OP should state brand and model of AGM battery. Sounds like a telecom surplus battery she got down where I got mine, but we don't know the brand and model number.
If it's like mine... DEKA brand surplus telecom AGAM, I set the voltage at the Mega Watt at 14.40 volts, with my in line RC charger meter, and let 'er run until the RC line charger shows a current rate at or very close to .75 amps. That specification is a 0.5% rating of the battery amp hour rating at a 20 hour discharge rate. 150 amp hrs x .005 = .75 amp hours current rate is considered fully charged, or close enough.
An AGM does better just disconnected after being fully topped off charged coming back from a camping trip. Discharge rate in fall, winter and spring, are quite low per month. They will require more frequent top charge in the summer, if stored in higher heat. Once a month then is sufficient.
