Since you have the HF panels, you could a couple different ways.
1.)Tie 5 in series and attach them directly to the batteries, with 15 watts input your not likely to over charge them for a long time.
These normally reach above the 12V each that they are supposed to be for but will quickly drop once loaded.

2.) Tie them together into a 12V array and use a voltage multiplier circuit to get it up to charging voltage. the current will still be low enough to not have to worry too much about overcharging. NOTE: just make sure the diodes and caps for the multiplier is above the voltage of the output i.e. up around 100V for the caps and 24V minimum rating for diodes would be a good safe level.