Designing an effective extensible solar cell + car battery setup
I would recommend sourcing a proper MPPT battery charger, designed for the job.  You don't want to over-charge the battery when it's not being used.

If you connect panels in parallel, ensure they are protected by diodes, to prevent one panel forcing current backwards through the others.  Some panels may have diodes already built in.

Or use a charger designed to handle a larger input voltage and wire the panels in series.  But if wiring in series, the current is limited by the weakest panel.  Beware of issues with shading of the panels by nearby trees.