This circuit is intended for charging sealed lead-acid batteries with a solar panel in small and portable applications. The customary diode that prevents the battery from discharging through the solar panel has been replaced by a FET-comparator combination. The charger will stop charging once a pre-set voltage (temperature compensated) has been reached, and recommence charging when the voltage has dropped off sufficiently. The load is disconnected when the battery voltage drops below 11V and reconnected when it gets back to 12.5V.
If you like these design features, also have a look at OpenD's Solar Charger, it has some very interesting solutions. For a simple solution for using a solar panel for USB charging, look here.
The circuit has the following features:
Note that the charging current is limited only by the solar panel used.
Here's the circuit:
It has been working fine for a year now (unlike my first attempt), although I suspect an unwanted state still occurs every now and again. Suggestions for improvement are welcome.
I've made a PCB design, it's in CorelDraw 4 format. After the usual handiwork, I ended up with this:
Note that I added three DC/DC-converters (for 9, 6 and 3V) on the PCB, the actual charger is less than half of the PCB. I don't have a silk screen, so if you want to build this, you'll have to figure out the PCB for yourself.
With all the components into place, some additional electronics to get the DC/DC-converters in and out of standby, two small SLA-batteries (2.2Ah each), some odd parts and wiring, a fuse, a front plate and housing, the end result looks like this:
A final word: you build everything at your own risk, no functionality is implied, the resulting apparatus might not work, be unfit for drying pet dogs, and contain small parts that could suffocate children when swallowed or inhaled.