2-Batteries

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

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

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. For redundancy, I used two
in parallel on each battery. The diodes are available in the TO-220
package and it's easy to incorporate a small heat sink, but I have no
reason to believe they ever attained any temperature at all.

With two batteries connected with switches, if one battery does really
"die" then any time spent with both switches in the "on" position causes
the good battery to attempt to charge the "dead" battery to no avail, so
ultimately you're wasting what power you do have during this time. The
diodes eliminate any chance for cross charging...or discharging. Used
this system for several years, never experienced a single power issue.

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. For redundancy, I used two
in parallel on each battery.

If it is always drawing from the best battery, what is the time
interval between switching.
What controls the switching. Please advise.
Udo

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.


At 19:48 25 December 2006, Udo 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. For redundancy,
I used two
in parallel on each battery.

If it is always drawing from the best battery, what
is the time
interval between switching.
What controls the switching. Please advise.
Udo

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. For redundancy, I used two
in parallel on each battery.

If it is always drawing from the best battery, what is the time
interval between switching.
What controls the switching. Please advise.
Udo

There is no switching. As the higher voltage battery drops in value,
gradually a greater % will start drawing from the other. From that point,
they will discharge equally. Assuming there is slightly different capacity,
current will gradually increase from the battery with the greater capacity.

Most diodes have about a .6 volt drop, which may be significant. Therefore,
you will need diodes with a low threshold. Schottky diodes should be about
..3 volts drop. You can measure the actual drop using a digital
volt-ohmmeter. There is a diode test range and it will show the voltage
drop.

Colin Lamb

Gary Emerson wrote:
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

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. For redundancy, I used two
in parallel on each battery. The diodes are available in the TO-220
package and it's easy to incorporate a small heat sink, but I have no
reason to believe they ever attained any temperature at all.

With two batteries connected with switches, if one battery does really
"die" then any time spent with both switches in the "on" position causes
the good battery to attempt to charge the "dead" battery to no avail, so
ultimately you're wasting what power you do have during this time. The
diodes eliminate any chance for cross charging...or discharging. Used
this system for several years, never experienced a single power issue.

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).

Steve

"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?"

Well, that is a problem. You do not know. Everything is automatic. If you
have one battery that has lost most of it's capacity, the good battery will
do all the work. The only way you will know is when the total capacity of
both batteries has been reduced. And, then, you will not know whether one
battery has done 50% of the work and they are both down in storage ability,
or one battery is still 90% and the other one is 10%.

So, you will have to determine the capacity of each battery separately. You
could test the battery capacity by switching either off during discharge, or
even by using 2 ammeters - but it is becoming more complex once we try to
extract more information. You can also determine the capacity during
charge.

Colin

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.

COLIN LAMB 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?"

Well, that is a problem. You do not know. Everything is automatic. If you
have one battery that has lost most of it's capacity, the good battery will
do all the work. The only way you will know is when the total capacity of
both batteries has been reduced. And, then, you will not know whether one
battery has done 50% of the work and they are both down in storage ability,
or one battery is still 90% and the other one is 10%.

So, you will have to determine the capacity of each battery separately. You
could test the battery capacity by switching either off during discharge, or
even by using 2 ammeters - but it is becoming more complex once we try to
extract more information. You can also determine the capacity during
charge.

Colin



During any one flight, you don't need to know. What is a good idea is
to test the batteries if you think you are losing capacity. During the
week charge them up Monday and Tuesday, and Wednesday put a load on each
one and see how long it lasts. Ideally, you'd like the load to test the
batteries for as long as you'd normally fly. Say 4-6 hours unless you
are typically flying for longer. If either one can't go the 4-6 hour
distance and mainain a suitable voltage, then it's due for replacement.
After the Wednesday test, charge them back up so you're ready for the
weekend assuming they pass. I'd certainly do this in the spring and
before any big contest during the year. 7Ah batteries are cheap in the
grand scheme of things.

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 thought all loggers just started again after a momentary power loss,
and continued the same flight file. It sounds like yours isn't behaving
properly, or are some brands a bit touchy?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org