, Dave

On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

On 4/8/20 10:58 AM, Martin Gregorie wrote:
On Wed, 08 Apr 2020 08:57:43 -0600, kinsell wrote:

Runs an embedded linux, they're not storing any data during normal
operation, so baffling why that's so hard to fix.

When does it commit data to non-volatile memory, i.e. does in
intentionally buffer it and only commit at intervals or as part of
shutting down?

FWIW The same problem occurs with Raspberry Pis.

As you may or may not know, these run a Debian Linux clone as their OS
and use SD cards for non-volatile memory by default. Much of the time
people can get away with simply pulling the plug when the Pi appears to
be idle, but if you do that while the Pi is flushing its caches to its SD
card or, more rarely, the card is in the middle of a wear-levelling
process, then the SD card will become corrupted and possibly permanently
damaged if it was wear-levelling when the power vanished. Which is why
everybody soon learns to shut the Pi down with a 'sudo stop' command
before powering it off. I think its worse with SD cards simply because
their internal controllers and cheap, rather basic and have no power
buffering. Use an SSD instead or, even better, a hard drive and the
problem largely goes away because ext4 is a journalling filesystem, so
has built-in recovery.

I agree this is a tricky problem, and maybe best solved with some sort of
cheap'n cheerful UPS. Here's a suggestion along those lines:

Pimoroni sell the PowerBoost 1000 Charger, a small and fairly cheap UPS
circuit ($US 19.39 from Amazon), which you connect between a 5v power
supply and the device you want to power via a UPS socket. You also
connect a suitable sized 1S (3.7 volt) Lithium-ion battery to it - the
sort used to power small RC models would be fine - and there's a power
buffer for any UPS-powered device that doesn't have an internal battery.
Its good to supply up to 1000mA provided that the battery is rated for
that current. Some soldering is needed.

It comes with a selection of sockets, but they're all sat loosely in
place on the board so you can solder the ones you want on, sling the
others and solder any permanent connections you need.

I have one but haven't used it yet - I'm planning to make a PDA from a 4"
touch screen and a Pi Zero WL and use this Pimoroni plus an RC model 1S
LiPO battery to power it. Add a case made from epoxyboard and it should
be ready to rock'n roll.



The Stratux project does run on a Rspberry Pi using the micro-SD for
storage. Yes, running other software on that hardware can lead to the
same file system corruption problem. What's interesting is I can load
FlightAware software on the same hardware and never see the problem. In
both applications, there is no user data that needs to be stored to
flash memory, other than a tiny bit of configuration data when they are
first set up. It is possible to run on a read-only file system, if some
provision is made for the configuration data. People have experimented
with this, but the images distributed for Stratux have never had a
solution that I'm aware of.

So many devices these days have a computer with flash memory. If your
smart TV becomes unbootable after a power fail, would that be considered
acceptable? Of course not. But for some reason Stratux doesn't seem to
get fixed. Putting a UPS on a microcomputer seems like an ugly Band-Aid
(rtm) that shouldn't be necessary. Just a simple file system check on
power up might be adequate. There's a Sentry Mini receiver available
now for just $300, I'll bet it doesn't lose its mind if you just pull power.

-Dave

On Wednesday, April 8, 2020 at 2:51:22 PM UTC-7, 2G wrote:
On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately.. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

What switch did Schleicher use on your 31? Mine originally had that NKK switch, searching for a replacement I could not find any with better specs than the NKK MRT. There are many different manufacturers of an equivalent MRY106, but all seemed to mirror the spec.

On Wednesday, April 8, 2020 at 2:51:22 PM UTC-7, 2G wrote:
On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately.. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

200 mohms seems like it would still send a lot of current through the switch, at least briefly. 70A?

On Friday, April 10, 2020 at 4:55:12 PM UTC-7, jfitch wrote:
On Wednesday, April 8, 2020 at 2:51:22 PM UTC-7, 2G wrote:
On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

What switch did Schleicher use on your 31? Mine originally had that NKK switch, searching for a replacement I could not find any with better specs than the NKK MRT. There are many different manufacturers of an equivalent MRY106, but all seemed to mirror the spec.

I don't know what they used, but it is definitely not the NKK model you posted.

Tom

On Friday, April 10, 2020 at 4:58:04 PM UTC-7, jfitch wrote:
On Wednesday, April 8, 2020 at 2:51:22 PM UTC-7, 2G wrote:
On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

200 mohms seems like it would still send a lot of current through the switch, at least briefly. 70A?

