How big is the battery.
If it is a reasonable size, any of the "maintenance" type solar panel chargers should work, without requiring a charge controller. However, you do want to make sure that the panels have a blocking diode, otherwise, the panels will eat a lot of electricity at night, when the current reverses and flows through the panel.
The real problem is that you really need to figure out how much of the batteries capacity you use on a trip on average, as well as what it loses per day on average just sitting there...and then to get a panel that is going to be strong enough to top the battery up and keep it there.
A 2W panel, like the one Jotun has picked, only provides less than an amp-hour per day. This is barely enough to keep up with the normal loss from a battery just sitting... and likely too small for your needs. A 20W panel,
might be a better choice, since it would act to top off the batteries, as well as keep them topped off—providing 4-6 amp-hours per day.
It also depends on where in the world you are... in the higher latitudes, during the winter, the hours are much shorter, and the corresponding charging capabilities are reduced.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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