Are the panels capable of sourcing more power than the batteries/system can use? In other words are you using the solar panels for charging or range extending? You could use your arduino (or an additional one just for charge control) to monitor the battery level and to turn off the connection to the solar panel when the batteries are charged and then turn it back on when the charge level drops. If the panels can't supply more power than the system uses then you could probably get away with just using the battery and other circuits as load and not worry about overcharging. Put 2 switches - one for the solar panel and one for the battery. That way you could use the solar cells to charge when the gizmo is not running.