Syndini,
Glad you got 2 fresh batteries!
I read somewhere that Northstar battery was started by a couple disgruntled Enersys/Odyssey employees. No Idea if this is indeed true.
NS does make many Similar claims as Odyssey as to life expectancy, and their group 31 batteries have identical Specs, or nearly so. Even the design of the battery casing is very similar between the two.
Info about Northstar batteries and their own documentation is not as Good as Odyssey, but I've been using Odyssey charging recommendations, regarding the 40% initial bulk charge rate. Absorption and float voltages are listed as the same as Odyssey.
I'd have no fear of using the Odyssey charger on a flooded battery, but 13.6v is too high a float/maintenance voltage for a flooded battery, and good size flooded batteries can still easily handle 30 amps of charge current. Just do not leave the odyssey charger hooked indefinitely to a lead acid battery as a maintenance/float charger.
You might want to try the 50% discharge followed by the 30 amp recharge letting the Odyssey optimizer charger do the specific Odyssey voltage algorithm. Before doing this, the NS battery was not able to achieve 13.06 resting volts with the Northstar, and that first key turn after that 40% bulk rate recharged from 50%, was shockingly fast, and impressive.