What is the usual or recommended circuit arrangement for incorporating a
solar panel as a backup or trickle charger for motor and/or secondary
batteries? My hope is that a small solar collector would be enough to
keep the batteries charged when the boat is not being used and to charge
the second battery (over a 12-hour period) after using the second
battery to power an auto-steering unit, depth finder, etc., for several
hours during a day's sailing, when the motor isn't being extensively
used and when shore power isn't available. I would also plan on having a
110V-powered battery charger for use when 110 AC was available.

This is a relatively small vessel that will likely be used on cruises of
3-5 days and not for extended blue water cruising, and I don't want to
invest in a large, high power solar collector. (Perhaps 10 watts or so.)
One concern is whether I need a regulator or shutoff device to prevent
overcharging of the batteries if the solar panel is left in a charging
mode for long periods. What's necessary for providing an appropriate
charging time, level, etc., or is it not that critical?

Thanks for any suggestions.

Jim