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

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

COLIN LAMB wrote:
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.

I'm flying with the diode setup. The best diodes I was able to find in
the UK were Fairchild MBR1035 Schottky diodes. These handle 35 amps and
come in TO-220 packages. Their spec quotes a 0.57 v drop, which seems
about right: with 2 x GPS II+, EW-D logger, SDI C4 and B.40 varios all
on the C4's internal voltmeter shows 11.6 v on two fully charged 7 AH cells.

I need to fit a radio and am thinking of fitting a Filser ATR-500. Can
anybody tell me if that will be OK on the end of the diodes or would I
be better to discard the diodes and use one battery to drive the ATR-500
and the other to run the GPS, logger, and varios?

Another reason I'm wondering about rewiring to separate the batteries is
that if we get landed with transponders I assume I'd be better off using
one for radio and transponder and reserving the other for the
GPS/logger/vario setup. Comments?

The other choice of radio would be a Microair 760. I assume that, as its
rated for 12-14v it would be quite unhappy on the end of the diodes and
just barely usable on a separate battery. I see that Maxim sell a range
of solid state voltage boosters (there is a model that can output 14v at
2 amps). Has anybody tried using one of these to drive a Microair radio?
If so, how well did it work?




Colin Lamb




--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

The Filsner is unlikely to have a problem through the diode setup you
have now. (not that I like the diode setup but it should work - BTW you
should be able to find lower voltage drop diodes).

For the Microair I would not read too much into a nominal voltage spec.
It is hard to tell wether the manufacturer is quoting a real absolute
voltage range or the nominal voltage. On the other hand I'd not want to
inflict anybody with a Microair radio (oops I can feel the flames
already). They have a very bad reputation where I fly. The several I've
seen installed have had problems, especially apparent heat related
problems with the displays getting garbled and then finally the whole
radio going out to lunch (and I'm not talking really hot days). Other
brands seems to be much more reliable. Why not go with Becker?

Personally I'd stay away from trying to get too fancy with regulators
etc. I want the supply to be as dumb and simple as possible. Just
batteries and circuit breaker or fuse and master switches. As few
connectors or solder joints as possible. And I personally I like
running one battery at a time - I want to see the health of each
battery and know about how much capacity I have in both batteries and
know I can switch in a reserve if I run a battery down -- which I may
not notice until it is too late. Like many of us in the Western USA I
fly over pretty desolate areas and I really want to know roughly what
battery capacity in reserve.

If you are goigg to install a transponder, have lots of avionics toys,
expect long cold flights it may well be you need to look at moving up
to larger capacity batteries or installing solar panels.

Darryl Ramm


Martin Gregorie wrote:
COLIN LAMB wrote:
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.

I'm flying with the diode setup. The best diodes I was able to find in
the UK were Fairchild MBR1035 Schottky diodes. These handle 35 amps and
come in TO-220 packages. Their spec quotes a 0.57 v drop, which seems
about right: with 2 x GPS II+, EW-D logger, SDI C4 and B.40 varios all
on the C4's internal voltmeter shows 11.6 v on two fully charged 7 AH cells.

I need to fit a radio and am thinking of fitting a Filser ATR-500. Can
anybody tell me if that will be OK on the end of the diodes or would I
be better to discard the diodes and use one battery to drive the ATR-500
and the other to run the GPS, logger, and varios?

Another reason I'm wondering about rewiring to separate the batteries is
that if we get landed with transponders I assume I'd be better off using
one for radio and transponder and reserving the other for the
GPS/logger/vario setup. Comments?

The other choice of radio would be a Microair 760. I assume that, as its
rated for 12-14v it would be quite unhappy on the end of the diodes and
just barely usable on a separate battery. I see that Maxim sell a range
of solid state voltage boosters (there is a model that can output 14v at
2 amps). Has anybody tried using one of these to drive a Microair radio?
If so, how well did it work?




Colin Lamb




--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

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

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.

Testing AH of batteries

Battery capacity is measured using a defined minimum voltage. That voltage
may or may not correspond to the minimum voltage your glider equipment will
operate on. If you use diodes, that minimum voltage will move slightly.
Capacity also depends upon a starting voltage - which is significantly
higher than 12 volts.

I often need to test batterys used in our search and rescue radios and have
found that I can test battery capacity quickly without a full discharge. I
charge the batteries, then let them sit for a day. Then, I put a load on
them and simply watch the decay of voltage over time. In a very short time,
you can make a graph that will indicate the trend of the battery and compare
with a new battery. A battery with reduced capacity will drop voltage much
more quickly.

You can determine the capacity of the battery during charge, too. Capacity
is the ability of the battery to resist change. That applies either way.
it means the battery will drop in voltage or charge more slowly. So, you
can simply time the charge of two batteries and learn the comparative
capacity of each.

Colin