[aprssig] balloon tracker

[Scott Miller]

> Energizer e^2 lithium AA.  Spec sheet says good to -40C, but doesn't
> give a voltage vs. temperature curve.  I was driving the transmitter
> and amp directly with 6 of these in a nominal 9V battery, as well as
> driving the OT's built in regulator.

Let's see, that looks like about 0.31 watt/hours per gram.  Not bad... but I
think the fact that they've got to keep the voltage at 1.5 to work in
consumer devices adds some limitations to the design.  I'd trust the higher
voltage industrial batteries more for extreme stuff like this.

Tadiran gives a voltage vs. temperature curve for their 2.1 Ah AA cells
here:  http://www.tadiranbat.com/prodpdf/viewpdf.php?datasheet=TL-2100.pdf.

> >   Are you sure it was the batteries freezing that killed it?
>
> Not absolutely sure, but I can't think of anything else.  The output
> looked like this:

I think some people have reported GPS receivers freezing up, too.  I'm sure
some folks here could tell you more about packaging them to keep them warm.

> It's entirely possible that they didn't freeze entirely, but the
> voltage got so low that the transmitter didn't have enough to drive it.

That'll do it.  I find it's pretty easy to do basic cold testing with a
cheap foam cooler and 10 or 20 pounds of dry ice.  Praxair carries it around
here, pretty cheap.  You can control the temperature to some extent by
raising the device under test up and away from the dry ice a bit.

> For this reason as well as size and weight, I need to do a DC-DC
> converter and just drive the rig from two or three of these.  Just
> haven't had time...  If I don't have time before the next flight, I may
> use a 12V pack instead of a 9V, and hope the voltage stays high enough
> to suit the transmitter.

I just looked up the specs on the lithium D cells - 19 amp hours at 3.6
volts!  But they're about $16 each, and Digi-Key only carries them by the
case (120).  I want a ruggedized LED flashlight with some of these, so I can
keep it on the shelf for 10 years and still have it run for weeks.  =]  Even
with NiMH D's, my converted maglite runs for 6 days before it starts getting
dim.

Anyway, packaging the payload to keep it warm is probably the easiest way to
go.  It's just not as much fun as playing with exotic batteries and surplus
NASA heating elements.  =]

Scott
N1VG