Solar Powered
Hidden Transmitter
December 7, 2008
Bob, WB6JPI

 

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.

Strangely, the 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 horizon.
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.

The 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.

 

Jippy