Cheeep home made solar battery for wireless sensor nodes made from bargain store LED garden lights on a plastic pail lid.
Ingredients:
7 solar LED lights
1 plastic disk
1 battery snap
1 diode
wire
Instructions:
Mount everything. Wire solar cells in series. Wire NiCd cells in series. Wire solar string to battery string using diode. Wire battery snap to battery string. Weatherproof and install as desired.
Here is wireless sensor node from Wicked Device transmitting battery voltage over almost 3 days.
After sunrise voltage increases, each night the node drains the battery, each day a little more voltage is accumulated.
Schematic for Solar Battery
The diode is not critical, I used a 1N4148 switching diode, a 1N4007 or similar world work as well.
A few important details:
For a power source, the wire colors are reversed on the battery snap.
There is no charge regulator in this very simple circuit. In order to guarantee the battery stays full, it must be consistently overcharged. Batteries that can tolerate this are lead acid and NiCd, and only at a small trickle current.
WARNING DO NOT ATTEMPT THIS WITH OTHER BATTERY TYPES SUCH AS LI OR NIMH WHICH CAN RUPTURE< BURN< EXPLODE AND EXHIBIT GENERALLY RUDE BEHAVIOR
The solar cells from the LED lights are pretty small and weak, in full direct sun the maximum battery charge current is 22 mA. The NiCd cells are 1200 mA-hour, so the maximum is around a 50 hour rate or C/50.
For NiCd cells, limit the maximum rate to somewhere between C/20 (1200/20 = 60mA) and C/100 (1200/100 = 12 mA). Above 60 mA the cells may lose electrolyte and dry out. If the charge current never rises above 12 mA the electrodes may crystallize excessively and reduce capacity.
This battery provides plenty of power to perpetually run small wireless motes such as Dirt Cheep Dumb Wireless and Wicked Node. The solar power collected over 24h easily exceeds the demands of the node, and the trickly current is low enough that the NiCds should last for years.
The same principle can be applied to larger solar arrays and loads, sizing the solar array to the load and making sure the battery is sufficient to handle the overcharge. At some point you have enough $$$ in the solar and battery that a real honest charge regulator becomes worthwhile investment.
I believe that was meant to say that you don't have to be too fussy about WHICH diode you use, but not that you don't have to bother with SOME diode, which I fear some readers may infer?
