Lou is almost right. Bulk mode is a constant current mode with a rising voltage and absorption (or acceptance) mode is constant voltage of about 14.4V with a falling current. This is taken from the manual for our Link 1000/Heart 20D monitor and inverter/charger.
Ned is almost right. (couldn't resist that one
) In Bulk mode most smart chargers will
hold a constant current by forcing
the output voltage to as high as reasonably necessary. The actual current flow is determined by the SOC of the battery being charged. That current will remain high (and constant) as long as the battery will accept/demand it. The voltage rises as the current decreases. Some chargers simply use a timer for bulk mode and some (the real smart ones) actually sense the charge on the battery by monitoring the current flow and/or battery voltage.
What ever the criteria, the second stage (absorption or acceptance) is, as Ned said, a period of constant voltage (13.8 to 14) and will continue until the current drops to a predetermined low level.
My point is that during the first two stages, it's really the battery that is in charge (pun intended). When all is said and done, if the charger will not lower it's output voltage to near 13.2-5 for a constant "float", then all is for naught, as the battery will eventually over charge and begin to dry up.