Converters there are some that are fantastic (Progressive Dynamics Wizard controlled for example) Some are **** (Magnetek 6300) the Parallex 7300 is an ok charger but may well never top off the batteries.. Plugged in it puts out as the other poster said... It is a single stage.. Parallex also makes a 7300T which is a two stage timed first stage. It can fully charge a battery but... Well a good 3 stage is better.. That said so long as it's working I'd not upgrade.
External chargers... Some are ****. some are decent. Mine is 2 stage and very good at it (No need for the 3rd stage on that type of charger)
It is a Deltran Battery Tender 2-20-70 amp (70 is for jump starting and I've never gotten it to work) but it sure did the job when my Progressive Dynamics died (till i could get a new one a couple days later.. Thankfully PD was only a slight detour for me that week.. i'm serious about the slight detour (Alternative route is all) IT was fun driving through my old stomping grounds.