2-Batteries

On Tue, 26 Dec 2006 01:39:44 +0000, Steve Paavola wrote:

Gary Emerson wrote:

wrote:
Better yet is to use diodes so that both batteries will always be "on"
in parallel and you're always pulling from the best battery with no
fiddling required from the pilot. Relatively low voltage drop diodes
are available with 18 Amp forward capacity.

If both batteries are on-line all the time, how do you know when one is
getting weak and needs to be replaced? Or do you replace both batteries
when voltage is marginal at the end of a flight?

I used two small diodes to provide a "fail safe" supply to the
Volkslogger, but the radio, vario, transponder etc get theirs via a 2 way
toggle switch.

Then when one battery dies in flight, I will notice and switch to the next
one manually. But in the meantime the logger always draws current from the
highest voltage battery and hence no interruption to the trace.

The diodes are equivalents of IN 5817 "Shottky rectifying diodes". They
are cheap and tiny. At the low currents drawn by the logger, the forward
voltage drop is very small (less than 0.2V). No cooling is required. I
soldered them directly in line with the wires from the fuse holder to the
toggle switch, without a circuit board.

I test the voltage on each battery during my pre-flight by setting the
Volkslogger on its battery voltage function and disconnecting the
batteries one at a time. (If the electric vario had a battery voltage
function, I could switch it between the batteries and measure their
voltage in flight.)

The weak link in this setup is the toggle switch which has to be sized to
handle a significant DC current.

(The u/c warning buzzer is now also running off a 2 diode "fail safe"
supply - but there is another story behind that ...).


Regards


Ian

"Regarding the 3 position switch. The simple solution is to put a
capacitor between ground and the output side of the switch. The
capacitor must be large enought to provide the current needed during
the switch switching time. This should not be a very large capacitor
for our purposes."

This brings up another issue. Switches may be rated for ac or dc (or both).
Many of the switches sold at the local electronics supply house are not
rated for dc. And, some may be less reliable than others. Many are not
rated for the current you want to run through them. That means likely a
failure. It could also be worse, with a possible fire.

When you get to rotary switches, contacts may be configured to make before
break, which means the rotor will contact the new contacts before
disconnecting from the old. This is another subject.

Colin

This is how I finally set up my glider--with two main battery switches.
But if you want to run this way without bouncing the logger, the two
switches should be on for the whole flight. You don't want to run one
battery down like a fuel tank, then switch when it is empty. If you do,
you will get a big current spike when you switch them both on. At best,
this wastes energy from the new battery by dumping it into the dead
one. You can also burn-out your switch, weld the contacts, or blow a
fuse.

What you want to do is run two identical batteries, bought at the same
time from the same production batch. Then always charge and run them
together. They should age together, and share the load properly this
way. You only want to use the two switches in opposite positions to
separate the batteries for testing, or if one battery fails.

wrote:
By all means a second battery should be installed in our electrically
driven modern sailplanes. After many years of quickly flipping my
3-position battery switch, and trying not to have my logger to
momentarily dropout, I have concluded that is best to use 2 single-pole
battery switches. That way one can have either or both batteries
connected at the same time.

I saw the light when Jim Hendrix brought his sailplane to Caddo Mills
for Wing Deturbulator flight testing, and it was wired like that. You
will hear much more about that amazing new invention at the coming SSA
Convention.

Thermally,

Dick Johnson

Steve Paavola wrote:

If both batteries are on-line all the time, how do you know when one is
getting weak and needs to be replaced? Or do you replace both batteries
when voltage is marginal at the end of a flight?

I've been using the 2 toggle switch solution for years. I replace a
battery when it discharges within 5 hours. This means I've always got at
least 10 hours worth of battery (assuming I've recharged the batteries).

It doesn't really matter, when one goes bad, you should replace both.
Not only will a new battery not load share properly with an old one, if
one battery in the set failed, the other can't be far behind.

The same thing goes for your two bulb light fixture at home. If you
only change one bulb at a time, you will just be changing bulbs twice
as often.

Hi,

I agree that it would be a nice product to offer. Larry Goddard reads this
group, hopefully he'll come up with a good solid product to offer. I'll
copy him on this note as well.

I must admit that I'm not sure I like the automatic switching approach - I
guess I just like to be in control of which battery is being used. The idea
of 2 battery switches and diodes to stop flow seems OK to me, but I'm not
sure it is necessary. I have always used 1 single 3 position switch (Batt
1/Off/Batt 2). I have never had a problem making a fast switch from battery
A to battery B. I wonder if there is a perceived need for something that
really isn't necessary. But I'm not an expert on batteries or diodes.

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com


"Gary Emerson" wrote in message
. net...
A quick Google of Schottky Diode and TO-220 yielded some sources.

