My boat has munched through three Victron 12v 90ah starter batteries in 9 years. By rights these high quality low internal resistance batteries should last 7 or more years. The battery is mounted on a shelf immediately above the engine and it gets pretty hot in there. Enough is enough and I’m moving the battery despite the complications. When they fail the voltage display still reads 12.8v but it can’t deliver enough power to crank the engine and operate the electronics interface. Interestingly when it has failed to start the voltage drops to 12.4 / 12.5 and after a short time creeps back up to 12.8 without any charging current. So voltmeters are no indicator of real health in these batteries. My Victron AGM 12v 165ah batteries are lasting 10 or more years but they are away from the engine heat.
My money is on a temperature problem with the M3W AGM batteries. I use a standard Varta on my M3W but have to top it up regularly otherwise it’s spot on.
