Circuit for charging RC car battery with solar
As you have described it, your solar cell configuration will not be able to charge the battery. To fully charge a "6 volt" NiCd battery you need a charging voltage that is somewhat higher than that...something like 8.4 volts.

Charging a 700mAh battery at 240mA could be dangerous, it's hard to know for sure without having any specifications for the battery. To make sure the charging is safe you would want to limit the current to about 1/10 of the battery's capacity, or 70mA in this case. Of course, that means it would take at least 10 hours to charge the battery and probably more like 20 hours.

Charging at higher rates requires pretty careful monitoring of voltage, current, and/or battery temperature. This is turning into a very broad topic so I suggest you do some more research and come back with specific questions.