I just tried the first one and then found this:

http://www.onsemi.com/PowerSolutions...do?id=MBR2515L

looks like even slightly better than what I had bought.

You'd lose about 0.3 volts during normal operation.

Datasheet at:

http://www.onsemi.com/pub/Collateral/MBR2515L-D.PDF

I'm sure some googling will find that someplace like DigiKey will have
these and will take small ordering quantities.

Is Paul Remde out there? Might be a nice "kit" to add to your line. Kit
includes 4 diodes, 4 heat sinks, 4 insulating washers some heat shrink
tubing, etc.??

Good luck.




Gary Emerson wrote:
Nyal Williams wrote:

I'd like to see a full write-up with diagrams, photos,
and names/addresses of places to buy parts. This would
make a good article for Soaring.


Here is a sketch

http://www.mydatabus.com/public/emer...y/e/sketch.JPG

and a pic of my install...

http://www.mydatabus.com/public/emer...y/e/diodes.jpg

There is a aluminum bracket which is lightly mounted to the bulkhead. The
PC board just provides insulation and some support for the wires.

Each diode has a heat sink, but I don't really think it was necessary.

I don't have the part numbers handy, but I'll see if I can't find
something.

With 18Amps of forward current capability (each), there isn't really any
"need" for the two diodes in parallel, but for an extra $5 and a few
extra minutes of wiring, if any one diode ever did fail "open" the other
would still provide power. Most likely overkill.

Paul Remde wrote:
Hi,

I agree that it would be a nice product to offer. Larry Goddard reads this
group, hopefully he'll come up with a good solid product to offer. I'll
copy him on this note as well.

I must admit that I'm not sure I like the automatic switching approach - I
guess I just like to be in control of which battery is being used. The idea
of 2 battery switches and diodes to stop flow seems OK to me, but I'm not
sure it is necessary. I have always used 1 single 3 position switch (Batt
1/Off/Batt 2). I have never had a problem making a fast switch from battery
A to battery B. I wonder if there is a perceived need for something that
really isn't necessary. But I'm not an expert on batteries or diodes.

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

I have the same setup as Paul, and it works fine. However, I have a 302
-- when I had a Volkslogger, this setup would create two separate IGC
files when you switched from one battery to the other.

Seems to me that in order to take care of the Volkslogger,
the ideal switch would be a 3-positionswitch: 1.
Battery A; 2. Battery A&B in parallel; 3. Battery B.
Switching rapidly from 1. through 2. to 3. would prevent
interruption in power without raising voltage or otherwise
causing damage, and position 2. could be used when
and if both batteries are somewhat low.

Does such a switch exist that provides such an overlap?


At 18:12 27 December 2006, Greg Arnold wrote:
Paul Remde wrote:
Hi,

I agree that it would be a nice product to offer.
Larry Goddard reads this
group, hopefully he'll come up with a good solid product
to offer. I'll
copy him on this note as well.

I must admit that I'm not sure I like the automatic
switching approach - I
guess I just like to be in control of which battery
is being used. The idea
of 2 battery switches and diodes to stop flow seems
OK to me, but I'm not
sure it is necessary. I have always used 1 single
3 position switch (Batt
1/Off/Batt 2). I have never had a problem making
a fast switch from battery
A to battery B. I wonder if there is a perceived
need for something that
really isn't necessary. But I'm not an expert on
batteries or diodes.

Good Soaring,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com

I have the same setup as Paul, and it works fine.
However, I have a 302
-- when I had a Volkslogger, this setup would create
two separate IGC
files when you switched from one battery to the other.

Greg Arnold wrote:
I have the same setup as Paul, and it works fine. However, I have a 302
-- when I had a Volkslogger, this setup would create two separate IGC
files when you switched from one battery to the other.

The 302 has an internal capacitor across the power supply lines, the
Volkslogger does not. Volkslogger owners can solve this problem by
adding an external capacitor, as is suggested in the manual...

Marc

I have the same setup as Paul, and it works fine. However, I have a 302
-- when I had a Volkslogger, this setup would create two separate IGC
files when you switched from one battery to the other.

I agree as well.
I used to switched an L nav on each flight with no problem.
Now that I use a 10.5 amp/h battery I rarely switch.
Hence I only have to take out one battery for charging.
I would use the same battery for about 5 flights and then rotate.
Udo

Nyal Williams wrote:
Seems to me that in order to take care of the Volkslogger,
the ideal switch would be a 3-positionswitch: 1.
Battery A; 2. Battery A&B in parallel; 3. Battery B.
Switching rapidly from 1. through 2. to 3. would prevent
interruption in power without raising voltage or otherwise
causing damage, and position 2. could be used when
and if both batteries are somewhat low.

Does such a switch exist that provides such an overlap?

Sure it's called "make before break" some one already
has mentioned it.
...lew...