deep cycle agm battery not charging

If you have flatten a battery very low to a point when a charger will not charge it the best thing to do is connect a 13.8v power supply to it and let it run for 20-40 minutes, this will put enough voltage back in for a battery charger to detect the battery and start charging.

Running a battery too low is not a good idea and over time you will destroy the battery.

There are a number of issues you may be encountering.

1/ ya battery may be buggered.

2/ these so called smart chargers can be easily fooled....in a number of situations they will not connect and chage a battery.
I would not have a smart charger as my only charger....many times I have connected one of mine to a battery and it would not play.
As mentioned above a 13.8 volt power supply or any old dumb charger might kick the battery in the guts hard enough to get enough charge so the multistage charger will play with it.

3/ batteries with calcium in the plates ( that is all sealed batteries) can sometimes develop charge resistance when dishcarged too far or left flat for too long....a higher charge voltage than usual is required to overcome this charge resistance.....varioulsy some say 15 or 16 volts.......your charger may have a setting for this.

BUT......BE CAREFULL...if you run a high charge voltage, observe the battery careffully and donet leave it on the high charge voltage for too long

cheers

Glenn, the easiest way to kick start this process of charging the two above are talking about may be to connect a set of jumper leads from a running vehicle to the battery to spark (no pun intended) it into life.