[dcb]

I might suggest a dc-dc converter of some sort. Conceptually you take the solar panel output and drive an oscillator with it, then run that through a transformer to step it up to the voltage you want (> 48 volts), then rectify that and charge the batteries with it.

So basically a 555 and a ~30volt dc wall wart should have everything you need (24v would probably work fine too).

There is probably a chip that does it too, that takes a little digging though. I'll see if I can sketch up what Im talking about.