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.