Battery Charging Currents
Ideally, charging current 'is decided' by the manufacturer's recommendation--but really, it's a pretty well known electro-chemical reaction that has been extensively studied/tested to determine the best way to charge such batts.  Most LA batts can be charge with a higher charge current, but a gentler charge is usually best.  I know some people quick charge LA batts with 10A or higher, but I would never recommend that.  Personally, I use a regulated bench power supply &amp; charge at 2A or less.  It takes longer, but I think it is less destructive to the batt plates.
The 'decay' is not decided.  What happens during the 'top charge'--which I prefer to think of as the 'saturation charge' stage is that the charge V is held at 14.1V and the batt takes as much charge as it is able to take.  As the batt becomes saturated, it increasingly accepts less current all on it's own--you don't need to do anything except keep the V at 14.1V (on a typical 12V flooded-LA batt).
Essentially, once the batt is fully charged (i.e. saturated), you do not want to continue to charge it @ 14.1V.  Depending on the batt composition, continuing to charge at that rate simply causes unnecessary gassing (electrolysis of the electrolyte into H2 &amp; O2) &amp; unnecessary heating of the batt.  Personally, when my charging current drops to ~300 mA, I lower the charging V to 13.5V.  The reason I use 13.5V is that once the batt has rested for 24h, I want the resting V of the batt to be @ ~12.7V.  If you set the float charge below 13.5, your resting batt V after 24h will likely be &lt; 12.7V. It's not a big deal though.  Some automatic chargers set the float V to 13.2V instead.  Usually 13.2V is better for newish batts compared to oldish batts--which seem to need a bit more charge V.


hth, best regards!