You should be able to pump 6 amp into that battery load 24/365 and not do any damage. A 45 watt panel on a good day will be lucky to do 3 amps. But lets say 4 amp, still under the 6 amp. I would not even use a controller. Wire direct with fuses. You need to check that the panel you get has blocking diodes. If you drop down to an 100 amp hour battery system you would need a controller.