There are basically 3 types of converters for this discussion.
The Magnetek 6300 Series is the one that is well known for boiling batteries dry, It also will not work without either a battery or a "Battery Simulator" which of course is no longer sold.... Under that name.
You can get a hardening condenser or hardening capacitor (Same thing) at a high end audio shop. these are typically multi-farad capicators with around a 20 volt rating,, They are... The same as the Magnetek simulator.
Most modern singnle stage converters do not necessarly bring the battery to full, but they also do not boil it dry.
A Progressive Industries, Intellapower 9200.. now that is one heck of a converter. From the research I have done, if you have flooded wet cells. it is #1,, A few others fall into the same quality band but none better.