Charging batteries directly from solar panels is tricky because of the mismatch between pack voltage and panel voltage. To further complicate matters, you will only get peak power from your solar panels by carefully managed their output voltage and current (called 'finding the knee of the I-V curve'). More about getting the most from solar panels here:http://en.wikipedia.org/wiki/Maximum_power_point_tracking#I-V_curve
Normal practice is to use a maximum power point (MPP) tracker device between the solar panel(s) and the battery bank, but that practice works best for relatively safe SLA deep cycle batteries. Most solar panels are engineered to work with SLA voltages. Even then, using a MPP tracker still
does not provide proper 'end-of-charge'
voltage limit and current tapering required by LiFePO4 batts.However
, there may
be an better alternative to a MPP tracker that could work with LiFePO4 batteries... (YMMV)
be worth it to try using the solar cells paralleled together to provide a 12-18v input to a RC LiPo charger. Make sure you use a charger that can accept a range of inputs between 12-24v, there are lots of models that do so. Then setup the RC charger so that it limits amp (current or 'I') draw to 1 amp for every ~18 watts of rated power from your solar array.
The RC charger will correctly provide the finishing voltage and current tapering your LiFePO4 batts need, all while producing very close to max power from your solar panels.
In fact, the RC charger's input power current limiting feature may
give you free bonus MPP tracking capability.
After you've estimated the appropriate current limit for your solar array in amps and begun charging, try changing the input current limit. First +1 amp, then -1 amp. Watch how much power is produced by the RC charger as the input current changes. You should be able to quickly find the input amps setting that maximizes the power output of the charger. This number will vary with sunlight (of course), panel placement (perpendicular to the sun in best), and the outside temperature (cold panels make more voltage). Experiment, and experience will be your teacher.
In effect, you will become your own MPP tracker. Congratulations, you've replaced a machine! Take that, Drone Army!
Cheers, and enjoy that sunshine!