10w solar panel charging

[pollenface]

You can upwards of 120ah with a blocking diode (so it doesn't back-feed at night).

I found with batteries less than 120ah the voltage would eventually get up to 15ish as the days went by, which is not ideal methinks.

Less than 120ah I would use a 5w panel.

You should choose a blocking diode which is rated 200% of the solar panels short circuit current (isc).