Solar Battery Charger V2
After building Solar Battery Charger V1, I decided I didn't like the approach of constantly dumping current into the battery after it was fully charged. What I really want is a float charger. The V2 design is the next attempt.
The idea behind V2 is that a microcontroller periodically samples the solar panel voltage and the battery voltage. If the panel voltage is less than the battery voltage, no charging takes place, and the microcontroller goes to checking the panel voltage once every 10 minutes. As the microcontroller is being powered by the battery at this point, we want to absolutely minimize the current drain.
Once the panel voltage goes above the battery voltage, the battery voltage is periodically checked. If it's below the float voltage (corrected for the current temperature), then the charger is enabled. Although a 5 watt panel is being used, which intrinsically limits the charge rate to around 325ma (well below the .1/C charge rate for most batteries), the circuit limits the charge rate to around 25ma. Since the charger is only used for keeping the battery topped off and not for a hard charge, there's no reason for a higher charging rate.
Several LEDs indicate the current state of the charger. A green LED indicates the panel voltage is present and sufficient for charging. A yellow LED indicates the battery is being charged. A red LED indicates the panel is providing sufficient voltage but the battery voltage is less than 10 volts (and therefore probably not connected).