Everything I've read about it recommends using float type maintenance chargers on gel and agm batteries; I do, and they've worked very well for me. If you have the battery in the vehicle, and something is drawing a current, your battery can go dead with a maintenance charger on it because the charging current is so low. It's normally very cold in winter where I live, so I remove all batteries that won't be used for the winter and store them in the basement to reduce cold stress. I have more batteries than chargers, but just swapping the chargers around every couple or three days keeps them charged just fine.

 

May I suggest to the OP to google Deltran and read up their chargers. They are very reliable. If you deplete an AGM battery they rarely come back, to use a smart charger on a depleted battery it's a crap shoot.