If you don't use a controller, you will need to put a diode in series between the panel and the batteries. A basic 1 amp diode at radio shack should do the trick.
Place the diode on the + side of the panel. The diode will have a symbol " >| " indicating Anode and Cathode. Place the lead with the "|" side to the battery and the other to the panel.
The diode will allow current to flow from the panel to the battery, but not from the battery to the panel when there is no sunlight. If you don't place a diode in series, the panel could will draw from the battery at night, so the net effect of the panel in reduced.
If you live in an area that has limited sunshine ( or snow)during the winter months, a 15 watt panel minimum will be needed to maintain two batteries.