This is the latest one I've added to my collection. Ctek 3300
CTEK Multi US 3300 - CTEK Battery Chargers
I like that it is compact (relative to output current), very well sealed for outdoor use, and has a cold weather mode that bumps the voltage up (same setting is used for AGM batteries at normal temperatures).
I might not leave it connected for 6 months straight at a time, but I consider it friendly enough to leave connected for long periods at a time.
You can find it on sale for no more than half of list price. Rgds, D.
P.S. - old thread on Ctek, etc.... :
http://www.tractorbynet.com/forums/owning-operating/270368-ip65-plug-n-forget-battery.html
I leave my CTEK's and Battery Tenders on 24/7 continuously on my Jeep GC, garden tractor and 8kw generator. And previously on my 1990 Nissan 300ZX TT with 90k miles on it. Never an issue. These are well regulated and do not boil the water out of the batteries.