If your converter is constantly putting out 13.6VDC to your battery terminal then keep in mind this is right at the DC VOLTAGE level that will boil out your battery fluids in a weeks time. When I had the single mode converter charging system i always checked my battery fluids about every week and added distilled water to keep the fluid level just above the cores.
When I changed everything out for the smart mode converter chargers this solved all of those problems for me. I still check my battery fluid level out of habit but never find them low anymore...
Of course a normal shorted cell failure on the battery will also boil out your fluids... Once the fluids go away the battery is usually beyond coming back.
Roy Ken