Four stage chargers typically have four active stages plus equalization option.  The four stages are Bulk, Adsorption, Float, and Rest(also know by other terms).  In Bulk and Absorption the objective voltage is typically 14.5 volts for a 12 volt battery. This may vary according to the battery type setting (Flooded, AGM, Gel).  In Float the voltage is typically 13.5 volts.  After some period of time in Float a four stage charger will switch to Rest where charging stops until the battery falls a bit below the fully charged rest voltage of 12.65 volts (again varies by type). At that time the charger will return to Float mode then cycle back to Rest once certain Float parameters are met. The Rest mode overcomes the various disadvantages of maintaining a battery on Float voltage forever. Tom Collins - Misty Sea