Battery switching without tears

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

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 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

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