when should a powermax pm4b drop into float mode

As you run out of sulfur on the lead plates to put back into ionic solution in the battery acid/electrolyte, at constant voltage, the amps will taper. The acid runs our of chemical reaction capacity as your Specific gravity reaches maximum... there is no more usable sulfur on the plates to go back into the solution. No sulfur to put in solution, no amps can flow, except those used for hydrolysis.