December 7, 2008
|As part of the GeoTransmitter project (http://www.thunter.org/geothunt)
I installed a solar panel (1.5 W unit from Harbor
Freight) into the GeoTran (Squawkbox + NiCd battery).
This note investigates the issues surrounding this
effort. There are two questions to be answered: Can the
panel keep up with the transmitter and How much battery
is required for 24/7 365 operation.
transmitter is a constant-current load. It draws 65 mA
when transmitting and 5 mA when in standby for any
source voltage from7 to 18 V. For a duty-cycle, Du, of
0.1 the current requirement for the transmitter is: Iave
= (1-Du)*5+Du*65 or 11 mAH. or 264 mAH/Day. This has to
come from the battery or from the solar panel or both. A
fully charged 2000mAH battery will last about 7 days.
My requirement for the transmitter was that as it was a
new idea for the region, that it had around 15-20
seconds of voice to explain what it was and what to do
about it. This made the repeating interval at 3 minutes.
This is about as long as you want to go as the hunting
of something that is not on for long periods is harder
than you may want.
Now dealing with the solar panel is a complex affair.
The little panel is a constant current source for load
voltages from 12 to 0 volts and is rated at 12 volts for
1.5 watts. This is the max with bright noon sun straight
on the panel. This comes out to a current of 125mA. As
the sun rises and sets the current will vary like a half
cosine of the sun angle due to the geometry of the
illuminated panel surface and it will also be slightly
less than this as the sun is less intense near the
|The solar folk simplify this be conceiving a square
wave that has the same area under the curve as the
squished cosine curve with the peak of the square wave
being the max output of the panel and the duration of
the square wave being such as to match the area between
the two curves. Typically, in the summer this comes out
to be 7 hours and in the winter 4 hours. Of course this
will vary from place to place. Here in Southern
California I measured 5 hours on Dec 6, very near the
winter solstice (shortest day) on a bright sunny day,
but on a cloudy overcast day it was 0.4 hours.
solar folk also use 80% as the efficiency of charging
batteries. So the 125 mA available from the panel will
result in 100mA of charge. For the 5 hour sunny day it
would yield 500 mAH of charge. This is more than the 264
mAH daily consumption by 236 mAH or about enough to run
almost another day without any sun at all. Even a cloudy
day will make some electricity. But not much. My cloudy
test day only made 60 mAH. This is a deficit of 204 mAH
The next question is how many days do you need to run
the transmitter without sun. Here in Southern California
it is about 5 days. This is _____mAH of charge. The
batteries should be capable of at least this capacity.
Now it the batteries do go down they will come backup
when once again the sun shines. No harm other than no
signal for a while. On the other end the battery
capacity should not be too small. The peak charging
current of 125 mA should not exceed 1/10 of the battery
capacity (by much) so at least 1000mAH is the minimum
battery size. The overcharging is not a problem at this
0.1C current level as the cheap Harbor Freight panel has
no shutoff for a charged battery. You can get packs of
NiCd sub-C cells at 7.2 V in 1200, 1500, 1800, 2000,
2500, 3000, and 4000 mAH with prices from $8.00 to
$75.00 for use in R/C cars. You can also parallel them
for more AHs.