The thing about AGM batteries is that you should never let them go completely dead. I had AGM batteries in my Yamaha for the past 10 years and they last several years with proper maintenance. I have been using a "Battery Fighter" (odd name) charger. I always disconnect it when it reaches "maintenance mode" (referred to as "float charge" here). I try not to let my battery go more than a month without being charged.
Battery Fighter® ’s 4-Step Constant Current Charging Program:
Step 1) Qualification:
Qualification check to ensures safety by verifying battery status prior to charging. Now you will know if batteries need replacing without guessing.
Step 2) Bulk Charge:
Constant current at 1.25 Amps to raises battery voltage to be approximately 80% recharged. Then current drop gradually until voltage reaches 14.4 VDC. When battery voltage reaches 14.4 VDC, will transit to Step 3, Absorption Charge.
Step 3) Absorption Charge:
Holds voltage at 14.4VDC and forces current to fully charge the battery. When battery-charging current drops below 0.1 Amp or until 6 to 8 hours have elapsed, will transit to Step 4, Float Charge.
Step 4) Float Charge:
Maintain fully charged battery at 13.2VDC float voltage. If an external load is applied to the battery while the charger is in Step 4, Float Charge, and if the battery voltage drops below a range between 12.0 to 12.5 VDC, then the charge cycle will restart.