The battery only "accepts" the amps it can handle, so the charger's voltage can remain the same while the battery is actually self-controlling what it uses.
When the 12v system has various loads running off the converter, they get first call on the output and the battery gets whatever is left over. (proper explanation is wrt the voltage differences between the loads and the converter being more than the difference between and the battery and the converter---or something like that!)
Modern converters do not have that transfer switch thing or relay like the old Magnetek 6300s. Your choice is between a single stage or a multi-stage modern one and how many amps size. Note that the amps size limit is how many watts (VA actually) your generator can do
If you get a power factor corrected charger you can do more amps size on the same generator. In the 75-100amp size range, the PowerMax converters do have PF correction. Other brands like PD and Iota may have features you want instead if generator is not the limit.
Mex here, has noted that you do get more amps even with the single stage converter because AGMs accept more at the same voltage than Wets do, which is interesting if you are choosing to upgrade from a single to a multi just to get more amps and you now have Wets. of course you can do both--get AGMs and the multi.
