Battery switching without tears

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 forgot to mention that aluminum electrolytic capacitors have very significant inductance at frequencies above 10 KHz. Inductance (also called a choke because they choke off current) limits inrush current. The joules of energy being transferred is pretty low (about 0.6 J), but I have no problem with adding a small series resistor (1 ohm - don't worry about the wattage as very little power is supplied by the cap).

Tom

On 4/7/20 10:07 PM, Andy Blackburn wrote:
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


For people who didn't sleep through their EE101 class, they can figure
out how to parallel a diode across the resistor to allow very slow
charging of the capacitor, yet rapid discharge to power the panel
temporarily. Charging the cap in seconds instead of milliseconds would
give oh about a factor of 1000 reduction in the inrush current. There's
better ways of doing it, but this would be an adequate solution.

The batteries Tom uses are both capable of delivering several hundreds
of amps at a reasonable voltage to a starter motor, so that should give
an idea of how much current limiting they provide.

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

At 04:20 08 April 2020, 2G wrote:
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
p=
ower. Typically I'll shut off some non-essential equipment to lower the
dra=
w if I really don't want a computer reset.
=20
Andy
=20
On Sunday, April 5, 2020 at 8:45:30 AM UTC-7, jfitch wrote:
=20
What is the inrush current when you first switch the power on? Must
n=
ot be enough to blow the fuse, but that'd be something I'd want to
O'scope
=
with a current probe.
=20
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
r=
esistance, switch contact resistance and the equivalent series resistance
(=
ESR) of the capacitor.

I forgot to mention that aluminum electrolytic capacitors have very
signifi=
cant inductance at frequencies above 10 KHz. Inductance (also called a
chok=
e because they choke off current) limits inrush current. The joules of
ener=
gy being transferred is pretty low (about 0.6 J), but I have no problem
wit=
h adding a small series resistor (1 ohm - don't worry about the wattage
as
=
very little power is supplied by the cap).

Tom

Has anyone reported this problem to LXV, and if so what was their response
please? There seems to be an opportunity for a firmware fix to this
problem, not requiring any additional hardware.

In the specification for Flight Recorders, a new IGC file is not started if
power has been interrupted for less than one minute. This is to allow a
change of battery, breaker reset or change a fuse. Exactly the problem we
are discussing.

If the LX firmware were enhanced to inhibit re-calculating QNH where power
has been lost for less than n seconds, this problem goes away.

Discuss.

Has anyone reported this problem to LXV, and if so what was their response
please? There seems to be an opportunity for a firmware fix to this
problem, not requiring any additional hardware.

In the specification for Flight Recorders, a new IGC file is not started if
power has been interrupted for less than one minute. This is to allow a
change of battery, breaker reset or change a fuse. Exactly the problem we
are discussing.

If the LX firmware were enhanced to inhibit re-calculating QNH where power
has been lost for less than n seconds, this problem goes away.

Discuss.


The LXNav manual says that the internal logger will continue to operate 'for a short time' if power is lost. The manual also says that the proper shutdown procedure should always be used, eg when changing batteries, since if power is suddenly cut the operating system may be scrambled.

On Wed, 08 Apr 2020 06:43:22 -0700, towsked wrote:


Has anyone reported this problem to LXV, and if so what was their
response please? There seems to be an opportunity for a firmware fix to
this problem, not requiring any additional hardware.

In the specification for Flight Recorders, a new IGC file is not
started if power has been interrupted for less than one minute. This is
to allow a change of battery, breaker reset or change a fuse. Exactly
the problem we are discussing.

If the LX firmware were enhanced to inhibit re-calculating QNH where
power has been lost for less than n seconds, this problem goes away.

Discuss.


The LXNav manual says that the internal logger will continue to operate
'for a short time' if power is lost. The manual also says that the
proper shutdown procedure should always be used, eg when changing
batteries, since if power is suddenly cut the operating system may be
scrambled.

What OS does LXNav use these days?

I'm asking because this sounds remarkably like similar problems with
WinCE, where it was well-known that powering off without shutting down
properly was almost as certain to corrupt the SD card as yanking the
card out of a running PDA.


--
Martin | martin at
Gregorie | gregorie dot org

On 4/8/20 7:54 AM, Martin Gregorie wrote:
On Wed, 08 Apr 2020 06:43:22 -0700, towsked wrote:


Has anyone reported this problem to LXV, and if so what was their
response please? There seems to be an opportunity for a firmware fix to
this problem, not requiring any additional hardware.

In the specification for Flight Recorders, a new IGC file is not
started if power has been interrupted for less than one minute. This is
to allow a change of battery, breaker reset or change a fuse. Exactly
the problem we are discussing.

If the LX firmware were enhanced to inhibit re-calculating QNH where
power has been lost for less than n seconds, this problem goes away.

Discuss.


The LXNav manual says that the internal logger will continue to operate
'for a short time' if power is lost. The manual also says that the
proper shutdown procedure should always be used, eg when changing
batteries, since if power is suddenly cut the operating system may be
scrambled.

What OS does LXNav use these days?

I'm asking because this sounds remarkably like similar problems with
WinCE, where it was well-known that powering off without shutting down
properly was almost as certain to corrupt the SD card as yanking the
card out of a running PDA.



The Stratux ads-b receiver has the same issue, high probability of
corrupting the memory if you don't shut it down manually before removing
power. Developers are aware of the problem, but haven't come up with a
proper fix.

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

-Dave

I havent had to switch batteries in flight lately but my power flarm will reset and start a new log it interupted in flight. Very annoying as I use it almost exclusively for logging as its download is very fast and simple. My backup logger is the venerable SN10B, it does not reset and continues right where it left off, Thanks YO! I have thought about adding a cap to the circuit for the powerflarm. Lots of good ideas here so far but I am trying to keep it as simple as possible.

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.

On Wednesday, April 8, 2020 at 12:00:15 PM UTC-4, wrote:
... the venerable SN10B, it does not reset and continues right where it
left off, Thanks YO!

Yes, I designed SN10 this way decades ago, because, Big Surprise!!
....glider instruments see power interruptions !!!
The SN10 has an internal short-term backup supply.
When the power is interrupted, it does a smooth shut-down (OS I wrote).
SN10 memory has a long-term backup battery, and next time power is applied,
the SN10 resumes wherever it left off. SN10s never do a hard reboot unless
new software is installed.

Of course, we have had SN10s damaged when customers added a capacitor
to their logger (powered thru SN10), and the inrush current eventually
blew up the SN10 power switch. Also some loggers had giant internal caps
to try prevent corruption, however...

Sensible design dictates avionics have internal short-term supplies
with power-interrupt sensing and implement smooth shutdown, especially
where they write to flash for logging, saving configuration, etc.
Otherwise things like SD cards or internal flash get corrupted with
data loss when power is interrupted during a write operation.
Of course, lots of design out there is not sensible...

Hope that helps clear some of the confusion,
Best Regards, Dave

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.


--
Martin | martin at
Gregorie | gregorie dot org