Solar charging a battery with Arduino?

I am making a solar rover robot using an arduino duemilanove. I have a 12 volt solar panel and a 9.6 volt nicad battery. I am wondering if this circuit I sketched http://www.flickr.com/photos/56541142@N02/5229716662/ would allow me to prevent the battery from over charging. The battery is monitored by dividing the voltage and connecting it to analogue input on the arduino. When the battery is fully charged, the arduino sets an output pin HIGH and breaks the connection between the panel and the battery using a PNP transistor. Does the transistor subtract current from the circuit? In the open state will there be more than 9.6 volts entering the battery?

here are some links to the robot so far! (without solar installed)
http:// http://www.flickr.com/photos/56541142@N02/with/5229716662/

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