Hi,
The approach, afaik, is (...knowing what the batteries will be powering and checking they will be "large" enough and will last as long as needed...) to start at the end with the load, i.e. the batteries: voltage and current draw (presumably mA/h or A/h, peak and steady if so), from there perhaps it's advisable to put together a solar panel parallel or series string, unless one panel will suffice, to meet those requirements plus all housekeeping circuitry's (on the PCB) power needs, and from there select a suitable battery (solar) charger PCB as the bridge between both ends. Good luck. I'd avoid products with no datasheet or user manual.
Also, TI, STmicro, Analog Devices et al all have application notes or IC datasheets or even evaluation PCBs maybe that are very informative and help to understand these kind of circuits.