I don't know of a better maintainer/charger then the battery tenders. They will charge a battery at normal charging voltages, then switch into a float/maintain mode, which will not hurt a battery, and can be left connected all the time. It should work fine w/ the Big Crank AGM, or any other lead acid battery. How often ?? You can leave it connected all the time, but the main thing is to keep any battery as near full charge as possible at all times. The closer a battery stays to full charge, the less it will internally sulfate, which will shorten its life.
For my bike, because its conveinent, I connect a float charger anytime I won't be riding it for a couple weeks or more. I have had batteries last 10+ years, just by keeping them fully charged on a maintainer/float charger. Others experiences may differ, but the experts will tell you the importance of keeping a battery fully charged, one way or another.
rl153 wrote on 02/07/10 at 20:06:34:Routy ,does a deltran battery tender junior work good with a big crank,and how often would you use it? Thanks