well the panels came with a 10 amp standard diode (I suppose just for the sake of it) I'd prefer to used a skottky one.
If I'm to build my own I need to know the "logic" of charging batteries, I'm sure it can be done with comparators and possibly some digital logic (ok ok a pic can do the lot but my pic skills go as far as knowing the wonders they could do for me if I knew how to program them).
As for the diode well, if i was designing my own I'd be using the panel to power the SMPS first so that surely would act as a block anyway from the battery discharging into the panel. So I'd step down the voltage hopefully increasing the current by about 20 % and charge the battery via the appropriate "logic" which as far as i know is when "x" voltage is reached its charged but I'm sure i need to know more than that....