A few things.
All basic solar panel (an array of solar cells) like yours just provides voltage and current. Wire them in series for more voltage and in parallel for more current. Just like batteries.
A 12V solar panels typically have an open voltage of ~20V DC (18V is close enough). Meaning open circuit with no current draw. So if you test it with a volt meter with nothing connected you should see ~15-20V.
Once you put a load (FlashFish) on there, the voltage will droop to something much less. You need to be at about 13.5V to charge a 12V lead acid battery. The charge controller should take care of that and I would assume the FlashFish has a charge controller in there it should be able to connect it directly.
I would
- Check the polarity (+/-) of your connector. There is no standard and it could be positive or negative polartiy .
The polarity of the plug should be in the documentation that came with the FlashFish. If it's not in there contact them and ask them.
- Make sure the the Solar panel is perpendicular to the sun with sufficient sunlight.
- Check the voltage at the panel. It should be 15-20V with no load and about 13.5-14V when plugged in.
It will take forever to charge with that solar panel. FlashFish shows the input as 15V @ 2.5A. This is vague, is that min/nominal/max rating? Not that you need to answer. Lets assume it's max 15V max input and it will draw a max of 2.5A? So if you are providing 1.5A max and they say it's 7-12 hrs (assuming 2.5A) it could be 20 hrs or more to fully charge it from the solar panel.
If you have doubts, I would ask FlashFish how to connect the solar panel.