I had a similar question. I'm planning on charging a bank of NiCa's . What I was gonna do was to build a voltage regulator for a 6V solar panel to charge 4 NiCa's in series and using a separate small solar panel like found on $3 yard lights to activate a logic switch(I'm not sure that's what it's called). When the voltage drops do to lack of light the switch would turn on. Any hel[p would be appreciated. I talk big but know very little.