"AFAIK The current part of that graph doesn't represent the normal charging amps going into a depleted battery. As mentioned in this thread, the amps should start high and decline until float.
But I guess the Magnum does something different in Bulk, where it increases its applied V to the final bulk voltage?
Odd that they call that phase "constant current" when the voltage and current is increasing in Bulk.
Really don't understand what's happening there. "
It is just a normal charging profile for a "smart" charger.
In Stage 1 (Bulk) the smart charger is "current limited" to its rated current "size" so it does constant current while the battery's voltage rises to the charger's setting for Stage 2 (Absorption) voltage (Vabs)
The smart charger then holds its voltage at Vabs while the battery's voltage continues to rise, shrinking the difference between the two voltages, so current tapers.
At some point close to when the battery is "full" the charger's voltage needs to be reduced to a "Float" level. This is where you see a variety of profiles among charger types and brands as to just how far along they stay at Vabs before dropping to Float. Some chargers have no Float at all. They just shut off when the battery is nearly full ("automatic" as opposed to "manual" charger)
The contest now becomes how to get the battery "truly full" after the charger has shut off or dropped to Float voltage, which is normally at about 97% SOC. A variety of methods are available to do that 97-100% part, which have been the subjects of many threads on this forum.
BTW never confuse the above battery "stages" with the "stages" term used with some converters, such as "three-stage" Those stages are just three different voltage levels the charger does, and has little to do with the stages the battery goes through.
Some call their second stage "Absorption" and have a lower voltage for that stage than what they get the battery to in their Stage 1. (Iota and PowerMax do that) Others have a timer so it stays at the "high set point" Vabs two or four hours and then drops to the second stage voltage (which will be the battery's third stage)
A "single" stage (one voltage setting) smart charger will put the battery through two stages: Bulk and Absorption. Say you set your 60 amp charger at 14.8v and put it on a 220AH battery bank down 50%. The charger will do 60 amps (or a bit more usually) steady for about half an hour to get the battery voltage up to near 14.8, then amps taper while the charger stays at 14.8 and the battery gets ever closer to that voltage until amps pretty much stop flowing. Now you must lower that voltage or shut the charger off, and go to a "maintenance" charger to do the Float.