build a solar panel

Re: build a solar panel

You need to know the voltage and current required by the LED's you want to drive.

Typically, "simple" outdoor lamps use 2 NiCad AA batteries to power them. NiCads take deep discharge well (down to 0 volts) and can be charged without a complex regulator (or with smaller solar panels, without a regulator at all if less than C/5 or C/10 charge rate--i.e., more than 5-10 hours to charge the batteries).

If you need higher voltages to power your LED's--you either need to put more batteries in series or use a small "boost" converter (kits and completed boost converters designed to drive "flashlight" LED's are around and relatively cheap and efficient) to drive the LED from low voltage batteries.

The problem is that with more than two batteries in series--if you take them dead (fully discharged), the two "slightly larger capacity" batteries can reverse charge the one battery that went "dead" first--which pretty much kills the battery.

Also, LED's usually require some sort of current control for optimum operation (LED's are diodes, which are non-linear devices and will either not be very bright or too bright and overheat if directly connected to a large battery).

So--you can size the battery pack using NiCads to the size of your LED's, use a boost mode current controlled converter to drive the LED (kit or built), and a diode to block current flow back into the solar panel at night, and you probably want some photo control to turn the LED off during the day and on at night (I have not looked--but you can probably copy the circuit from an old outdoor yard light).

One big issue is picking your LED's--Many of the "cheap" LED's do not seem to last very long. They will drop to to 1/2 brightness or less in a year or less when used as yard lights.

-Bill