I have used a similar solar panel as mentioned by hcb3200 above. It is listed as 30 watt but the most it would output is about 20-25 watt in bright sun.
"OYMSAE 30 Watt 12 Volt Solar Panel Solar Trickle Charger 30W 12V Portable Solar Battery Charger & maintainer with Cigarette Lighter Plug & Alligator Clip"
It is convenient to just plug it into the cigarette lighter socket or use the alligator clips directly to the battery. I think a maintainer has circuitry to reduce the charging current when the battery is fully charged. This trickle charger just supplies the output from the solar panel to the battery regardless of the state of charge.
Mark
