Mex has nailed it.
Battery wants to rise to 14.6, hang there for a while and then stay at 13.8 until the cows come home. Then it will be full and happy.
Generator is good for partial charging, it won't bring it to full in an hour or two.
Converter is good for keeping it at 13.8. The best models will raise it to 14.6 but it's not intelligent enough to know for how long to keep it at 14.6 before dropping to 13.8, so the battery won't be completely happy but more-less satisfied.
So the best way is to plug a regulated DC power supply and babysit it, watching the current and changing the voltage.
Fortunately, there are better solutions today.
A decent solar controller will bring it to 14.6 and power down to 13.8 when the time is right. No need to do anything manually, microchip will switch to 13.8 when needed, not sooner or later. A half-decent one will still bring it to full by running 13.8 after an hour or two on generator, not a perfect way but better than most other scenarios.
