Solar panels can still generate power even under completely overcast days like today.
As you can see the panels are putting out a bit over 13vdc today. Not optimal, but should keep the deep cycle from draining as fast under load.
In this case I’m powering the Kenwood 2m while charging a phone at .5A; the red plug with the wire coming down is a usb hub. I plug the phone, kindle, etc into it to recharge for free. Under this light load and with losses in the freebie/primitive charger the deep cycle is holding steady at 12.8vdc. Again, not optimal but tolerable.
The setup that would shine here would be panels in series to get the voltage up, then run that into an MPPT (Maximum Power Point Tracking) solar charger. Example: the MPPT would run the panels at a voltage that gives maximum wattage, then DC-DC downconvert to whatever voltage it wants to use at a given stage of the charging cycle. Let’s say >= 14.5vdc . We can see that our 13.11 isn’t going to let the charger do what it really wants to do.
My cheap Harbor Freight panels (3 x 15w in parallel) won’t handle serial configuration without releasing magic smoke. I’ve got a higher end 60w panel but no mate for it. I also have two better chargers (one PWM, one MPPT) that I have stored. Saving them for a more interesting project.