Small battery for multiple small charges
I would have to agree with Elliot Alderson. supercapacitors would probably suit your needs the best. If the main source of charging are photo-voltaic cells, which will produce a varying voltage (depending on the light), then super-caps are by far the easiest to design with. 

According to an article by Battery University, it says that "supercapacitor[s], rated in farads, which is thousands of times higher than the electrolytic capacitor. The supercapacitor is used for energy storage undergoing frequent charge and discharge cycles at high current and short duration"

An RC car will require high amounts of current at certain times while taking no current at others. As well, a super-capacitor can tolerate the continuous pulses of high current needed to power the RC car. As long as the RC car is small enough and does not require too much current, I think the idea of powering it with supercapacitors is feasible. Now, I do not know who long you could run the RC car on a supercapacitor, but as long as it is pretty sunny, the idea remains mainly feasible. 

Now there is an issue on how one could charge the supercapacitor while also drawing current for the RC car. You could solve this by either having two supercapacitors and switching in between, one that gets charged and another that supplies power to the RC car, and when one of them gets drained, you switch. This would require complex circuitry, but yet again is feasible.