Buck converter charge battery control output voltage
If you used a synchronous converter, its duty cycle x input voltage is pretty much the output voltage when averaged. If you are intent on doing this without feedback you should strongly consider a synchronous solution - it has a high side and a low side MOSFET. In effect it is a half bridge solution.

Having said all that, you need to modify duty cycle on the fly to cater for changes in input voltage levels so this requires some analogue signal processing.