when should a powermax pm4b drop into float mode

Check the specs here for the 45 amp WFCO's input in watts and amps. the 55 amper's is 12 amps.

So 9.8 on that 45 of yours is actually low. Perhaps its PF is not as much as the WFCO's.

What it all means is that the way these different companies state their specs is not to a common standard, so it is very confusing.

http://www.bestconverter.com/WFCO-9845-45-Amp-Deckmount-Converter-Charger_p_42.html#.VDmMcmd0xjo