I have a summer place in new mexico that is totally solar powered. Harbor freight sells a 5 watt charger that 'theoretically' should have trickle charged my backhoe battery. However, without a controller, it fried it when left unattended for a few months. The controllers that sell for about 30 dollars will keep the battery from overcharging by shunting the solar power away after a preset voltage (appropriate for wet cell batteries) is reached. I've seen them at Northern Tool, West Marine, but I buy them from Northern Arizona Wind and Power.
What you end up with is a charger/maintainer that is WAY better than a plug in type, since it actually does taper to nearly nothing when the voltage is reached. For my home power, I use more complex MPPT charge controllers that allow incredible control over the battery. Also, Harbor Freight sells a 45watt combo with a not-so-great charge controller for $200. It's actually 3 separate 15 watt panels. Add 3 charge controllers, such as the Morninstar sungaurd controllers (about 30 dollars apiece) and you've got 3 charger/maintainers that will bring up a dead battery with no attendance or worry, just make sure there's water over the top of the cells (and no shorted cells). They can literally be left on all the time and whatever vehicle or implement will start when you want it to, especially when parking them far from the barn or shop. Solar chargers are cheaper and better than plug in types for my money, but you shouldn't use one without the controller.
The small 1.5 watt one does work for the average battery, but it relies on the resistance of the battery to keep it from overcharging, and doesn't have enough oomph to bring up a low battery in a few hours like the others I mentioned.
