In a nutshell:
A proper charger for lead-acid batteries does three steps:
(1) limit the initial charging current to a safe level specified by the battery maker, (and possibly to protect the charger itself)
(2) allow the battery voltage to rise to ~14.5V (2.42V/cell), hold it there until the charging current naturally decreases to ~10% of the initial charging current used in step 1, (could take hours to days, depending on state-of charge, the current used in step 1, and the A-H rating of the battery).
(3) then switch to the float voltage of ~13.2V (2.20V/cell, must be very accurately set, should be temperature compensated).
is the straight dope:
I do take issue with the following statement in that article:
"During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated."
Actually, all the charger needs to do is keep the voltage fixed at ~14.5V, while monitoring the current acceptance of the battery. The time to step from 14.5V to ~13.2V is after the current during step 2 drops below a few tens to hundreds of mA, depending on battery size.