I modeled it in Spice, included series inductance, and saw peak currents in the range of 10-20A for a few msec.

Tom

On Wednesday, April 8, 2020 at 12:55:49 AM UTC-4, kinsell wrote:

The really funny thing is, most people reading this have no need at all
to ever switch batteries. Instead of two small batteries, one big one
is so much easier to manage. If they have to be broken into multiple
units, then just wire them in parallel and let them all provide power
until depleted. If you switch them, then you risk switching too soon
and wasting capacity in the first one, or switching too late and ruining
a flight log or messing up a flight computer when you key the mike and
don't realize how weak the battery was.

-Dave

Funny that nobody responded to this comment from Dave. I think he's right on. Other than motorgliders that need an engine-starting battery separate from the avionics, why do we need a 2-battery setup? I've flown with a single battery for 25 years now, and have NEVER had a problem with that. Had plenty of other glitches in flight recorders etc, but not the battery's fault. Having two batteries (perhaps one in the tail) with separate wires to the panel can add capacity and redundancy even if they are simply paralleled within the panel.

Can one battery go bad (shorted cell) and load down the other one? Theoretically yes. Not likely, if you test your batteries once a season and stop using any that show a decline in capacity. But you can have a separate on/off switch for each battery, and normally have both turned on for the whole flight. If you have a voltmeter in the panel (some radios have it built-in) you can turn off one battery at a time just to check the condition of the other one.

On Saturday, April 11, 2020 at 1:39:09 PM UTC-7, wrote:
On Wednesday, April 8, 2020 at 12:55:49 AM UTC-4, kinsell wrote:

The really funny thing is, most people reading this have no need at all
to ever switch batteries. Instead of two small batteries, one big one
is so much easier to manage. If they have to be broken into multiple
units, then just wire them in parallel and let them all provide power
until depleted. If you switch them, then you risk switching too soon
and wasting capacity in the first one, or switching too late and ruining
a flight log or messing up a flight computer when you key the mike and
don't realize how weak the battery was.

-Dave

Funny that nobody responded to this comment from Dave. I think he's right on. Other than motorgliders that need an engine-starting battery separate from the avionics, why do we need a 2-battery setup? I've flown with a single battery for 25 years now, and have NEVER had a problem with that. Had plenty of other glitches in flight recorders etc, but not the battery's fault. Having two batteries (perhaps one in the tail) with separate wires to the panel can add capacity and redundancy even if they are simply paralleled within the panel.

Can one battery go bad (shorted cell) and load down the other one? Theoretically yes. Not likely, if you test your batteries once a season and stop using any that show a decline in capacity. But you can have a separate on/off switch for each battery, and normally have both turned on for the whole flight. If you have a voltmeter in the panel (some radios have it built-in) you can turn off one battery at a time just to check the condition of the other one.

That's a reasonable question and there is, of course, a reasonable answer. You need a backup battery if your main battery fails in flight.

On Friday, April 10, 2020 at 6:07:46 PM UTC-7, 2G wrote:
On Friday, April 10, 2020 at 4:58:04 PM UTC-7, jfitch wrote:
On Wednesday, April 8, 2020 at 2:51:22 PM UTC-7, 2G wrote:
On Wednesday, April 8, 2020 at 9:04:00 AM UTC-7, jfitch wrote:
On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
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

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do.. How much is the question. Quite easy to measure, hard to calculate accurately. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.

All decent suggestions. I can deduce the series resistance from the time constant of the waveforms, which is about 200 mohm. This includes the battery internal resistance, wiring resistance, and capacitor ESR. So forget about "hundreds of amps" flowing. Any more measurements will have to wait until I get out to the airport again, but including a series resistor of a few ohms, depending upon you total panel current drain, won't hurt. I did look up the NKK switch you mentioned and that is definitely not the one used in my panel.

Tom

200 mohms seems like it would still send a lot of current through the switch, at least briefly. 70A?

I modeled it in Spice, included series inductance, and saw peak currents in the range of 10-20A for a few msec.

Tom

Well, today I did measure the inrush current: the peak current was 9A, very close to what I had simulated with Spice. This current is very brief and totally within the capability of the switch to handle, but a few ohms of series resistance will cut it down to a couple of amps if you are anal about it.