Solar powered deer deterrent upgrade
Still no no signs of deer
damage to the garden this year.
I finally got around to
upgrading the solar
powered deer deterrent from the Alkaline D cell to a pair of
rechargeable AA batteries just in case we need a mechanical deer
deterrent on short notice.
The batteries were a little
undercharged at only 1.2 volts, but I'm trying a series circuit which
will double that to 2.4 volts DC. My thinking is that the motor stops
turning just under 1.3 volts, and even though it turns a bit faster
with 2 batteries it's not too fast and the increase should help boost
the longevity factor.
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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.
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Mark, what was the voltage of the original battery in the solar-powered light? If you deviate a lot from that it is likely that the power output from the panel will decrease sharply. Above a certain voltage, the power production of a solar panel drops like a rock.
If you don't know it, you can measure the power curve your the panel with a multimeter and some resistors. The linked webpage also does a good job of explaining what the maximum power is. BTW, with the small panel you're using for this, I don't think you need special high-power resistors to do the measurements.
There is also a webpage about building a solar battery charger that might be useful. Observe that you need a diode to prevent the batteries from discharging through the panel!
And this PDF tells you why and how to build an MPP tracker.
You really need to rip out that LED, and anything else you don't need. If there is a diode, it should have a model number on it. With that you should be able to find the datasheet, which will tell you what it can handle. Who knows, if you open the unit up, there might even be a model number or a spec on the solar cell?
But the best you can do is take two measurements of the solar cell with your multimeter. Disconnect the panel from everything, and then only connect the multimeter. First put it in DC voltage mode. What you measure in that case is the open circuit voltage ("Voc"). Next, switch the multimeter to DC current mode. What you then get is the short circuit current ("Isc"). Note that both Voc and Isc will vary according to both temperature and sunlight! At least measure them at different amounts of sunlight (e.g. full sunlight vs. overcast day). The maximum power point is usually just below Voc, with a current that is just below Isc. (Google for "solar panel IV curve" to get an impression of how these curves look.)
If the measured Voc is much less than your battery voltage, you won't get much charging done unless you add a boost convertor. If it is much higher, you might damage the batteries, unless you add at least a voltage regulator (78xx) in the charging system or add more batteries.