How satisfied one will be is only in a few years. The previous Zenith AGM (no Heavy Duty) have only held 2 years. Maybe I've also connected them wrong. I know now, plus supply line must be connected to 1. and minus lead to last battery. I had both on 1 and the other batteries parallel. Possibly. I have too often deeply discharged - 3 days anchoring loose 150 to 180Ah as I now white now since Batt.Monitor installed. In the previous boat the AGM has held 10 years.