Ive used a 5-watt solar panel, made by ICP, to charge two wet cell batteries for years. This low power unit has a blocking diode to prevent the battery from discharging at night, however, I added a fuse to the hot wire as a precaution should the unit fail for some reason.

I clip the unit directly to one of my two batteries as a small unit this size will not maintain two batteries at the same time. I alternate the battery being maintained each time I go to the boat. I think it important that your batteries are fully charged, and can maintain a charge, before you start such a regimen.

Small solar units do a good job of maintaining batteries that are in good condition, but will not do much for a battery in a discharged state.