A 40 Watt panel will need a charge controller or you could damage your battery.
In full sun your panel could produce over 15 volts (as I have seen on mine frequently) which could overheat your electrolyte.
Additionally, without a charge controller (with internal blocking diodes) your battery will discharge back through the panel at night wrecking your good work during the day.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW