What effect does replacing the load on a switch-mode power supply with a battery have on design?
Here's a really rough conceptual model of your system (in a real switcher SW1 would be a FET).  The important thing to note is that if D1 is really a diode, then it simply won't conduct backwards.
To gain efficiency, modern switching converters use FETs for both the switch and the diode, in which case the thing could back-conduct from the battery to the solar cell -- but you could take care of that in your control circuitry.



simulate this circuit &ndash; Schematic created using CircuitLab